Development of pretreatment technology and enzymatic hydrolysis for biorefineries. Anne Kallioinen
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This dissertation examines the development of pretreatment technologies and enzymatic hydrolysis for biorefineries. Several new lignocellulosic feedstocks, such as reed canary grass and barley straw, were studied and found to be suitable for pretreatment by steam explosion and enzymatic hydrolysis. A new alkaline oxidation pretreatment method was developed that fractionated biomass into a carbohydrate-rich fiber and lignin-rich dissolved fraction. This fiber could be efficiently hydrolyzed by enzymes. Optimization of enzyme mixtures was also studied to improve hydrolysis efficiency and decrease enzyme dosage requirements. The research contributes to making biorefineries more economically feasible through the development of new pretreatment methods and optimization of enzymatic hydrolysis.
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view. J. Basic Microbiol. 50, 5–20.](https://image.slidesharecdn.com/s56-140515004323-phpapp01/85/Development-of-pretreatment-technology-and-enzymatic-hydrolysis-for-biorefineries-Anne-Kallioinen-94-320.jpg)
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sugars. These include endoglucanase (EG), which hydrolyses the internal β-1,4-glucosidic
bonds randomly in the cellulose chain; cellobiohydrolase (CBH), which moves processively
along the cellulose chain and cleaves off cellobiose units from the ends of the chain; and β-
glucosidase (βG), which converts cellobiose and soluble cellodextrins into glucose. Individual
cellulases have only limited hydrolytic effects, whereas a mixture of cellulases can exhibit a
synergistic action in which the hydrolytic activity of the cellulase mixture is greater than the
sum of the activities of the individual enzymes [1]. Enzymes hydrolysing hemicellulose are
also required for complete hydrolysis of lignocellulosic biomass [2, 3] and they act synergis-
tically with cellulases [4-7].
The enzyme cost is important for the economy of biofuel processes. Optimized
enzyme mixtures can provide maximal synergistic effect, and thus, more efficient
hydrolysis can be obtained with minimized enzyme dosages. Several studies
concerning optimal enzyme mixtures have been published with various lignocellulosic
substrates such as pretreated barley straw [8], pretreated wheat straw [9], pretreated
corn stover [10-14], pretreated switchgrass, Miscanthus, distillers grains, poplar wood
[2] and pretreated douglas fir [15]. The optimization of enzyme mixtures has been
performed for a core set of cellulases [14], cellulases and xylanases [13], for multi-
component mixtures [2] containing several accessory enzymes and for mixtures of
commercial enzymes [16, 17]. Banerjee et al. [2] reported hitherto the only compar-
ison of the effect of raw material and pretreatment on optimal enzyme mixtures.
Optimized enzyme mixtures of Trichoderma reesei enzymes were shown to be dom-
inated by CBHI and EGI for most of the raw materials and pretreatments, whereas the
proportions of β-glucosidase and xylanase varied considerably depending on the raw
material and pretreatment. According to their results, hydrolysis of NaOH-pretreated
corn stover and switchgrass required less xylanase than ammonium fibre explosion
(AFEX) and alkaline peroxide-pretreated materials. The high proportion of xylanase
was generally compensated by lower Cel7A (CBHI) and Cel7B (EGI) contents. Wide
variation was also observed depending on how many and which enzyme components
were included in the optimization. Therefore, it is important to include all the most
essential enzymes in the experimental design, although this will make the design more complex.
The optimal enzyme mixtures have also been shown to be dependent on the total enzyme
loading [13] and hydrolysis time [9].
Thermostable enzymes have several advantages in the hydrolysis of lignocellulose, such as
the potential to operate in higher hydrolysis temperatures, reduction of the enzyme
amount needed due to higher specific activity in elevated temperature and higher
stability enabling longer hydrolysis times [18]. Higher hydrolysis temperature de-
creases the viscosity, enabling higher substrate concentrations and potentially increas-
ing product yields and consequently reducing processing and capital costs. A pre-
hydrolysis that liquefies the high viscosity slurry is one suggested application for thermo-
stable enzymes [18, 19].
The aim of this study was to develop optimal enzyme mixtures of thermostable enzymes for
hydrolysis of three different pretreated lignocellulosic raw materials: hydrothermally pretreated
wheat straw and alkaline oxidised or steam-exploded bagasse. The effects of raw material
type and different pretreatments on the optimal mixture were investigated. The work
was carried out in two stages: first two-component synergistic systems were used to
narrow down the ranges for optimization, and secondly, optimal mixtures were
developed using statistically designed multi-component optimization protocols.
Mixtures of thermostable enzymes were also compared with mixtures of T. reesei enzymes
and commercial enzyme products.
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Materials and Methods
Raw Materials
Hydrothermally pretreated wheat straw was provided by Inbicon. Pretreatment was carried out
at the Inbicon pilot plant (Skærbæk, Denmark) by the method described in Petersen et al. [20].
Straw was cut into pieces up to 6–10 cm long and pre-soaked in 3 g l−1
acetic acid at 80 °C for
10 min. After that, the feedstock was fed to the pretreatment plant at a rate of 50 kg h−1
and
steamed in the hydrothermal reactor at 190 °C for 12 min. The pretreatment was conducted at a
water/straw ratio of 5:1 and the pretreated straw was pressed to a higher final dry matter (d.m.)
content of 33 %. For hydrolysis experiments, handsheets were prepared from pretreated
wheat straw after homogenization by cold dispersion. Laboratory handsheets (target
density 120 g/m3
) were formed in a sheet mould producing 165 mm×165 mm sheets. The
sheets were pressed at 490 kPa for 4 min and dried at 60 °C for 2 h. The d.m. content of
pretreated wheat straw sheets was 93 % after drying. Handsheet disks with approximately 6 mm
diameter were cut from the sheets using a paper punch.
Alkaline oxidised (AlkOx) bagasse was prepared according to Kallioinen et al. [21]. Sugar
cane bagasse was pretreated with 0.25 M Na2CO3 at 120 °C for 20 h at 5 % d.m. content.
Material from ten 20 g batches was washed and mixed. Sticks remaining in the material
(approx. 1 % of d.m.) were removed to provide a homogeneous material. The d.m. content of
AlkOx bagasse was 30 %.
Steam explosion (SE) of bagasse was carried out at 200 °C for 5 min [21]. Sugar cane
bagasse was immersed into dilute sulphuric acid (0.5 % w/w) at room temperature for 30 min,
the excess acid was drained and the material was stored overnight at 4 °C before SE
treatments. After SE, the material was washed with water. The d.m. content of SE-pretreated
bagasse was 32 %. The monosaccharide sugar compositions of the washed insoluble fractions
of the pretreated feedstocks after total acid hydrolysis [22] were analysed by high performance
anion exchange chromatography (HPAEC-PAD) using a CarboPac PA-1 column in a Dionex
DX 500 series chromatograph equipped with pulse amperometric detection [23]. The carbo-
hydrate contents of water-washed pretreated wheat straw and bagasse are presented in Table 1.
Enzymes
T. reesei cellulases Cel7A (CBHI), Cel6A (CBHII), Cel7B (EGI) and Cel5A (EGII) were
produced and purified as described by Suurnäkki et al. [24]. Xylanase (XYL II, i.e. family 11
xylanase with pI 9) was purified by the method of Tenkanen et al. [25], but omitting the last gel
filtration step. Aspergillus niger β-glucosidase was purified according to Sipos et al. [26].
Table 1 Carbohydrate composi-
tion of washed insoluble solids of
hydrothermally pretreated wheat
straw and alkaline oxidised and
steam-exploded bagasse (% of
d.m.), used in the experiments
Pretreated
wheat straw
AlkOx
bagasse
Steam-exploded
bagasse
Glucose 65.4 67.9 59.3
Xylose 3.6 23.9 3.0
Mannose 0.2 0.14 0.1
Galactose 0.06 0.1 0.1
Arabinose 0.05 1.3 0.4
Monosaccharides total 69 93 63
As polysaccharides 62 83 57
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The thermostable glycosyl hydrolases were kindly provided by Roal Oy (Rajamäki,
Finland): three CBHs originating from strains of Acremonium thermophilum, Chaetomium
thermophilum and Thermoascus aurantiacus; one EG originating from T. aurantiacus; two
xylanases originating from strains of T. aurantiacus and Nonomuraea flexuosa; and a βG from
T. aurantiacus. The enzymes (Table 2) were heterologously produced in a genetically modified
T. reesei strain, deficient in the four major native cellulases (Δcbh1/cel7A, Δcbh2/cel6A,
Δegl1/cel7B, Δegl2/cel5A), under the control of the strong cbh1/cel7A promoter of the host
fungus [27]. The TaCel7A (CBHI) construct contained the cellulose-binding module (CBM)
from T. reesei Cel7A (CBHI), and TaCel5A (EGII) contained the CBM from C. thermophilum
Cel7A (CBHI). Thermostable enzymes were heat-treated at 60 °C, pH 6 for 2 h to inactivate
the thermolabile side activities. For mixture 1, the CBHI component TaCel7A+CBM was
purified by one-step anion exchange chromatography, using columns and chromatography
media available from GE Healthcare. The preparation was first desalted and equilibrated with
0.02 M sodium phosphate buffer (pH 6) using a Sephadex G-25C column. After that, the
sample was loaded on a DEAE Sepharose FF column, equilibrated with 0.02 M sodium
phosphate at pH 6 and eluted using a gradient of 0–0.2 M sodium chloride. Protein fractions
with TaCel7A+CBM eluting in ca. 0.07–0.11 M NaCl were pooled and equilibrated with
0.02 M sodium acetate buffer (pH 5.0) using a Sephadex G-25C column. The CBHII
component of mixture 1 (CtCel6A) was expressed from the cbh1/Cel7A promoter in
the T. reesei strain VTT D-00775. The expression construct was transformed into
T. reesei as described in [28] and replaced the native cbh1/cel7A gene. The CtCel6A
enzyme was produced for purification in shake flask cultures in Trichoderma minimal
medium [28] supplemented with 4 % lactose and 2 % distiller’s spent grain extract at
Table 2 Components of the enzyme mixtures. Thermostable mixture 1 and Trichoderma mixture 1 were used in
preliminary optimization studies and thermostable mixture 2 and Trichoderma mixture 2 in final optimization
studies
Enzyme Abbreviation Used in mixture Purification
Trichoderma reesei enzyme mixtures
Trichoderma reesei Cel7A TrCel7A (CBHI) Mix 1, 2 [24]
Trichoderma reesei Cel6A TrCel6A (CBHII) Mix 1, 2 [24]
Trichoderma reesei Cel7B TrCel7B (EGI) Mix 2 [24]
Trichoderma reesei Cel5A TrCel5A (EGII) Mix 1, 2 [24]
Trichoderma reesei Xyn11A TrXyn11A (XYLII) Mix 1, 2 [25]
Aspergillus niger Cel3A AnCel3A (βG) Mix 1, 2 [26]
Thermostable enzyme mixtures
Thermoascus aurantiacus
Cel7A+CBM/TrCBHI
TaCel7A (CBHI) Mix 1 Heat-treated+purified
as described
Acremonium thermophilum Cel7A AtCel7A (CBHI) Mix 2 Heat-treated at 60 °C, 2 h
Chaetomium thermophilum Cel6A CtCel6A (CBHII) Mix 1 Heat-treated+purified
as described
Chaetomium thermophilum Cel6A CtCel6A (CBHII) Mix 2 Heat-treated at 60 °C, 2 h
Thermoascus aurantiacus
Cel5A+CBM/CtCBHI
TaCel5A (EGII) Mix 1, 2 Heat-treated at 60 °C, 2 h
Nonomurea flexuosa NfXyn11A Mix 1 Heat-treated at 60 °C, 2 h
Thermoascus aurantiacus TaXyn10 Mix 2 Heat-treated at 60 °C, 2 h
Thermoascus aurantiacus TaCel3A (βG) Mix 1, 2 Heat-treated at 60 °C, 2 h
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200 rpm and 28 °C. The culture filtrate with CtCel6A was desalted and equilibrated
with 0.01 M sodium phosphate buffer at pH 7.04 using a Sephadex G-25C column.
After that, the sample was loaded on a DEAE Sepharose FF column and equilibrated
with 0.01 M sodium phosphate (pH7.04), and the fractions with unbound protein were
collected. The purification was continued using an affinity column as described by
van Tilbeurgh et al. [29], eluting the CtCel6A protein with lactose. The affinity run
was repeated twice, and the fractions with CtCel6A were concentrated by ultrafiltra-
tion and at the same time washed to change the buffer to 50 mM sodium acetate,
pH 5. The thermostable enzymes and T. reesei enzymes used are presented in Table 2.
The protein contents of enzyme components were analysed using a Bio-Rad analysis
kit based on the Lowry method [30]. Purity of the used enzyme components was
analysed by SDS-PAGE using 12 % Tris-HCl gel (Bio-Rad) and pre-stained SDS-
PAGE Standards (Bio-Rad) (Fig. 1). Xylanase activity was determined by the method of Bailey
et al. [31], and β-glucosidase activity was measured according to Bailey and Linko [32].
Commercial enzymes Celluclast 1.5 L, Novozym188, Cellic Ctec2 and Cellic Htec were
obtained from Novozymes.
Hydrolysis Experiments
The hydrolysis experiments were performed at 1 % d.m. content in 0.05 M sodium acetate
buffer, pH 5.0, containing 0.02 % NaN3. The hydrolysis was carried out using a microassay
procedure with 0.5 g of reaction mixture in 96-well plates. For hydrolysis of pretreated wheat
straw substrate, two substrate disks (5 mg of dry matter) were dispensed to each well. With SE
and AlkOx bagasse, the washed insoluble fraction of pretreated material was homogenized and
diluted with buffer and the slurry was pipetted into the wells.
In hydrolysis, the microwell plates were closed with adhesive foil and incubated in a mixer
(Thermo micromixer Mxi4t, FinePCR, South Korea) shaken at 1,000 rpm at 45 °C
(Trichoderma mixture), 55 °C (Thermostable enzymes, mixture 1) or 52 °C (Thermostable
enzymes, mixture 2). The lower temperatures of 45 °C and 52 °C were selected for T. reesei
and thermostable enzymes, respectively, to minimize the effect of enzyme inactivation on
mixture optimization during prolonged incubation times. It has been shown that at higher
temperatures, the activity of cellulases can decrease significantly in 48 h in materials contain-
ing lignin [33]. After 24, 48 and 72 h, a plate containing four replicate samples for each test
point was withdrawn from the incubator, and the reaction was terminated by adding 10 μl of
10 M NaOH. The supernatants were collected for analysis after centrifugation. The hydrolysis
was monitored by measuring reducing sugars by the dinitrosalicylic acid (DNS) method using
glucose as a standard [34].
Preliminary Evaluation of the Ratios of the Main Cellulases
In the studies concerning the optimal ratio of CBHI and CBHII, the thermostable or T. reesei
enzymes, with components described in Table 2 (mixtures 1), were dosed on the basis of
protein content with a total dosage of 10 mg g−1
d.m. excluding β-glucosidases, which were
dosed at 1,000 nkat g−1
d.m. The amount of CBHs (CBHI and CBHII combined) was 75 % of
protein content (7.5 mg g−1
d.m.). The share of CBHII of total CBHs (CBHI+CBHII) was
varied in the range 0–100 %. Xylanase dosage was 2,500 nkat g−1
d.m. (0.59 mg g−1
d.m. for
thermostable xylanase and 0.26 mg g−1
d.m. for Trichoderma xylanase). The rest of
the protein was EG (1.91 mg g−1
d.m. for the thermostable mixture and 2.24 mg g−1
d.m.
for the T. reesei mixture).
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different variables and constrains. The optimal enzyme composition of Trichoderma enzymes for
hydrolysis of hydrothermally pretreated wheat straw resembled relatively well the results obtained
by Billard et al. [9] with steam-exploded wheat straw. Compared to the study with steam-exploded
wheat straw, the proportions of Cel7A (CBHI) and Cel6A (CBHII) and the sum of
endoglucanases were similar to those obtained with hydrothermally pretreated wheat straw,
whereas the ratio of Cel7B (EGI) and Cel5A (EGII) and the proportion of xylanase were more
significantly different (Table 4). As TrCel7B has significant xylanolytic activity [35, 36], it appears
that part of the requirement for xylanase activity was satisfied by a high proportion of Cel7B.
Compared to Trichoderma enzyme mixtures, the results obtained using thermostable
enzymes with all the lignocellulosic substrates were significantly different with respect to
the main cellulases (Table 4). The general finding was that the thermostable enzyme mixtures
required much more CBHI than any other optimal enzyme mixtures containing Trichoderma
enzymes. Although the enzymes used in thermostable enzyme mixtures were not purified and
the SDS-PAGE showed minor additional bands, the heat treatment almost totally inactivated
the xylanolytic and endoglucanase activities present before the heat treatment (data not
shown). Thus, the difference in the proportions is due to the different properties of thermo-
stable enzymes compared to T. reesei enzymes.
Because the experimental setup and differences in different studies appeared to affect
significantly the optimal mixtures, general trends of the effects of different raw materials
and pretreatments on the mixture composition were difficult to identify. However, some
conclusions could be drawn about the effect of pretreatment and raw material on optimal
mixtures of thermostable enzymes. Firstly, more AtCel7A (CBHI) was needed in the optimal
Table 5 Optimized enzyme mixtures for hydrolysis of various pretreated lignocellulosic materials
Raw
material
Pretreatment Cel7A Cel6A Cel7B Cel5A Xylanase βG Other Hydrolysis
yield (%)
Reference
Sugar cane
bagasse
AlkOx 49–58 14–30 – 5–13 12 3 – 67–88a, b
This study
Sugar cane
bagasse
SE 57–73 10–24 – 9–11 1–4 3-5 – 84–98a, b
This study
Wheat straw Hydrothermal 52–56 17–19 – 13–16 8–12 2-3 – 51–74a, b
This study
Wheat straw Hydrothermal 39–42 32–34 20–26 1–2 1 1-2 – 37–56a
This study
Wheat straw SE 40 27 15 6 12 –c
0 68d
[9]
Barley straw SE 27 47 27 0 – –c
–c
56e
[8]
Barley straw Hot water 20 43 37 0 – –c
–c
56e
[8]
Corn stover SE 27 35 21 – – 6 11 36f
[14]
Corn stover AFEX 29 19 35 – 14 –c
3 51e
[13]
Corn stover AFEX 35 4 26 – 19 12 4 44d
[2]
Corn stover Alkaline 49 4 34 – 4 5 4 41d
[2]
Corn stover Alk. peroxide 43 4 30 – 11 8 4 58d
[2]
SE steam explosion, AlkOx alkaline oxidation
a
Twenty-four- to 72-h hydrolysis
b
Thermostable enzymes
c
Present in the hydrolysis but not in variables of optimization
d
Forty-eight-hour hydrolysis
e
Twenty-four-hour hydrolysis
f
Seventy-two-hour hydrolysis
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mixture for SE bagasse than for AlkOx bagasse. Secondly, hydrolysis of AlkOx bagasse
required at least three times more xylanase than hydrolysis of SE bagasse. The difference can
partly be explained by the chemical composition of the materials. Bagasse pretreated by
alkaline oxidation had a very high xylan content (24 %), whereas the xylan content of SE
bagasse was only 3 %. Interestingly, Banerjee et al. [2] found that more xylanase (TrXyn10)
was needed after AFEX than NaOH and alkaline peroxide pretreatments. The role of xylanase
in improving the accessibility of cellulose to cellulases can be more significant in materials
with high hemicellulose (alkaline oxidised bagasse) and lignin contents (AFEX-treated
material) [4]. Thirdly, comparison of two raw materials pretreated with relatively similar
methods, wheat straw and bagasse, showed that more AtCel7A (CBHI) and a lower amount
of xylanase were needed with SE bagasse than with hydrothermally pretreated wheat straw,
although the carbohydrate compositions were similar. Higher enzyme dosage and degree of
hydrolysis with pretreated bagasse might have affected the composition of the optimal enzyme
mixture. Wheat straw substrate was also used as dried sheet disks, whereas the more homo-
geneous SE bagasse could be pipetted as a slurry and was used without drying. It is possible
that drying also decreased the accessibility of enzymes and thus increased the role of
xylanases. It appears that cell wall anatomy and microstructure can have a significant impact
on optimal enzyme proportions in addition to the content of lignin, cellulose and hemicellulose.
The Role of Cellobiohydrolases
The total amount of CBHs varied between 70 and 88 % of the protein in all the optimal enzyme
mixtures, being significantly higher than that produced by T. reesei [26]. Cel7A (CBHI) has an
important role in hydrolysis since it efficiently breaks down the bulk of crystalline cellulose of
the substrate. Relatively wide variation, 10 %, was allowed in the proportions of CBHI and
CBHII practically without effect in the hydrolysis yield (Figs. 3 and 4). Generally less Cel7A
(CBHI) was needed in longer 72 h hydrolysis than in 24 h. Probably more crystalline substrate
is available for CBH in the beginning than in the later part of hydrolysis. The only exception
was steam-exploded bagasse, for which the lowest optimal share of CBHI (57 %) was in
hydrolysis lasting 24 h. With SE bagasse, the high hydrolysis degree after 48 and 72 h resulted
in broad optimal areas instead of a single optimal point, and therefore, enzyme mixtures
containing 50–80 % Cel7A (CBHI) gave almost the same hydrolysis yield (Fig. 4). If enzyme
dosage is high enough and the hydrolysis time is long, even a suboptimal enzyme mixture can
produce high hydrolysis yields.
The Role of Endoglucanases
The role of endoglucanases was clearly less important than that of CBHs. The EG content varied
from 5 to 27 % in optimized mixtures. However, the addition of Cel7B (EGI) to T. reesei mixture
caused clear increase in the degree of hydrolysis. In preliminary optimization, only 35 % degree
of hydrolysis was obtained with a dosage of 10.5 mg g−1
without Cel7B (EGI), whereas in
optimization experiments, 56 % hydrolysis was reached with a dosage of 6 mg g−1
. The high
increase indicates the significant role of Cel7B (EGI) in hydrolysis with T. reesei enzymes. Similar
conclusions were also drawn in the studies of Billard et al. on steam-pretreated wheat straw [9].
In an optimal mixture of T. reesei enzymes, 20–26 % of Cel7B (EGI) and only 1–2 % of
Cel5A (EGII) were required. Banerjee et al. [2] suggested that the overlapping functions of
Cel7B (EGI) and Cel5A (EGII) can result in situations in which major changes in the relative
proportions of these two enzymes cause only small shifts in the hydrolysis yield. According to
the model, increasing the EGII content from 1 to 11 % and decreasing EGI from 26 to 16 %
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caused only 2 % decrease in hydrolysis yield. In the mixture of thermostable enzymes, the role
of EG was less significant and the mixtures contained 5–16 % of TaCel5A (EGII). However,
TaCel5A has been shown to be efficient in the liquefaction of pretreated wheat straw [19].
The Role of Xylanase
The amount of xylanase in the mixtures was limited to 1–12 % (or 1–15 % with AlkOx
bagasse) of total protein. A relatively high proportion of xylanase, 12 %, was needed in the
hydrolysis of hydrothermally pretreated wheat straw and AlkOx bagasse with the optimal
mixtures of thermostable enzymes. In AlkOx bagasse, the required high xylanase amount
could be explained by high xylan content of the substrate. Xylanase activity has been shown to
be important in the hydrolysis of hydrothermally pretreated wheat straw although its xylan
content is low [7]. Interestingly, only a minimal amount of xylanase was needed in the
hydrolysis of wheat straw with a T. reesei enzyme mixture, which may be due to the fact that
T. reesei endoglucanase Cel7B (EGI) has been shown to be active on arabinoxylan and xylan
[35, 36], and thus, a high content of Cel7B could at least partly replace xylanase or vice versa
[11]. The improving effect of T. reesei EGI on hydrolysis yield may thus also be due to its
xylanolytic activity.
The T. reesei and thermostable enzyme mixtures also contained xylanases from different
GH families, which can affect their role in the enzyme mixture. It has been reported that the
hydrolysis products of family 10 xylanases are shorter than family 11 xylanases and family 10
xylanases are less specific to xylan than family 11 xylanases [37]. Shorter hydrolysis products
giving potentially higher response in reducing sugars assay and broader specificity might have
made the role of TaXyn10A more important than that of TrXyn11 in this experimental setup.
In the preliminary optimization study, the xylanase (NfXyn11) dosage was 2,500 nkat g−1
,
corresponding to 5.7 and 2.5 % of the total protein for thermostable enzymes and Trichoderma
enzymes, respectively. It appears that more than 2,500 nkat g−1
was required in the efficient
hydrolysis of AlkOx bagasse and pretreated wheat straw. TaXyn10 has shown higher efficien-
cy in solubilizing xylan in pretreated wheat straw than NfXyn11 [7]. The too low xylanase
dosage might also have been one reason for the low degree of hydrolysis by the T. reesei
mixture in the preliminary optimization. The high amount of even more efficient xylanase in
the optimized mixtures of thermostable enzymes underlines the importance of an adequate
level of xylanolytic activities in total hydrolysis of the wheat straw substrate.
The Role of β-Glucosidase
The design of the experiments was made by limiting the βG content to 1–5 % of protein. Most
of the optimal mixtures of thermostable enzymes contained 2–3 % of βG. Interestingly, only
1–2 % of βG or even less was needed with T. reesei enzymes in the hydrolysis of wheat straw.
Zhou et al. [14] reported 6 % of βG in optimized mixtures for hydrolysis of SE corn stover,
whereas studies by Banerjee et al. [2] indicated that the optimal mixture for hydrolysis of
various pretreated substrates contained 4–12 % of βG when the minimum tested value was
4 %. It is possible that even a lower proportion of βG could provide better hydrolysis yields
with some materials. For pretreated dried distillers grains with solubles (DDGS), the need was
even higher (20–43 %) and it is clear that the required βG proportion for hydrolysis of
different biomasses can vary considerably. The present study as well as the optimizations in the
literature (Table 5) was carried out at 1 % d.m. content, and it is possible that the required
amount of βG would be higher in conditions with higher consistency, for example because of
the increased inhibition of βG by high glucose concentration.
Appl Biochem Biotechnol](https://image.slidesharecdn.com/s56-140515004323-phpapp01/85/Development-of-pretreatment-technology-and-enzymatic-hydrolysis-for-biorefineries-Anne-Kallioinen-171-320.jpg)
![IV/17
zation experiments, a maximum of 56 % hydrolysis yield was obtained with a dosage of
6 mg g−1
, whereas in pre-tests with a dosage of 10.5 mg g−1
, the maximal hydrolysis yield was
only 35 %. The corresponding increase was less marked with thermostable enzymes (from 70
to 74 %), but when considering the lower enzyme dosage in optimization (6 mg g−1
), the
improvement was clear. The optimized mixture of thermostable enzymes also showed higher
efficiency when compared to the hydrolysis yield obtained using the commercial Celluclast-
Novozym 188 mixture with a clearly higher dosage.
The performance of the optimal thermostable enzyme mixture was also evaluated in the
hydrolysis of wheat straw with never-dried substrate at 1 and 12 % d.m. contents; 1 %
substrate was hydrolysed with the dosage of 6 mg g−1
very easily, giving 70, 86 and 96 %
yield in 24, 48 and 72 h, respectively. It is obvious that the never-dried substrate was
hydrolysed more easily than the dried handsheet disks. Hydrolysis at 12 % d.m. content was
clearly slower and only 52, 61 and 66 % yield was obtained in 24, 48 and 72 h, respectively. It
is also possible that hydrolysis at high dry matter concentration would require a different
optimal enzyme mixture than hydrolysis at 1 % d.m. content.
The performance of the optimal enzyme mixtures of thermostable enzymes (mixtures for
72 h hydrolysis) in hydrolysis of bagasse was compared to that of a commercial enzyme Cellic
at 8 % d.m. content. With AlkOx bagasse, the hydrolysis was very efficient with both the
optimal mixture and the commercial enzyme (Fig. 5a). However, the commercial enzyme
mixture reached the maximum degree of hydrolysis clearly earlier than the optimal enzyme
mixture. With SE bagasse, the optimal enzyme mixture was more efficient in the beginning of
hydrolysis but the hydrolysis then slowed down more steeply (Fig. 5b). With AFEX-treated
corn stover, a six-component enzyme mixture could not reach the hydrolysis yield obtained by
commercial enzyme mixtures even with high enzyme dosages, whereas an 11-component
mixture was almost as efficient in the longer hydrolysis [12]. It has also been suggested that
the importance of accessory enzymes increases in the later stages of hydrolysis. The
rate and degree of hydrolysis, especially in the final stage of hydrolysis, was probably
improved by these accessory enzymes present in the full commercial product but not
in the thermostable enzyme mixture, composed essentially of five monocomponent
preparations. As a conclusion, by careful selection and optimization of the enzyme
components, it was possible to reach a hydrolysis performance close to that of the
commercial products, although it became evident that the optimal enzyme mixtures could
further be improved by including other additional enzymes needed for complete hydrolysis of
cell wall carbohydrates.
Conclusions
Enzyme mixtures were optimized by statistically designed experiments. Preliminary studies
showed that a CBH/EG ratio of 4:1 or higher gave maximal synergy in the hydrolysis of
hydrothermally pretreated wheat straw by thermostable enzymes. The composition of optimal
enzyme mixtures depended clearly on the substrate and enzyme system studied. The optimal
thermostable enzyme mixture contained Cel7A as a major component, whereas in the
Trichoderma reesei mixture, relatively high proportions of Cel6A and Cel7B were also needed.
Hydrolysis of wheat straw with thermostable enzymes also required more xylanase than
hydrolysis with T. reesei enzymes, in which the high proportion of Cel7B (EGI) appeared to
provide the required xylanase activity. The raw material and pretreatment also had an effect on
the optimal enzyme mixture. Even materials with relatively low xylan content required a high
proportion of xylanase, possibly due to otherwise decreased accessibility of the cellulose
Appl Biochem Biotechnol](https://image.slidesharecdn.com/s56-140515004323-phpapp01/85/Development-of-pretreatment-technology-and-enzymatic-hydrolysis-for-biorefineries-Anne-Kallioinen-173-320.jpg)
Development of pretreatment technology and enzymatic hydrolysis for biorefineries. Anne Kallioinen
- 1. •VISIONS •SCIENCE• TECHNOLOGY•R ESEARCHHIGHL IGHTS Dissertation 56 Development of pretreatment technology and enzymatic hydrolysis for biorefineries Anne Kallioinen
- 3. VTT SCIENCE 56 Development of pretreatment technology and enzymatic hydrolysis for biorefineries Anne Kallioinen Doctoral dissertation for the degree of Doctor of Science in Technology to be presented with due permission of the School of Chemical Technology for public examination and debate in Auditorium V1 in Material technology building, at Aalto University School of Chemical Technology, on the 28th of May, 2014 at 14:00.
- 4. ISBN 978-951-38-8143-6 (Soft back ed.) ISBN 978-951-38-8144-3 (URL: http://www.vtt.fi/publications/index.jsp) VTT Science 56 ISSN-L 2242-119X ISSN 2242-119X (Print) ISSN 2242-1203 (Online) Copyright © VTT 2014 JULKAISIJA – UTGIVARE – PUBLISHER VTT PL 1000 (Tekniikantie 4 A, Espoo) 02044 VTT Puh. 020 722 111, faksi 020 722 7001 VTT PB 1000 (Teknikvägen 4 A, Esbo) FI-02044 VTT Tfn. +358 20 722 111, telefax +358 20 722 7001 VTT Technical Research Centre of Finland P.O. Box 1000 (Tekniikantie 4 A, Espoo) FI-02044 VTT, Finland Tel. +358 20 722 111, fax +358 20 722 7001 Kopijyvä Oy, Kuopio 2014
- 5. 3 Development of pretreatment technology and enzymatic hydrolysis for biorefineries Esikäsittely- ja entsyymihydrolyysiteknologioiden kehittäminen biojalostamosovelluksiin. Anne Kallioinen. Espoo 2014. VTT Science 56. 107 p. + app. 64 p. Abstract The growing demand for energy, materials and food, depletion of fossil raw mate- rial reservoirs and increasing environmental concerns have all increased interest in renewable resources. Lignocellulosic biomass is an alternative for replacing fossil raw materials in the production of fuels, materials and various chemicals. Lignocellulose present in plant cell walls consists mainly of polysaccharides, cellu- lose and hemicellulose, and aromatic lignin. These major components form a complex structure that is resistant to microbial and enzymatic activity. Due to the recalcitrant structure of plant cell walls, lignocellulosic raw materials must be pre- treated before their enzymatic hydrolysis to monosaccharides. Various pretreat- ment methods; chemical, physical, biological or their combinations, have been developed. After pretreatment, polysaccharides can be hydrolysed enzymatically to monosaccharides, which in turn can be fermented to different products such as ethanol. Currently the first commercial scale lignocellulosic ethanol plants have started production. A secure supply of biomass is one of the key factors for a feasible biorefinery, and new alternative feedstocks are still required especially in northern climates in order to fulfil the raw material demands of biorefineries in a sustainable way. In addition, development of new pretreatment technologies and more efficient enzymatic hydrolysis are needed. New lignocellulosic feedstocks and improved pretreatment methods were stud- ied in the work described in this thesis. Reed canary grass and barley straw were found to be interesting carbohydrate-rich raw materials that could be pretreated by steam explosion and hydrolysed enzymatically with yields comparable to those obtained from wheat straw. Selection of the most favourable harvest time for reed canary grass, autumn or spring, was studied in relation to pretreatment and hy- drolysis yields. Spring harvested reed canary grass was found to be the more suitable raw material as it had a higher cellulose content and the pretreated fibre was hydrolysed more efficiently compared to autumn harvested material. A new pretreatment method using sodium carbonate and oxygen pressure was developed. The alkaline oxidation method fractionated biomass into a carbohy- drate-rich fibre and a dissolved fraction containing most of the lignin. The pro- duced carbohydrate-rich fibre could be efficiently hydrolysed by enzymes and the hydrolysis was also efficient at 12% dry matter content. Compared to the 52% total glucose yield obtained in enzyme hydrolysis of spruce after pretreatment by steam explosion, a significantly higher glucose yield of 84% was obtained in hydrolysis after alkaline oxidation. Different kinds of raw materials, such as spruce, birch and sugar cane bagasse, could be efficiently pretreated by alkaline oxidation.
- 6. 4 The main effects of alkaline oxidation pretreatment were dissolution and partial degradation of lignin and hemicellulose. Some galactoglucomannan and xylan was solubilised and further oxidised to other products, and therefore relatively low yields of hemicellulose were obtained. Organic acids were formed as degradation products of lignin and carbohydrates. Process conditions were partially optimized using spruce as raw material in order to improve the efficiency of alkaline oxidation. The pretreatment could be accelerated by increasing the treatment temperature, by the use of copper-phenanthroline catalyst, and by decreasing the particle size of the raw material. Further optimization of e.g. alkali dosage and the solid to liquid ratio is still required to improve hemicellulose yield and economical feasibility. Fibre fractions of alkaline oxidation could be hydrolysed by low enzyme dosages, 2–4 FPU/g dry matter. Significantly higher enzyme dosages were required in the hydrolysis of steam exploded materials, probably due to the inhibitory effect of the high residual lignin content after the pretreatment. The efficient hydrolysis of alka- line oxidised materials by low enzyme dosages can decrease enzyme costs or enable shorter hydrolysis time. In order to further improve the hydrolysis efficiency and decrease the required enzyme dosage, enzyme mixtures were optimized regarding the major enzymes needed in biomass hydrolysis. Optimized mixtures of thermostable enzymes were found to have significantly different proportions of cellobiohydrolases, endoglu- canases and xylanase than the optimized mixtures of Trichoderma reesei en- zymes. Although different, the significant role of cellobiohydrolases was demon- strated in both types of mixtures. The results also indicated that high xylanase activity was required in the hydrolysis of pretreated materials having decreased enzyme accessibility to cellulose due to high xylan content or possibly due to drying of the substrate. The hydrolysis performance of optimized enzyme mixtures of five thermostable enzyme components was shown to be close to that of state- of-the-art commercial mixtures. Keywords lignocellulose, pretreatment, enzymatic hydrolysis, optimal enzyme mixture
- 7. 5 Esikäsittely- ja entsyymihydrolyysiteknologioiden kehittäminen biojalostamosovelluksiin Development of pretreatment technology and enzymatic hydrolysis for biorefineries. Anne Kallioinen. Espoo 2014. VTT Science 56. 107 s. + liitt.64 s. Tiivistelmä Kasvavat energian, materiaalien ja ruuan tarpeet, fossiilisten raaka-ainevarojen vähentyminen ja huoli ympäristöstä ovat lisänneet kiinnostusta uusiutuviin luon- nonvaroihin. Lignoselluloosapohjainen biomassa on vaihtoehto fossiilisille raaka- aineille polttoaineiden, materiaalien ja monien kemikaalien tuotannossa. Kasvien soluseinän lignoselluloosa koostuu pääosin polysakkarideista, kuten selluloosasta ja hemiselluloosasta, sekä aromaattisesta ligniinistä. Nämä pääkomponentit muo- dostavat monimutkaisen rakenteen, joka on hyvin kestävä mikrobien ja entsyymien hajotukselle. Koska kasvien soluseinat ovat lujia, lignoselluloosapohjaiset raaka- aineet täytyy esikäsitellä ennen entsymaattista hydrolyysia monosakkarideiksi. Erilaisia kemiallisia, fysikaalisia ja biologisia esikäsittelymenetelmiä tai niiden yhdistelmiä on kehitetty. Esikäsittelyn jälkeen polysakkaridit voidaan hydrolysoida entsymaattisesti monosakkarideiksi, jotka voidaan edelleen fermentoida erilaisiksi tuotteiksi, kuten etanoliksi. Tällä hetkellä ensimmäiset kaupallisen mittakaavan lignoselluloosapohjaista bioetanolia valmistavat tehtaat ovat aloittaneet tuotannon. Koska kannattava biojalostamo vaatii turvatut raaka-ainelähteet, uusia ja vaihtoeh- toisia biomassoja tarvitaan edelleen erityisesti pohjoisessa ilmastossa täyttämään biojalostamoiden raaka-ainetarve. Myös uusien esikäsittelytekniikoiden kehitystä ja nykyistä tehokkaampaa entsyymihydrolyysiä tarvitaan. Tässä työssä tutkittiin uusia lignoselluloosaraaka-aineita ja kehitettiin esikäsittely- menetelmiä. Tutkimuksessa havaittiin, että ruokohelpi ja ohran olki olivat kiinnos- tavia korkean hiilihydraattipitoisuuden omaavia raaka-aineita, jotka voitiin esikäsi- tellä höyryräjäytyksellä ja hydrolysoida entsymaattisesti vehnän olkeen verrattavilla saannoilla. Ruokohelven korjuuajankohdan vaikutusta esikäsittelyyn ja hydro- lyysisaantoon tutkittiin syksyllä ja keväällä korjatulla ruokohelvellä. Keväällä korjatun ruokohelven havaittiin olevan sopivampi raaka-aine, sillä sen selluloosapitoisuus oli suurempi ja siitä saatu esikäsitelty kuitu hydrolysoitui paremmin verrattuna syksyllä korjattuun materiaaliin. Tutkimuksessa kehitettiin uusi esikäsittelymenetelmä, joka perustui alkaliseen käsittelyyn hapettavissa olosuhteissa käyttämällä kemikaaleina natriumkarbonaattia ja happea. Alkalihapetus fraktioi biomassan hiilihydraattipitoiseen kuituun ja ligniini- pitoiseen liuenneeseen fraktioon. Tuotettu kuitu voitiin hydrolysoida tehokkaasti entsyymeillä, ja hydrolyysi oli tehokas myös 12 %:n kuiva-ainepitoisuudessa. Verrattuna höyryräjäytetyllä kuusella saatuun 52 %:n glukoosisaantoon esikäsitte- lyssä ja hydrolyysisssä, merkittävästi korkeampi 84 %:n glukoosisaanto saatiin käyttämällä alkalihapetusta esikäsittelynä. Menetelmä soveltui erilaisten raaka- aineiden, kuten kuusen, koivun ja sokeriruokobagassin, esikäsittelyyn.
- 8. 6 Alkalihapetuksen päävaikutukset olivat ligniinin ja hemiselluloosan liukenemi- nen ja osittainen hajoaminen. Osa galaktoglukomannaanista ja ksylaanista liukeni ja hapettui edelleen muiksi tuotteiksi, ja siksi hemiselluloosasaanto oli suhteellisen alhainen. Alkalihapetuksessa muodostui ligniinin ja hiilihydraattien hajoamistuot- teina orgaanisia happoja. Prosessiolosuhteita optimoitiin alkalihapetuskäsittelyn tehokkuuden parantamiseksi käyttämällä kuusta raaka-aineena. Esikäsittelyä voitiin tehostaa nostamalla käsittelylämpötilaa, käyttämällä kupari-fenantroliini- katalyyttiä ja pienentämällä raaka-aineen partikkelikokoa. Hemiselluloosasaannon ja taloudellisen tehokkuuden parantamiseksi tarvitaan esimerkiksi alkaliannostuk- sen ja kiintoaine-neste-suhteen lisäoptimointia. Alkalihapetus-esikäsittelystä saadut kuitufraktiot hydrolysoituivat tehokkaasti jo alhaisilla entsyymiannoksilla, 2–4 FPU/g kuiva-ainetta. Merkittävästi korkeampia entsyymiannoksia tarvittiin höyryräjäytettyjen materiaalien hydrolyysissä, mikä johtui todennäköisesti korkean ligniinipitoisuuden aiheuttamasta inhiboivasta vai- kutuksesta. Alkalihapetettujen materiaalien tehokas hydrolyysi alhaisilla entsyymi- annostuksilla voi alentaa entsyymikustannuksia tai mahdollistaa lyhyen hydro- lyysiajan. Biomassan hydrolyysissa tarvittavien pääentsyymien seoksien koostumusta op- timoitiin, jotta hydrolyysiä voitaisiin edelleen tehostaa ja alentaa entsyymiannoksia. Lämpöstabiilien entsyymien optimoiduissa seoksissa oli eri suhteissa sellobiohyd- rolaaseja, endoglukanaaseja ja ksylanaasia kuin Trichoderma reesei -homeen entsyymeistä koostetuissa optimoiduissa seoksissa. Sellobiohydrolaasit olivat kuitenkin merkittävin entsyymi molemmissa seoksissa. Saadut tulokset viittaavat siihen, että korkeaa ksylanaasiaktiivisuutta tarvitaan sellaisten esikäsiteltyjen materiaalien hydrolyysissä, joissa entsyymien pääsy selluloosaan on heikentynyt korkeasta ksylaanipitoisuudesta tai mahdollisesti raaka-aineen kuivaamisesta johtuen. Viiden termostabiilin entsyymin optimaaliset seokset vastasivat hydro- lyysitehokkuudeltaan kaupallisia entsyymiseoksia. Avainsanat lignocellulose, pretreatment, enzymatic hydrolysis, optimal enzyme mixture
- 9. 7 Preface The studies presented in this thesis were carried out at VTT Technical Research Centre of Finland during the years 2005–2012. Financial support of the Finnish Funding Agency for Technology and Innovation, ClimBus programme (AGROETA 40333/05), Biorefine programme (projects Pre-Cu 2366/31/07 and SugarTech 40282/08), EU 7th framework programme (EU-HYPE, 213139), and VTT is grate- fully acknowledged. I wish to thank Vice Presidents R&D Bio and Process Technology, Prof. Juha Ahvenainen (-2007) and Prof. Anu Kaukovirta-Norja (2007-), Head of research area Dr. Raija Lantto, my former Technology manager Dr. Niklas von Weymarn, and my former Team leader Dr. Terhi Hakala for providing excellent working facili- ties. I am grateful to Prof. Maija Tenkanen and Associate Professor Venkatesh Balan for reviewing the manuscript of my thesis and for their constructive feedback and valuable suggestions. I warmly thank Prof. Matti Leisola and Prof. Heikki Ojamo from Aalto University School of Chemical Technology for the smooth coop- eration during the preparation of this thesis. I warmly thank my thesis advisory group, Principal scientist Matti Siika-aho, Research professor Kristiina Kruus, and Dr. Niklas von Weymarn for all their help and encouragement during the years. Especially I am thankful to Matti Siika-aho for his practical help, scientific support and patience during this work. Kristiina Kruus is thanked for her continuous support during writing this thesis and soft but demanding way to make me learn. I also wish to thank my co-authors Prof. Liisa Viikari, Jaana Uusitalo, Dr. Katri Pahkala, Markku Kontturi, Dr. Stella Rovio, Dr. Tarja Tamminen, Dr. Maija Hakola, Tiina Riekkola, Prof. Timo Repo, Prof. Markku Leskelä, and Dr. Terhi Puranen. I am thankful to Dr. Eemeli Hytönen for being helpful when I had questions on techno-economic issues. Planning Officer for doctoral affairs at Aalto University, Sirje Liukko is thanked for helping with university issues. Päivi Vahala and VTT publication services are thanked for helping with the technical editing and Michael Bailey for revising the English language of the thesis. I thank my colleagues and all the personnel in the laboratory for a friendly and helpful working environment. In particular, I wish to express my warmest thanks to Pirkko Saarelainen, Ulla Vornamo, Jenni Lehtonen and Riitta Pasanen for excellent technical assistance and for carrying out most of the laboratory work involved in this thesis. I also want to thank my two colleagues in the next door: Stina Grönqvist for listening the joys and challenges during this work and Ronny Wahl-
- 10. 8 ström for giving good advices in the university bureaucracy issues. I appreciate the patience and support of all my friends. Finally, I thank my parents, Sinikka and Pasi, my parents-in-law, Marja-Liisa and Tuomo, my sister Paula and brother Tero for love and all their help in combin- ing family and work. Most of all, I am grateful to my children Silja and Konsta for reminding me what is the most important in life, and my husband Harri for his love and for the many encouraging words I have received over the years. Espoo, March 2014 Anne Kallioinen
- 11. 9 Academic dissertation Supervising professor, Custos Professor Heikki Ojamo Aalto University School of Chemical Technology Espoo, Finland Thesis advisors Research Professor Kristiina Kruus VTT Technical Research Centre of Finland Espoo, Finland Doctor Niklas von Weymarn Metsä Fibre Ltd Espoo, Finland Preliminary examiners Professor Maija Tenkanen Department of Food and Environmental Sciences University of Helsinki Helsinki, Finland Associate Professor Venkatesh Balan Department of Chemical Engineering and Material Science Michigan State University Lansing, USA Opponent Professor Guido Zacchi Department of Chemical Engineering Lund University Lund, Sweden
- 12. 10 List of publications This thesis is based on the following original publications, which are referred to in the text as I–IV. The publications are reproduced with kind permission from the publishers. Some additional unpublished results are included. I Kallioinen, A., Uusitalo, J., Pahkala, K., Kontturi, M., Viikari, L., von Wey- marn, N., Siika-aho, M., 2012. Reed canary grass as a feedstock for 2nd generation bioethanol production. Bioresour. Technol.123, 669–672. II Rovio, S., Kallioinen, A., Tamminen, T., Hakola, M., Leskelä, M., Siika-aho, M., 2012. Catalysed alkaline oxidation as a wood fractionation technique. Bio- resources 7, 756–776. III Kallioinen, A., Hakola, M., Riekkola, T., Repo, T., Leskelä, M., von Wey- marn, N., Siika-aho, M., 2013. A novel alkaline oxidation pretreatment for spruce, birch and sugar cane bagasse. Bioresour. Technol. 140, 414–420. IV Kallioinen, A., Puranen, T., Siika-aho, M. Mixtures of thermostable enzymes show high performance in biomass saccharification. Appl Biochem. Biotechnol. In press.
- 13. 11 Author’s contributions I Anne Kallioinen had the main responsibility for preparing and writing the article, and is the corresponding author. She planned the study together with the co-authors. She designed the experiments with respect to enzymatic hy- drolysis and carbohydrate analysis and supervised the experimental work and was mainly responsible for the interpretation of the results. II Anne Kallioinen wrote the article together with the other co-authors. She planned the study together with the co-authors. She had the main responsi- bility in the design of oxidation treatments and supervised the carbohydrate analysis. She interpreted the results together with the co-authors. III Anne Kallioinen had the main responsibility for preparing and writing the article, and is the corresponding author. She planned the study together with the co-authors. She had the main responsibility in the design of experiments and supervised the experimental work. She was mainly responsible for the interpretation of the results. IV Anne Kallioinen had the main responsibility for preparing and writing the article, and is the corresponding author. She planned the study together with the co-authors. She designed the experiments, supervised the experimental work and carried out statistical analysis of the data. She was mainly responsible for the interpretation of the results.
- 14. 12 Contents Abstract ...........................................................................................................3 Tiivistelmä .......................................................................................................5 Preface.............................................................................................................7 Academic dissertation.....................................................................................9 List of publications........................................................................................10 Author’s contributions ..................................................................................11 List of abbreviations......................................................................................14 1. Introduction.............................................................................................16 1.1 Biomass as a raw material for chemicals and fuels ............................16 1.2 Lignocellulosic raw materials.............................................................17 1.2.1 Cellulose ...............................................................................20 1.2.2 Hemicellulose........................................................................20 1.2.3 Lignin ....................................................................................22 1.3 Enzymes degrading lignocellulose.....................................................23 1.3.1 Cellulases..............................................................................23 1.3.2 Auxiliary enzymes for degradation of cellulose........................25 1.3.3 Hemicellulolytic enzymes .......................................................26 1.3.4 Synergism in the hydrolysis of cellulose..................................27 1.3.5 Optimal enzyme compositions for hydrolysis of lignocellulosic materials .........................................................28 1.4 Pretreatment of lignocellulosic substrates..........................................34 1.5 Pretreatment methods ......................................................................34 1.5.1 Physical and biological pretreatments.....................................35 1.5.2 Thermo-chemical pretreatments.............................................35 1.5.3 Physico-chemical pretreatments.............................................36 1.5.3.1 Steam pretreatments...............................................37 1.5.3.2 Alkaline pretreatments.............................................38 1.5.3.3 Alkaline oxidative pretreatments...............................39
- 15. 13 1.6 Techno-economic considerations for sugar platform biorefineries.......40 1.6.1 Raw materials........................................................................40 1.6.2 Pretreatment..........................................................................41 1.6.3 Process configurations for hydrolysis and fermentation ...........44 1.6.4 Multi-product biorefineries......................................................46 2. Aims of the present study.......................................................................47 3. Materials and methods............................................................................48 3.1 Lignocellulosic substrates and pretreatments.....................................48 3.1.1 Preparation of materials.........................................................48 3.1.2 Analytical procedures for biomasses.......................................50 3.2 Enzymatic hydrolysis ........................................................................51 3.2.1 Enzymes ...............................................................................51 3.2.2 Hydrolysis experiments..........................................................53 3.3 Ethanol fermentation.........................................................................53 4. Results and discussion...........................................................................54 4.1 Potential Nordic raw materials for a sugar platform biorefinery ...........54 4.2 Development of alkaline oxidation pretreatment.................................57 4.2.1 Optimizing the treatment conditions for alkaline oxidation........57 4.2.2 Hydrolysis and fermentation of alkaline oxidised materials.......59 4.2.3 Effect of particle size in alkaline oxidation pretreatment...........60 4.2.4 Chemical reactions and modification of biomass in alkaline oxidation................................................................................62 4.2.5 Process conditions in alkaline oxidation..................................63 4.3 Improved enzymatic hydrolysis by efficient pretreatment and optimization of enzyme mixtures .......................................................65 4.3.1 Decrease in enzyme dosages by alkaline oxidation pretreatment..........................................................................65 4.3.2 Enzyme mixture optimization..................................................68 4.3.2.1 Supplementation of commercial enzyme mixtures ....68 4.3.2.2 Optimization of enzyme mixtures from monocomponent enzymes for the hydrolysis of pretreated raw materials..........................................69 5. Conclusions and recommendations for future studies..........................75 References.....................................................................................................78 Appendices Publications I–IV Supplementary material for Publication IV
- 16. 14 List of abbreviations AA Auxiliary activities (family) Ac Acetyl AlkOx Alkaline oxidation AFEX Ammonia fibre explosion Ara Arabinose ARP Ammonia recycle percolation At Cel7A Acremonium thermophilum cellobiohydrolase I G -glucosidase CDH Cellobiose dehydrogenase CBH Cellobiohydrolase CBM Carbohydrate binding module CBP Consolidated bioprocessing Cel Cellulase CrI Crystallinity index DDGS Dried distillers’ grains with solubles d.m. dry matter DNS dinitrosalicylic acid DP Degree of polymerization EG Endoglucanase EU European Union FPU Filter paper unit G Guaiacyl unit
- 17. 15 Gal Galactose GH Glycoside hydrolase Glc Glucose GlcA Methyl glucuronic acid H 4-Hydroxyphenyl unit HPAEC High performance anion exchange chromatography LCC Lignin-carbohydrate complex LPMO Lytic polysaccharide monooxygenase Man Mannose MESP Minimum ethanol selling price Mw Average molecular weight, mass average Mt Mega tons nd no data PAD Pulsed amperometric detection RI Refractive index S Syringyl unit SAA Soaking in aqueous ammonia SE Steam explosion SEC Size exclusion chromatography SHF Separate hydrolysis and fermentation SSF Simultaneous saccharification and fermentation Thermo mix Mixture of thermostable enzymes T. reesei Trichoderma reesei Tr Cel7B Trichoderma reesei endoglucanase II Tr Xyn10 Trichoderma reesei family 10 xylanase TrMix Mixture of Trichoderma reesei enzymes UV Ultra violet Xyl Xylose Xylp Xylopyranose Xyn Xylanase
- 18. 1. Introduction 16 1. Introduction 1.1 Biomass as a raw material for chemicals and fuels Worldwide interest in sustainable production of energy, fuels and chemicals has increased for many reasons during recent decades. Energy, food and material demands are continuously increasing due to the growing global population, the reservoirs of fossil raw materials are known to be depleting, global climate change has caused worldwide environmental concern, and many countries have national aims to increase energy independence by their own energy production. To fulfil the growing need for energy resources, more efficient ways to use the remaining fossil resources by e.g. the recovery of oil sands and shale gases have attracted wide interest. Alternative ways and advanced technologies to produce chemicals, fuels and energy from renewable resources to decrease greenhouse gas emis- sions are however also needed. Against this background, various national, EU, and worldwide targets have been set to increase the proportion of renewable energy and transportation fuels and to reduce greenhouse gas emissions. Biomass is a promising alternative to fossil raw materials for the production of liquid biofuels needed in transportation and for production of chemicals. A biore- finery is a facility which converts biomass to various products such as fuels, ener- gy and chemicals. Bioethanol and biodiesel are examples of biorefinery products that are currently produced and blended with gasoline and diesel. In first genera- tion biorefineries these products are mainly produced from biomass also suitable for food use, such as starch, sucrose from sugar cane or beet and vegetable oils. The sustainability of the utilization of agricultural land for production of fuels and chemicals in the first generation technologies has been questioned and the ob- tained greenhouse gas reductions are also under review. Second generation biorefineries utilize more sustainable raw materials, such as various lignocellulosic side-streams, which are generated when sugar cane, sugar beet, corn, grains (e.g. wheat) and wood are processed. Dedicated energy crops as well as industrial and municipal solid wastes are also seen as sustainable raw materials. However, the division into 1st and 2nd generation technologies is not completely clear and lignocellulosic side-streams can also be considered as raw materials for production of animal feed and food (Villas-Bôas et al., 2002). Ligno- cellulosic raw materials contain polysaccharides, cellulose and hemicellulose, which
- 19. 1. Introduction 17 can be hydrolysed to monosaccharides and converted to ethanol or various chem- icals by a microbial conversion. Hydrolysis of carbohydrates in lignocellulosic materials can be carried out by acids or enzymes. Acid hydrolysis requires severe, corrosive process conditions. Yield losses can also occur, as well as formation of compounds inhibiting fermentation organisms. Enzymatic hydrolysis is currently considered to be more promising due to its mild process conditions and lower formation of inhibiting compounds. The first commercial scale lignocellulosic bio- ethanol plant is now operational in Italy and four facilities are under construction (International Energy Agency, 2013). However, the technology is still in a devel- opmental phase and needs improvements. The major challenges are related to the recalcitrant, complex structure of plant biomass and the performance of en- zymes. Pretreatment of lignocellulosic raw materials is required before the enzy- matic hydrolysis to monosaccharides. Long hydrolysis time, high dosages and costs of enzymes are disadvantages of the enzymatic process. Low cost raw materials and efficient and optimized pretreatment and hydrolysis technologies are required for profitable production in sugar platform biorefineries. 1.2 Lignocellulosic raw materials Cell walls of woody and gramineous plants represent a huge resource of ferment- able carbohydrates that are potential sources for ethanol and other chemicals. It has been estimated that globally 1010 to 1011 tonnes of cellulose is synthesised and degraded annually (Hon, 1994). Botanically, lignocellulosic raw materials are divided into angiosperms and gymnosperms; angiosperms can then further be divided into monocotyledons and eudicotyledons. These three groups have differ- ent cell wall compositions. Gramineous plants (i.e. grasses, Poaceae) belong to monocotyledons, hardwoods to eudicotyledons and softwoods to gymnosperms. They all have cellulose, hemicellulose and lignin as the main constituents, alt- hough their proportions vary significantly (Table 1). Cellulose content can be 30– 41% of dry matter in gramineous plants and 40–47% in soft- and hardwoods. Lignin content is especially high in softwoods and in some hardwoods such as eucalyptus and poplar. In addition to the contents of the main components, the composition of hemicellulose (Section 1.2.2) and lignin (Section 1.2.3) varies con- siderably between hardwoods, softwoods and grasses. Pectins, proteins, miner- als, and various lipophilic compounds (or extractives) are also present as minor constituents in lignocellulosic biomass. Cell walls are built up of several layers: middle lamella, primary and secondary cell walls, and the warty layer (Sjöström, 1993). These layers differ from one an- other with respect to their ultrastructure as well as their chemical composition. The highly lignified middle lamella is located between cells and it binds them together. The thin primary cell walls consist of cellulose microfibrils (Section 1.2.1) that are embedded in a matrix consisting of hemicellulose, lignin and pectins (Harris and Stone, 2008). The primary cell wall is supported by the thicker secondary cell wall. Secondary cell wall consists of several sublayers: S1, S2, and S3 that are com-
- 20. 1. Introduction 18 posed of cellulose, hemicellulose and lignin (Harris and Stone, 2008; Sjöström, 1993). The secondary cell wall layers differ in respect to orientation of the cellu- lose microfibril network. A schematic presentation of the lignocellulosic cell wall structure is presented in Figure 1. Figure 1. Schematic presentation of a lignocellulose structure. Cellulose microfi- brils with crystalline and paracrystalline (amorphous) regions are embedded into a matrix consisting of lignin and hemicellulose in the cell wall (US DOE, 2005).
- 21. Table 1. Chemical composition of various lignocellulosic raw materials (percent of dry matter). Raw material Cellulose Hemicellulose Lignin Extractives Ash References Softwoods Norway spruce 45–47 24–26 27–28 0.4–0.9 nd Bertaud and Holmbom, 2004 Pitch pine 43 24 29 nd nd Park and Kim, 2012 Hardwoods Eucalyptus 42 19 30 nd nd Park and Kim, 2012 Birch 40 27 25 nd nd Goshadrou et al., 2013 Aspen 45 21 21 nd 0.5 Xu and Tschirner, 2012 Salix 43 21 26 nd 1.0 Sassner et al., 2006 Poplar 44 20 29 3.6 1.1 Wyman et al., 2009 Gramineous plants Sugar cane bagasse 41 21 26 0.6 1.6 Rabelo et al., 2011 Wheat straw 35 22 16 nd 7 Østergaard Petersen et al., 2009 Corn stover 38 26 18 nd 7 Kim and Lee, 2006 Switchgrass 30–35 23–26 19–22 nd 3–4 Kim et al., 2011 nd = no data 19 1.Introduction
- 22. 1. Introduction 20 1.2.1 Cellulose Cellulose is one of the most abundant materials in the natural world and it is the structural material in plants. Cellulose is a linear polymer of glucose composed of glucopyranose units coupled to each other by -(1 4)-glycosidic bonds. The number of glucose units in cellulose molecules varies and the degree of polymeri- zation ranges from 300 to 15000 depending on the source and treatments carried out (Fengel and Wegener, 1989). Every glucose unit is 180° rotated with respect to its neighbours in the cellulose chain and thus the repeating unit is a cellobiose residue. In nature, the individual cellulose chains adhere to each other along their lengths by intra- and intermolecular hydrogen bonding and van der Waals interac- tions to form elementary fibrils (Nishiyama et al., 2002; Parthasarathi et al., 2011). The hydrogen bonding is not homogeneous, but differs from the centre chains to the outer chains. Although not fully confirmed, the elementary fibrils are consid- ered to consist of 24–36 cellulose chains adhered to each other. These considera- tions are based on scattering data and information on cellulose synthase proteins (Endler and Persson, 2011; Fernandes et al., 2011). The elementary fibrils are highly crystalline and they further aggregate to form microfibrils. However, the structure of cellulose is not uniform. There are crystalline and more disordered or amorphous regions in the structure and in addition there are several types of sur- face irregularities (Cowling, 1975; Fan et al., 1980). In native cellulose the crystal structure contains tens of glucan chains in a parallel orientation with their reducing chain ends at one terminus of the crystal and their non-reducing chain ends at the other. Seven different crystal structures have been identified for cellulose, which are designated as I , I , II, IIII, IIIII, IVI and IVII (O'Sullivan, 1997). In nature cellulose is present in two forms, I and I , which are the most abundant crystal forms generally available. Cellulose I and I differ mainly in the packing arrangement of their hydrogen bonded sheets (Nishiyama et al., 2003). The other structures are formed from native cellulose after chemical or thermal treat- ments, but how the changes in crystal structure occur is not yet fully understood. Cellulose is almost always present in nature together with other biopolymers, primarily with lignin and hemicellulose. The cellulose produced by the bacterium Acetobacter xylinum is an example of pure cellulose (Marchessault and Sundara- rajan, 1993). The crystallinity and the matrix structure make the cellulosic structure highly recalcitrant. 1.2.2 Hemicellulose In contrast to cellulose, hemicelluloses are heteropolymers consisting of the pen- toses D-xylose and L-arabinose, the hexoses D-mannose, D-glucose and D- galactose, and uronic acids (Saka, 1991). Hemicelluloses can be extracted from plant cell walls by dilute alkali. The average degree of polymerization of hemicellu- loses varies between 70 and 200 depending on the source (Fengel and Wegener,
- 23. 1. Introduction 21 1989). Hemicelluloses can be grouped into xylans, mannans, xyloglucans and mixed linkage glucans on the basis of the main sugar residue in the backbone. Xylans and mannans are the main groups of hemicellulose. The basic structures of xylan and mannan are presented in Figure 2. Xylans, polymers of (1 4) -linked -D-xylopyranosyl (Xylp) units, are abun- dantly found from many cell wall types in both hardwoods, softwoods and gramin- eous plants. The xylan backbone is usually substituted by side chains of L- arabinose or glucuronic acids (Ebringerová and Heinze, 2000; Timell, 1967). In hardwoods, xylans constitute 10–35% of the wood and are substituted by methyl glucuronic acid residues at every tenth Xylp residue (Whistler and Chen, 1991; Willför et al., 2005b). Hardwood xylan is esterified with acetyl groups (~3.5–7 acetyl groups per ten Xylp residues). In softwoods, xylans consist of 7–15% of the wood. Softwood xylan contains methyl glucuronic acid substituents at every 5–6 Xylp residues and arabinose substituents at every 8 Xylp residues (Sjöström, 1993; Timell, 1967; Willför et al., 2005a). Softwood xylans are not acetylated. In gramineous plants arabino(methylglucurono)xylans are prevalent and the content of xylan can vary from 20% in the primary cell wall up to 50% in the secondary cell wall (Vogel, 2008). The xylan backbone contains arabinose and glucuronic acid substituents and oligosaccharide side chains consisting of arabinose, xylose and galactose (Naran et al., 2009; Wende and Fry, 1997; Wilkie, 1979). Ferulic acid and small amounts of p-coumaric acid and sometimes sinapinic acid are esterified to arabinose groups as single substituents or in oligosaccharide side chains (Harris and Trethewey, 2010; Iiyama et al., 1994; Wende and Fry, 1997). Mannans, galactoglucomannans and glucomannans, are found particularly in softwoods, where they are usually the predominant non-cellulosic polysaccharides (Sjöström, 1993; Timell, 1967). Galactoglucomannans comprise approximately 12–18% of softwood. Galactoglucomannans have a linear chain of (1 4) linked - D-mannopyranosyl and -D-glucopyranosyl residues, with -D-galactose substitu- ents (Lundqvist et al., 2002). The ratio of glucose:mannose:galactose varies in the approximate range 1 : 3–4 : 0.1–1.0 (Sjöström, 1993). The higher galactose con- tent is found in the water soluble fraction and the lower galactose content in the alkali soluble fraction of softwood galactoglucomannan. In addition, galactogluco- mannans are acetylated, the degree of acetylation depending on the species. Hardwoods contain 2–5% of glucomannan, which is not substituted with galactose (Timell, 1967). The glucose-mannose ratio is usually 1:2 and acetyl groups can be attached to mannose (Teleman et al., 2003). Only low amounts of glucomannan are present in gramineous plants (Vogel, 2008).
- 24. 1. Introduction 22 Figure 2. Schematic structures of polymeric xylan and galactoglucomannan and presentation of enzymes participating in their hydrolysis. 1.2.3 Lignin Lignin is a complex aromatic polymer present abundantly in nature and is an inte- gral component of plant cell walls. Softwoods contain approximately 27–29% of lignin, whereas hardwoods, grasses and cereals usually have lower amounts (Table 1). The exact structure of lignin in the native form is still unclear. The struc- ture of isolated lignins is always modified to a certain extent. Lignin is a three dimensional heteropolymer of methoxylated phenyl propane units, for which the precursors, monolignols, are p-coumaryl, coniferyl and sinapyl alcohol. Monolig- nols differ in respect to methoxylation. The corresponding aromatic constituents of these alcohols in the polymer are called 4-hydroxyphenyl (H), guaiacyl (4-hydroxy- 3-methoxyphenyl, G) and syringyl (4-hydroxy-3,5-dimethoxyphenyl, S) units. The composition of lignin varies widely with species. Softwood lignin is dominated by G units, whereas hardwood lignin is a mixture of G and S units (Fengel and Wegen- er, 1989; Sjöström, 1993). Lignin from grasses typically contains all three types of monolignols with different ratios (Buranov and Mazza, 2008). Different types of linkages connect the phenylpropane units. The most common is the -O-4´ linkage making up more than half of the lignin linkages in soft- and hardwoods (Dorrestijn et al., 2000; Sjöström, 1993). The amount, composition, and chemical bonds of lignin vary between plants, in different plant parts and cell types or even within a single cell wall (Campbell and Sederoff, 1996). In lignocellulosic biomass lignin is Arabino xylan in grasses Hardwood xylan Softwood galacto glucomannan feruloyl esterase endoxylanase acetyl xylan esterase -glucuronidase -arabino- furanosidase endomannanase acetyl glucomannan esterase Xyl XylXyl Ara AraAra Ara Xyl Xyl AcAc GlcA Ph Xyl Xyl Xyl XylXyl Xyl Xyl AcGlcA Xyl Xyl AcAc -galactosidase AcGal AcAc Man ManGlc Man Man ManGlc Man Xyl Ac = acetyl Ara = arabinose Ph = phenolic group Xyl = xylose Glc = glucose Man = mannose Gal = galactose GlcA = (methyl)glucuronic acid endoxylanase endoglucanase
- 25. 1. Introduction 23 cross-linked with carbohydrates by ether or ester linkages via e.g. arabinose- ferulic acid or glucuronic acid (Takahashi and Koshijima, 1988). 1.3 Enzymes degrading lignocellulose Lignocellulosic biomass is degraded and utilized in nature by the action of micro- organisms as part of the carbon cycle (Lundell et al., 2010; Lynd et al., 2002). Fungi and bacteria produce and secrete enzymes that are needed to degrade lignocellulosic materials into a more widely utilisable form. Cellulose is degraded by an enzyme system consisting of hydrolytic cellulases. Recent studies also suggest that oxidative mechanisms participate in the degradation of cellulose (see Sections 1.3.1 and 1.3.2). Hydrolysis of hemicellulose requires various enzyme activi- ties, e.g. xylanases and mannanases (Section 1.3.3). Several enzymes modifying and degrading lignin through oxidative mechanisms are also produced by microbes, e.g. laccases, lignin peroxidases and manganese peroxidases (Hatakka, 1994). One of the most characterized producers of biomass-degrading enzymes is the filamentous fungi Trichoderma reesei. It is also largely used in industrial produc- tion of cellulases and hemicellulases as well as being a heterologous host for protein production. Cellulases and hemicellulases are often modular (Henrissat and Davies, 2000). They consist of at least one catalytic domain, which may be attached to a carbo- hydrate binding module (CBM) via a highly glycosylated peptide linker (Tomme et al., 1988). CBMs have been shown to improve the hydrolysis of insoluble sub- strates (Tomme et al., 1988) and they have also been proposed to contribute to amorphogenesis (Arantes and Saddler, 2010). Cellulases, other glycoside hydro- lases and auxiliary enzymes have been classified into different families. Enzymes in the same family share similar protein folding and three dimensional structure as well as similar reaction mechanisms (Henrissat, 1991; Levasseur et al., 2013). Today, 133 families of glycoside hydrolases (GH) and 11 auxiliary activity (AA) families including other enzymes participating in the degradation of lignocellulose have been identified and listed in the Carbohydrate Active Enzyme database (CAZy, 2013). Enzymes known to play a role in cellulose degradation are found at least in 20 glycoside hydrolase families, in families 1, 3, 5, 6, 7, 8, 9, 10, 12, 18, 19, 26, 30, 44, 45, 48, 51, 74, 116 and 124. 1.3.1 Cellulases Cellulases are glycoside hydrolases that catalyse the cleavage of -(1 4) glycosidic bonds in cellulose and produce glucose, cellobiose and cello-oligosaccharides as primary products. They are found in various organisms, predominantly in Prokary- otae and Fungi. Cellulases have traditionally been divided into cellobiohydrolases, endoglucanases and -glucosidases. Cellobiohydrolases (exo- 1,4- -glucan cellobiohydrolases, CBHs) sequentially release cellobiose units from the cellulose chain either from the reducing end (EC
- 26. 1. Introduction 24 3.2.1.176) or the non-reducing end (EC 3.2.1.91). They can attack the crystalline parts of the substrate, primarily producing cellobiose and decreasing the substrate degree of polymerization (DP) only very slowly (Irwin et al., 1993; Kleman-Leyer et al., 1996). The CBHs attack cellulose crystals on the hydrophobic faces (Liu et al., 2011). Most CBHs belong to GH families 6 and 7. Cellobiohydrolases have been shown to have a tunnel shaped active site composed of several loops, which limits their activity to chain ends (Divne et al., 1994; Rouvinen et al., 1990). On the other hand, CBHs can have some endo-type activity (Kurašin and Väljamäe, 2011; Ståhlberg et al., 1993). CBHs are generally considered as processive enzymes, initiating their action from the ends of the cellulose chains and continuing along the cellulose chain until the end (Divne et al., 1994; Teeri et al., 1998). In strongly processive cellobiohydrolases (e.g. Trichoderma reesei CBHI) a substrate binding tunnel enhances the probability for the enzyme to remain bound after a catalytic cycle. T. reesei CBHII is suggested to be less processive (Igarashi et al., 2009). The processive action may be restricted by obstacles formed by other molecules on the cellulosic substrates which prevent the CBHs from gliding along cellulose chains (Kurašin and Väljamäe, 2011). The blocked CBH molecule can also pre- vent action of other CBH molecules (Igarashi et al., 2011). Endoglucanases (endo-1,4- -glucanase, EGs, EC 3.2.1.4) attack the cellulose chain at internal positions (Medie et al., 2012), resulting in a rapid decrease in the DP of the substrate (Kleman-Leyer et al., 1994; Kleman-Leyer et al., 1996). Be- cause the glucan chains can remain associated with the rest of the cellulose crys- tal after a single bond cleavage at the surface, it takes a relatively long time before soluble products are observed after an endo-type of attack. Endoglucanases are present in many GH families e.g. in 5, 6 and 7. Endoglucanases usually have an open active site, a substrate binding cleft, which can bind and act in the middle of the chain (Henriksson et al., 1996; Kleywegt et al., 1997). The cellulose chain segment interacts with multiple subsites (4–7) in the cleft. Processive endoglu- canases have also been reported (Irwin et al., 1993; Wilson and Kostylev, 2012). -Glucosidases ( -D-glucoside glucohydrolase, Gs, EC 3.2.1.21) have an im- portant role in the hydrolysis of cellobiose and short oligosaccharides to glucose, thereby decreasing the product inhibition of cellobiohydrolases (Gusakov and Sinitsyn, 1992). The hydrolysis rates of Gs decrease markedly as the substrate DP increases (Zhang and Lynd, 2004). Gs can be found in GH families 1, 3, 9, 30 and 116, although most of the microbial Gs employed in cellulose hydrolysis belong to GH family 3. Structures of Gs have not been thoroughly studied. Family 1 Gs have been shown to have loops around active cavities (Hakulinen et al., 2000). The loops are suggested to be involved in substrate binding. The GH1 Gs having broader cavities are suggested to have wider substrate specificity. The GH3 Gs have been reported to have clear differences in structure (Suzuki et al., 2013). Crystal structures of Aspergillus aculeatus G contained a long cleft suita- ble for longer oligosaccharides, whereas Hordeum vulgare G had a binding pocket suitable for various disaccharides. Because cellobiose is a strong inhibitor of CBHs, the G activity in cellulase mixtures must be optimized to overcome the product inhibition of CBHs (Gusakov and Sinitsyn, 1992). The inhibition of Gs by
- 27. 1. Introduction 25 glucose must also be considered because accumulation of glucose will lead to the accumulation of cellobiose and CBH inhibition (Seidle et al., 2004; Teugjas and Väljamäe, 2013). Many Gs also have transglycosylation activity, which competes with hydrolysis (Bohlin et al., 2013; Seidle et al., 2004). 1.3.2 Auxiliary enzymes for degradation of cellulose In 2007 Merino and Cherry found that a Thielavia terrestris GH61 protein en- hanced the activity of T. reesei cellulases and in 2008 Moser et al. reported the same with a Thermobifida fusca CBM33 protein. Since then the structure, mecha- nisms and role of fungal GH61 and bacterial CBM33 proteins in cellulose degrada- tion have been intensively studied. It is currently known that these new oxido- reductive enzymes, lytic polysaccharide mono-oxygenases (LPMOs, AA9 and AA10, formerly referred to as GH61 and CBM33, no EC classification) participate in the degradation of cellulose in addition to CBHs and EGs in the conventional models. They catalyse the oxidative cleavage of cellulose using low molecular weight reducing agents such as ascorbate, gallate, reduced glutathione, and even fragments from lignin (Quinlan et al., 2011; Westereng et al., 2011). LPMOs have been shown to cleave the cellulose chain in crystalline cellulose by an oxidative mechanism. Oxidation of hydroxyl groups at the C1, C4 or C6 position has been reported (Beeson et al., 2012; Langston et al., 2011; Quinlan et al., 2011). Cellobiose dehydrogenases (CDH, EC 1.1.99.18) are known to catalyse oxida- tion of the reducing end of cellobiose, cellodextrins, or certain other oligosaccha- rides to the corresponding lactone, which spontaneously converts to the aldonic acid (e.g. cellobionic acid) (Cameron and Aust, 2001; Henriksson et al., 2000; Zamocky et al., 2006). CDHs employ a wide spectrum of electron acceptors in oxidation reactions (Henriksson et al., 2000). Recently it has been reported that CDH enhances the depolymerisation of crystalline cellulose in a synergistic mech- anism together with LPMOs (Langston et al., 2011; Sygmund et al., 2012). How- ever, not all enzyme systems contain CDHs and their exact role in the degradation of lignocellulose is not known. The proposed model for cellulose degradation is presented in Figure 3. CBHs hydrolyse the cellulose chains from reducing or non- reducing chain ends, endoglucanases form new chain ends in the amorphous regions and LPMOs in the crystalline parts of cellulose. Gs further hydrolyse the cellobiose formed into glucose.
- 28. 1. Introduction 26 Figure 3. Synergistic model for enzymatic degradation of cellulose. Swollenins and expansins have also been suggested to participate in cellulose degradation. Fungal swollenins have exhibited clear effects on insoluble cellulosic substrates, such as swelling and decrease in particle size and crystallinity, without the formation of reducing sugars (Chen et al., 2010; Jager et al., 2011; Saloheimo et al., 2002). Disruption of the hemicellulosic fraction has also been reported (Gourlay et al., 2013). Bacterial expansin exhibits similar cellulose binding and weakening activities (Kim et al., 2009). No catalytic activity has been detected so far except for a weak endoglucanase activity (Chen et al., 2010). Combination of swol- lenins and expansins with cellulases and xylanases has been reported to accelerate the hydrolysis of cellulose and xylan (Gourlay et al., 2013; Kim et al., 2009). 1.3.3 Hemicellulolytic enzymes Hemicellulose is a group of carbohydrate polymers consisting of different mono- saccharides, sugar acids and non-carbohydrate subunits, and having different branching and structures. A variety of enzymes is needed for the complete hydrol- ysis of hemicellulose. Depolymerizing enzymes act on the backbone sugar chain i.e. xylan, gluco- mannan or glucan chain. Endo-1,4- -xylanases (EC 3.2.1.8) solubilize xylan whereas an endo-1,4- -mannanase (EC 3.2.1.78) cleaves the galactoglucoman- nan main chain to produce oligosaccharides (Sørensen et al., 2007; Stålbrand et al., 2004; Vrsanská et al., 2007). The produced mixed xylo- and manno- oligosaccharides are hydrolysed by debranching enzymes and by the enzymes hydrolysing the oligomers. -L-Arabinofuranosidases (EC 3.2.1.55) remove arabi- nose groups bound to the xylan backbone and can be specific for either mono- or disubstituted xylose residues and short or long oligomers (Kühnel et al., 2011; Van Laere et al., 1997). -Glucuronidases (EC 3.2.1.139) are needed for the removal
- 29. 1. Introduction 27 of glucuronic acid groups from xylan (Tenkanen and Siika-aho, 2000) and - galactosidase (EC 3.2.1.22) for the removal of galactose substituents in galac- toglucomannans (Luonteri et al., 1998). Acetyl groups ester-linked in xylan and glucomannan can be hydrolysed by acetyl xylan esterases (EC 3.1.1.72) or acetyl glucomannan esterases (EC 3.1.1.6), respectively (Biely, 2012; Stålbrand et al., 2004). In addition, feruloyl esterases (EC 3.1.1.73) and p-coumaroyl esterases (EC 3.1.1.-) are needed to cleave the corresponding ester linkages in xylan (McCrae et al., 1994). The depolymerizing and debranching enzymes acting on xylan and galactoglucomannan are presented in Figure 2. -Glucosidases (EC 3.2.1.21), -xylosidases (EC 3.2.1.37) and -mannosidases (EC 3.2.1.25) are essential for the complete hydrolysis of hemicellulosic oligosaccharides to mono- saccharides (Ademark et al., 1999; Rahman et al., 2003). Cellulases can also have hemicellulolytic activity towards xylan and other hemicelluloses (Tomme et al., 1995; Vlasenko et al., 2010). Several family 7 endoglucanases are active against xylan, arabinoxylan and xyloglucan, whereas family 5 endoglucanases have activity on mannan and galactomannan (Vlasenko et al., 2010). Having a broad specificity may be beneficial for a biomass-hydrolysing enzymatic system, because of the heterogeneity of the substrate. 1.3.4 Synergism in the hydrolysis of cellulose Cellulose degradation requires sets of secreted enzymes that work in synergy. Synergism occurs when the activity exhibited by mixtures of components is great- er than the sum of the activity of these components evaluated separately (Henrissat et al., 1985; Walker and Wilson, 1991). It is known that endoglucanases and cellobiohydrolases work synergistically in the so called endo-exo synergy. Endoglucanases produce new chain ends by cleaving the cellulose chains, thus creating new starting points for cellobio- hydrolases (Henrissat et al., 1985; Nidetzky et al., 1994; Nidetzky et al., 1993). On the other hand, CBHs make the substrate more accessible to EGs (Irwin et al., 1993; Väljamäe et al., 1999). Two cellobiohydrolases (CBHI and CBHII) have also reported to have synergy due to their different specificity for reducing and non- reducing ends (Fägerstam and Pettersson, 1980; Igarashi et al., 2011; Medve et al., 1994; Nidetzky et al., 1994). -Glucosidase reduces inhibition by cellobiose and has a synergistic effect on the hydrolysis with cellobiohydrolases or endoglu- canases (Gruno et al., 2004; Lamed et al., 1991). Some oxidoreductases have recently been shown to synergistically enhance the hydrolysis by cellulases. Lytic polysaccharide monooxygenases (LPMOs) in family AA9 and some members in the family AA10 can act synergistically with cellulases (Forsberg et al., 2011; Harris et al., 2010). The synergistic effect ap- pears to be related to the ability of LPMOs to act on highly crystalline areas, gen- erating new ends for hydrolases and disrupting the crystallinity of cellulose (Langston et al., 2011). Synergy of LPMOs with cellobiose dehydrogenases has
- 30. 1. Introduction 28 also been reported and the combination is able to significantly stimulate the deg- radation of cellulose by cellulases or even by -glucosidase (Langston et al., 2011). Hydrolysis of hemicellulose by hemicellulolytic enzymes can synergistically im- prove the hydrolysis of cellulose. Addition of xylanase, feruloyl esterase and acetyl xylan esterases has increased the release of glucose in addition to xylose (Hu et al., 2011; Kumar and Wyman, 2009; Murashima et al., 2003; Selig, 2008; Zhang et al., 2011). The boosting effect of xylanases and mannanases is also reported even with lignocellulosic substrates having only a low xylan content (Hu et al., 2011; Varnai et al., 2011a; Várnai et al., 2010) and may be due to removal of residual hemicellulose on the cellulose fibres, or increased porosity and fibre disintegration (Arantes and Saddler, 2010; De Jong et al., 1997; Suurnäkki et al., 1997). 1.3.5 Optimal enzyme compositions for hydrolysis of lignocellulosic materials The hydrolysis of lignocellulosic substrates requires numerous enzymes working in synergy. The variation in structure and composition between raw materials from different sources and after different types of pretreatment further increases the complexity. The optimal enzyme composition for a given raw material can vary due both to the different chemical bonds to be broken and to structural limitations such as crystallinity, pore size, fibrillation, and content and location of hemicellu- lose and lignin. In addition, the applied enzyme components, their specificity and synergistic interactions as well as their varied susceptibility to inhibition, inactivation or non-productive binding on lignin can affect the optimal enzyme composition. The aim of enzyme mixture optimization is to hydrolyse lignocellulosic materials efficiently using lower enzyme dosages. Enzyme mixture optimization has been conducted for various substrates (Table 2). In addition to almost pure cellulosic substrates (Andersen et al., 2008; Baker et al., 1998; Boisset et al., 2001; Gusakov et al., 2007; Kim et al., 1998), optimized minimal enzyme mixtures have been developed for lignocellulosic substrates such as pretreated barley straw (Rosgaard et al., 2007), pretreated wheat straw (Billard et al., 2012), pretreated corn stover (Banerjee et al., 2010a; Banerjee et al., 2010b; Banerjee et al., 2010c; Gao et al., 2010a; Gao et al., 2010b; Zhou et al., 2009), pretreated switchgrass, Miscanthus, distillers’ grains, poplar wood (Banerjee et al., 2010b) and pretreated douglas fir (Gusakov et al., 2007). Studies on the optimization of enzyme mixtures for the hydrolysis of lignocellu- losic substrates indicated that the composition of enzyme mixtures can vary con- siderably. It has been claimed that the optimal ratio of cellulolytic activities in the hydrolysis of lignocellulose can differ remarkably from that of the cellulases se- creted by T. reesei (Rosgaard et al., 2007). Generally, a high proportion of Cel7A (CBHI) and Cel7B (EGI) has been reported in studies carried out using the main Trichoderma cellulases and endoxylanase (Banerjee et al., 2010b; Banerjee et al., 2010c; Billard et al., 2012; Gao et al., 2010a; Rosgaard et al., 2007). On the other hand, the optimal proportions of xylanase, -glucosidase and Cel6A (CBHII) ap- pear to vary considerably depending on the study and raw material.
- 31. 1. Introduction 29 Banerjee et al. developed optimal enzyme mixtures for the hydrolysis of various pretreated substrates (Banerjee et al., 2010b). They found that the optimal mixture of T. reesei enzymes for the hydrolysis of NaOH-pretreated corn stover and switchgrass contained clearly less endoxylanase than for the ammonia fibre ex- plosion (AFEX) and alkaline peroxide pretreated materials, and that the increased proportion of xylanase was generally associated with lower proportions of Cel7A (CBHI) and Cel7B (EGI). In the case of barley straw, hydrolysis of material pre- pared by acid-catalysed steam explosion required less endoglucanase than mate- rial pretreated by non-catalysed steam explosion or hot water treatment (Rosgaard et al., 2007). The number of enzyme components in reported studies has varied from 3 to 16 and both purified and commercial enzyme mixtures have been used as mixture components (Table 2). The optimizations have been performed for the main cellu- lase activities (Zhou et al., 2009), for cellulases and xylanases (Gao et al., 2010b), for multi-component mixtures containing several accessory enzymes (Banerjee et al., 2010c), or for mixtures of commercial enzymes (Berlin et al., 2007; Garlock et al., 2012). Banerjee et al. found that the optimal enzyme mixture varies substan- tially depending on how many and which enzyme components were included in the optimization in addition to a core set of enzymes (Banerjee et al., 2010b). In fact, the number of enzyme components and the raw material appeared to have more significant effect on the enzyme proportions than the pretreatment carried out for the raw material (Banerjee et al., 2010b; Banerjee et al., 2010c; Billard et al., 2012; Gao et al., 2010a; Zhou et al., 2009). Thus it is important to include all the most essential enzymes in experimental design, although this makes the de- sign more complex. The optimal mixtures have been shown to be clearly different for glucose and xylose release (Banerjee et al., 2010a), and to be dependent on the total enzyme loading (Gao et al., 2010a). Despite relatively high enzyme dosages (10–30 mg/g of dry substrate), many of the optimization studies resulted in very low hydrolysis yields (10–50%; Table 2). Recalcitrant raw materials and inefficient pretreatment appear to be the most probable reasons for low hydrolysis yields. In such a situa- tion no significant benefit can be obtained by optimization. In addition to the de- gree of hydrolysis, the enzyme dosage can affect the optimal enzyme ratio. Gao et al. found that within T. reesei enzymes the role of Cel7B (EGI) was more important in the case of lower total protein loadings (Gao et al., 2010b). The hydrolysis time also influenced the optimal enzyme ratio (Billard et al., 2012). In the beginning of the hydrolysis of steam exploded wheat straw less xylanase and Cel5A (EGII) were needed than in the later stages of the hydrolysis.
- 32. Table 2. Studies on optimal enzyme compositions for hydrolysis of lignocellulosic raw materials. Raw material Pretreatment a Source of enzyme components Number of enzyme com- ponents Statistical method (+/-) Hydrolysis conditions (d.m. content / hydrolysis time / mixing speed) Max. yield b Reference Avicel - Chrysosporium lucknowense, T. reesei, Aspergillus japonicus 7 - 5% / 24–72 h / n.d. 90% Glc Gusakov et al., 2007 Cotton - Chrysosporium luck- nowense, T. reesei, Aspergillus japonicus 7 - 2.5% / 24–72 h / n.d. 84% Glc Gusakov et al., 2007 Filter paper - Thermomonospora fusca, T. reesei 7 + 0.85% / 24–72 h / n.d. 60% Kim et al., 1998 Bacterial cellulose - Humicola insolens, A. niger 4 - 0.1% / 1–14 h /0 rpm 56–90% Boisset et al., 2001 Barley straw SE (acid impr.) T. reesei, A. niger (Novozym188) 5 + 1% / 24 h / n.d. 56% Glc Rosgaard et al., 2007 SE (water impr.) T. reesei, A. niger (Novozym188) 5 + 1% / 24 h / n.d. 33% Glc Rosgaard et al., 2007 Hot water extraction T. reesei, A. niger (Novozym188) 5 + 1% / 24 h / n.d. 50% Glc Rosgaard et al., 2007 Wheat straw SE T. reesei, A. niger 6 + 1% / 0.5–48 h / 175 rpm 66% Billard et al., 2012 1.Introduction 30
- 33. Raw material Pretreatment a Source of enzyme components Number of enzyme com- ponents Statistical method (+/-) Hydrolysis conditions (d.m. content / hydrolysis time / mixing speed) Max. yield b Reference Rice straw NaOH Celluclast, A. aculeatus (enzyme exract), Bacillus subtilis (expansin) 3 + 1% / 48 h / 200 rpm 78% Suwannarangsee et al., 2012 Corn stover AFEX A. nidulans, A. niger T. reesei, 6 + 0.2% glucan / 24 h / 200 rpm 80% Glc, 56% Xyl Gao et al., 2010a AFEX Dictyoglomus tur- gidum, C. thermocellum, Geobacillus spp. T. reesei 6 - 0.2% glucan / 24 h / 200 rpm 94% Glc, 62% Xyl Gao et al., 2010b AFEX T. reesei, A. niger T. longibrachiatum, 6–16 + 0.2% glucan/ 48 h / 10 rpm 44%– 52% Glc Banerjee et al., 2010a; Banerjee et al., 2010b; Banerjee et al., 2010c NaOH T. reesei, A. niger T. longibrachiatum, 6 + 0.2% glucan/ 48 h / 10 rpm 41% Glc Banerjee et al., 2010b Alkaline peroxide T. reesei, A. niger T. longibrachiatum 6–16 + 0.2% glucan/ 48 h / 10 rpm 58%– 69% Glc Banerjee et al., 2010b SE T. viride 7 + 5% / 72 h /210 rpm 80% Glc Zhou et al., 2009 Dilute acid Celluclast1.5 Novozym188 Multifect xylanase Multifect pectinase 4 + 0.6% / 24 h / 600 rpm 99% Glc, 88% Xyl Berlin et al., 2007 Ball milling Accellerase1000 CellulaseZSL-1300 Xylanase 3 + 10% / 95 h/ 120 rpm 95% Lin et al., 2011 1.Introduction 31
- 34. Raw material Pretreatment a Source of enzyme components Number of enzyme com- ponents Statistical method (+/-) Hydrolysis conditions (d.m. content / hydrolysis time / mixing speed) Max. yield b Reference Switchgrass AFEX T. reesei, A. niger T. longibrachiatum, 6 + 0.2% glucan/ 48 h / 10 rpm 24% Glc Banerjee et al., 2010b AFEX Novozym188, Spezyme SP, Multifect xylanase Multifect pectinase 4 + 1% glucan /72 h / 200 rpm 80% Glc, 76% Xyl Garlock et al., 2012 NaOH T. reesei, A. niger T. longibrachiatum 6 + 0.2% glucan/ 48 h / 10 rpm 26% Glc Banerjee et al., 2010b Alkaline peroxide T. reesei, A. niger T. longibrachiatum 6 + 0.2% glucan/ 48 h / 10 rpm 39% Glc Banerjee et al., 2010b Miscanthus AFEX T. reesei, A. niger T. longibrachiatum, 6 + 0.2% glucan/ 48 h / 10 rpm 23% Glc Banerjee et al., 2010b NaOH T. reesei, A. niger T. longibrachiatum, 6 + 0.2% glucan/ 48 h / 10 rpm 18% Glc Banerjee et al., 2010b Alkaline peroxide T. reesei, A. niger T. longibrachiatum, 6 + 0.2% glucan/ 48 h / 10 rpm 32% Glc Banerjee et al., 2010b DDGSc AFEX T. reesei, A. niger T. longibrachiatum, 6–16 + 0.2% glucan/ 48 h / 10 rpm 23%– 52% Glc Banerjee et al., 2010b NaOH T. reesei, T. longibrachiatum, A. niger 6 + 0.2% glucan / 48 h / 10 rpm 24% Glc Banerjee et al., 2010b Alkaline peroxide T. reesei, A. niger T. longibrachiatum, 6 + 0.2% glucan/ 48 h / 10 rpm 30% Glc Banerjee et al., 2010b 1.Introduction 32
- 35. Raw material Pretreatment a Source of enzyme components Number of enzyme com- ponents Statistical method (+/-) Hydrolysis conditions (d.m. content / hydrolysis time / mixing speed) Max. yield b Reference Poplar AFEX T. reesei, A. niger T. longibrachiatum, 6 + 0.2% glucan/ 48 h / 10 rpm 14% Glc Banerjee et al., 2010b NaOH T. reesei, A. niger T. longibrachiatum, 6 + 0.2% glucan / 48 h / 10 rpm 10% Glc Banerjee et al., 2010b Alkaline perox- ide T. reesei, A. niger T. longibrachiatum, 6–16 + 0.2% glucan/ 48 h / 10 rpm 11% Glc Banerjee et al., 2010b Douglas fir Organosolv Chrysosporium lucknowense, T. reesei, A. japonicus 7 - 5% /24–72 h / n.d. 96% Glc Gusakov et al., 2007 a SE = steam explosion, AFEX = ammonia fibre explosion b Glc = glucose yield, Xyl = xylose yield c DDGS = dried distillers grains with solubles n.d. = no data 1.Introduction 33
- 36. 1. Introduction 34 1.4 Pretreatment of lignocellulosic substrates Lignocellulosic matrix is an important structural element in plants, and provides resistance against e.g. wind and cell rupture caused by osmotic pressure. Plants also have different kinds of defence mechanisms for protection against microbial and enzymatic attack, for example waxy layers on leaves and bark (Himmel et al., 2007). The cell wall matrix as such is also resistant to microbial degradation. The structure of cellulose, with highly ordered and water-excluding crystallites, retards the action of cellulases. Cell wall microfibrils are surrounded by hemicellulose that is covalently linked to lignin. The recalcitrant matrix of polymers is the main reason why plant biomass has high resistance to chemical, mechanical and enzymatic treatment. For an efficient and complete hydrolysis, biomass needs first to be pretreated. Various pretreatment methods have been developed to decrease the recalcitrance of lignocellulosic materials, and to make the cellulose more susceptible to enzy- matic hydrolysis (Mosier et al., 2005b; Sun and Cheng, 2002; Zhao et al., 2012). Pretreatments can also be used to fractionate the lignocellulosic raw materials by separating lignin and hemicellulose from cellulose. The aim of the pretreatment is to open up the cell wall structure and make cellulose and hemicellulose more accessible to enzymes during hydrolysis. Cell wall structure can be opened up by reducing the particle size and by increasing the pore size and accessible surface area. The pretreatments should also modify lignocellulose matrix structure by reducing the cellulose crystallinity, and by modification and solubilisation of lignin and hemicellulose. From a process technology and economy point of view, the overall aim of the pretreatment of lignocellulose is to get high product yield in the whole process, including pretreatment, hydrolysis, fermentation and downstream processing. To achieve this, pretreatment should minimize the degradation or loss of carbohydrates, and avoid formation of by-products that are inhibitory to en- zymes and fermentation microbes. In addition, pretreatment should have low capi- tal and processing costs and be suitable for various different raw materials. 1.5 Pretreatment methods Various different kinds of processes have been developed for the pretreatment of lignocellulosic biomass (Galbe and Zacchi, 2012; Haghighi Mood et al., 2013; Mosier et al., 2005b). The methods can be approximately grouped into physical, chemical, physico-chemical and biological pretreatment processes, although it is in many cases difficult to place a specific method into only one category. For ex- ample, thermal treatments can cause chemical changes in biomass and on the other hand it might also be difficult to avoid shear force effects during chemical treatments.
- 37. 1. Introduction 35 1.5.1 Physical and biological pretreatments Physical pretreatments can be defined as methods using physical processing on biomass. Thus mechanical and thermal processes and methods using irradiation and ultrasound treatments can be defined as physical pretreatments. Mechanical treatments aim at particle size reduction by chipping, milling and grinding. Commi- nution techniques include e.g. wet disk milling, ball milling and hammer milling (da Silva et al., 2010; Mani et al., 2004). Milling is also often combined with other pretreatments (Gao et al., 2012; Lin et al., 2010; Teramoto et al., 2008). In addi- tion to particle size reduction, decrease in crystallinity of the lignocellulosic materi- als has been observed (Chang and Holtzapple, 2000; da Silva et al., 2010). High energy requirements are typical for efficient mechanical treatments, thus decreas- ing the economic feasibility of these methods. Biological pretreatment uses microorganisms, mainly brown, white or soft-rot fungi, to degrade lignin and hemicellulose. Biological pretreatments have been applied for gramineous plants such as wheat straw (Hatakka, 1983; Lopez- Abelairas et al., 2012; Zeng et al., 2011) and corn stover (Song et al., 2013). The suggested advantages of biological methods are low energy consumption and the absence of chemical demands, mild process conditions and low capital costs (Alvira et al., 2010; Talebnia et al., 2010). On the other hand, pretreatment with microorganisms is slow and significant yield losses occur. Addition of metal ions and Tween has been suggested to improve the pretreatment efficiency by increasing the production of lignin-degrading enzymes (Song et al., 2013; Zeng et al., 2011). Com- bination of biological pretreatment with other pretreatments such as organosolv cooking is also one proposed option to improve its efficiency (Itoh et al., 2003). 1.5.2 Thermo-chemical pretreatments Chemical processes alone are not very often applied and chemicals are usually combined at least with thermal processing. Acid pretreatments are typically carried out with dilute mineral acids such as sulphuric acid at temperatures ranging from 120 to 200°C (Nguyen et al., 2000; Saha et al., 2005). The residence time can vary from minutes to an hour. Acid degrades hemicellulose, cellulose and lignin and toxic degradation products can be formed (Chundawat et al., 2010; Larsson et al., 1999). Alkaline treatments (Section 1.5.3.2) are usually carried out at lower temperatures and using longer residence times than acid pretreatments. Ozone treatments are purely oxidative treatments that solubilize lignin and small amounts of hemicellulose at room temperature and normal pressure, but their efficiency appears to be limited to grasses or agricultural residues such as sugar cane ba- gasse, wheat straw or rye straw (García-Cubero et al., 2012; Lee et al., 2010; Travaini et al., 2013). Due to the low temperatures in ozone treatment, formation of toxic compounds such as furfural or hydroxymethylfurfural is low (Travaini et al., 2013). The amount of ozone required in the treatment is important for the process
- 38. 1. Introduction 36 economy. Oxidative chemicals can be applied as such, or more usually combined with alkaline treatments (Section 1.5.3.3). One attractive chemical pretreatment method is organosolv pretreatment, which includes systems containing combinations of solvents e.g. ethanol, acetic acid, acetone, butanol or formic acid (Arato et al., 2005; Huijgen et al., 2010; Rio et al., 2010; Teramoto et al., 2009; Vanderghem et al., 2011) and acidic, neutral or alka- line catalysts (Mesa et al., 2010; Park et al., 2010). Treatment temperature can vary from 80°C to 210°C and time from 20 min to a few hours. Organosolv pre- treatments remove extensively lignin and almost completely hemicelluloses (Perez-Cantu et al., 2013; Rio et al., 2010; Teramoto et al., 2009). They are also effective for biomass with a high lignin content. Treatments with organic acids cause esterification of cellulose (Pan et al., 2006; Zhao et al., 2009). At the same time, degradation products such as furfural and hydroxymethylfurfural, acetic acid and formic acid are formed (Huijgen et al., 2010; Rio et al., 2010; Teramoto et al., 2009). Disadvantages of organosolv treatments are the high cost of reactors and solvents that need to be recycled. Use of ionic liquids (ILs) as solvents for pretreatment of cellulosic biomass has recently received much attention. ILs are salts, typically composed of large organic cations and small inorganic anions, which exist as liquids at temperatures below 100°C (Dadi et al., 2007; Stark, 2011). Their solvent properties can be varied by adjusting the anion and the cation. In the pretreatment with ionic liquids, biomass is dissolved in IL and then precipitated by addition of antisolvent e.g. water (Dadi et al., 2006; Dadi et al., 2007). After washing, the precipitate can be hydrolysed by cellulases. Carbohydrates and lignin can be dissolved in ILs simultaneously. The first studies were carried out with pure crystalline cellulose (Dadi et al., 2006; Dadi et al., 2007; Kuo and Lee, 2009), but pretreatments with lignocellulosic biomass, even wood, have also been reported (Kilpeläinen et al., 2007; Liu and Chen, 2006; Singh et al., 2009). The pretreatment can be carried out at moderate temperatures (100–120°C) and the treatment time ranges from 30 min to 3 h. The disadvantages of ILs in general are related to the price, purity, toxicity and recyclability of the ILs (Stark, 2010). The ILs also have limited compatibility with cellulases, which hinders their use as a pretreatment for enzymatic hydrolysis (Datta et al., 2010). 1.5.3 Physico-chemical pretreatments Combinations of physical and chemical methods are currently regarded as the most promising approaches. These technologies include e.g. steam pretreatments (Section 1.5.3.1), ammonia fibre explosion (AFEX), and wet oxidation (Section 1.5.3.3). The AFEX process includes ammonia treatment and steam explosion at tem- peratures from 90 to 180°C and high pressure (Garlock et al., 2012; Holtzapple et al., 1991). AFEX is a dry process and does not produce any separate liquid stream. In contrast, other pretreatment methods yield separate solid and liquid fractions. The main effects of AFEX are physical disruption, swelling and decrys-
- 39. 1. Introduction 37 tallization of cellulose and repositioning of cell wall decomposition products (Chundawat et al., 2011a). Low formation of inhibitors has also been reported (Chundawat et al., 2010). Recovery of ammonia is required for an economical process. Ammonia recycle percolation (ARP) and soaking in aqueous ammonia (SAA) are different variants of ammonia treatment occurring in aqueous solution (Kim et al., 2003; Yoo et al., 2013). The following sections present a deeper description of steam treatments, alka- line, and alkaline oxidative pretreatments that were also studied in this work. 1.5.3.1 Steam pretreatments Steam pretreatment is a general term used in this thesis to describe physico- chemical technologies exploiting high pressure steam. These methods include steam explosion and hydrothermal and liquid hot water treatments. The main difference between steam explosion and hydrothermal or hot water treatments is that steam explosion generally involves a higher dry matter content, whereas hydrothermal and liquid hot water treatments have more liquid water present dur- ing the pretreatment (Allen et al., 2001; Bondesson et al., 2013; Kristensen et al., 2008; Kumar et al., 2010). Steam pretreatments involve treatment of the biomass at high temperature (160–260°C, typically 180–215°C) and pressure. High- pressure and high-temperature steam is introduced into a reactor containing ligno- cellulosic material. The treatment time can vary from seconds to several minutes (typically 5 to 15 minutes). In steam explosion the pressure is rapidly released, causing the steam to expand in the lignocellulosic matrix, whereas in hydrothermal and liquid hot water treatments a similar explosion is not described in the litera- ture, although the pressure will drop after the reaction. Steam treatments can be intensified with chemical catalysts such as SO2 or sulphuric, acetic, phosphoric or lactic acid (Linde et al., 2006; Pitarelo et al., 2010; Tengborg et al., 1998; Xu et al., 2009b). Acid increases the solubilisation of hemicellulose sugars and it improves the enzymatic hydrolysis of the solid fraction (Hahn-Hägerdal et al., 2006). In steam pretreatments acids are also formed from acetyl groups present in biomass, causing autohydrolysis (Fengel and Wegener, 1989). Steam treatment can be applied for a wide range of raw materials including agricultural residues, softwoods and hard- woods (Bondesson et al., 2013; Kumar et al., 2010; Schütt et al., 2011). Softwoods usually require more drastic steaming conditions than hardwoods and straws. During the steam pretreatment hemicellulose is solubilised, and after the treat- ment it is located in the liquid phase as oligomers and monomeric sugars (Cui et al., 2012; Østergaard Petersen et al., 2009). Increase of accessibility of lignocellu- losic biomass by steam treatments appears to be mainly due to the removal of hemicelluloses (Mosier et al., 2005b). On the other hand, steam treatments have been shown to have a major effect on the distribution and the chemical structure of lignin (Marchessault, 1991; Ramos, 2003). Degradation of -ether linkages of lignin and of the chemical bonds in lignin-carbohydrate complex (LCC) have been reported, as well as chemical modifications of lignin. Depolymerisation of lignin in lower severity steam treatments can occur, whereas in more severe conditions
- 40. 1. Introduction 38 repolymerization has been reported (Li et al., 2007; Wang et al., 2012). Most of the lignin remains insoluble but the solubility of syringyl lignin increases more than that of guaiacyl lignin (Shimizu et al., 1998) and the S/G ratio has been shown to de- crease in steam explosion (Rahikainen et al., 2013). Steam explosion has also been reported to increase the oxygen/carbon–ratio and thus to remove surface lignin and reduce the hydrophobicity of biomass (Kumar et al., 2009; Negro et al., 2003). Structurally, steam treatments reduce biomass particle size and increase pore volume after explosive decompression (Mosier et al., 2005b). Increased crystallini- ty index (CrI) of the substrate has been reported due to removal of hemicellulose (Kumar et al., 2009). Significant decrease in the DP of cellulose in addition to shortening of cellulose fibrils has also been observed. Steam treatments cause partial hemicellulose degradation and the generation of inhibitors that may affect the subsequent hydrolysis and fermentation (Allen et al., 2001; Kim et al., 2013; Palmqvist et al., 1996). The major inhibitors generated during steam pretreatments are furan derivatives (furfural and hydroxymethylfurfu- ral), weak acids and fragments from lignin. Treatment temperature and the amount of acid should be optimized to minimize the formation of inhibitors. Varying steam treatment temperature profiles (Monavari et al., 2010) and two step steam explo- sion have been developed to decrease degradation of pentoses and formation of inhibitors (Söderström et al., 2003; Zhang et al., 2012). Mosier et al. reported that maintaining the pH value in the hydrothermal method between 4 and 7 during the process decreased the amount of inhibitors formed (Mosier et al., 2005a). 1.5.3.2 Alkaline pretreatments Alkaline treatments are widely used in the fractionation of wood in pulp and paper manufacturing. The dominating alkaline pulping method is sulphate cooking, the Kraft process, that is carried out using sodium hydroxide and sodium sulphide as cooking chemicals (Fengel and Wegener, 1989). The temperature is typically 155– 175°C and the pressure 7–11 bar. The cooking time varies between 2 and 5 hours. For lignocellulose pretreatments various alkaline treatments have also been developed using varying temperatures and times. Alkaline pretreatment can be performed at room temperature and pressure conditions, when the time ranges from hours to days (Kim and Holtzapple, 2006; Park and Kim, 2012; Sharma et al., 2013). The treatment time can be shortened to 0.5–10 hours by applying higher temperatures (85–160 °C) (Chang et al., 1998; Chen et al., 2013; Sharma et al., 2013). Alkaline treatments can be combined with oxidative conditions (see Section 1.5.3.3) to improve the efficiency of high lignin content materials. Sodium, potassi- um, calcium and ammonium hydroxides as well as sodium carbonate are possible chemicals for alkaline pretreatments (Chen et al., 2013; Jin et al., 2013; Park and Kim, 2012; Sharma et al., 2013). Typically, chemical dosages are high, with a range as high as 5–100% of biomass (w/w). High chemical consumption in alka- line pretreatments decreases the profitability of the method (Chen et al., 2013) and thus recycling of alkali is needed.
- 41. 1. Introduction 39 Alkaline treatments cause significant changes in the chemical composition of lignocellulose. They have been reported to dissolve lignin (Kim and Holtzapple, 2006; Kumar et al., 2009; Sun et al., 1995) and to cause the depolymerization of lignin molecules by cleavage of inter-molecular - and -aryl ether linkages, which essentially contributes to lignin degradation (Gierer, 1982; Gierer and Noren, 1980). In addition to lignin, dissolution of hemicellulose has also been observed (Jin et al., 2013; Sun et al., 1995). Alkaline pretreatments have also been shown to remove acetyl-substituents from hemicellulose (Kim and Holtzapple, 2006; Selig et al., 2009), which has been reported to improve xylan and cellulose accessibility and hydrolysis by xylanases and cellulases (Grohmann et al., 1989; Kumar and Wyman, 2009; Selig et al., 2009). On the other hand, deacetylation and removal of arabinosyl residues might increase the adsorption of xylan to cellulose (Kabel et al., 2007). Alkaline conditions also cause modification of uronic acid groups in xylan (Teleman et al., 1995). Due to its crystallinity and linearity, cellulose is more resistant to chemical changes than hemicellulose. However, degradation of cellulose also occurs in alkaline conditions by peeling from the reducing ends and alkaline hydrolysis of glycosidic bonds (Theander, 1980). The degree of polymerization of cellulose has been reported to decrease after NaOH and lime treatments (Chundawat et al., 2011b; Kumar et al., 2009; Mittal et al., 2011). Structural changes of lignocellulose are also remarkable after alkaline treat- ments. The crystallinity of lignocellulosic materials has been reported to increase due to removal of less crystalline lignin (Kim and Holtzapple, 2006; Kumar et al., 2009). The ratio of amorphous and crystalline cellulose has been shown to in- crease in lime treatment of corn stover and poplar (Kumar et al., 2009). Swelling and crystal change from cellulose I to cellulose II occurs during NaOH treatments, whereas ammonia treatments cause change from cellulose I to cellulose IIII or IIIII (Mittal et al., 2011; O'Sullivan, 1997). 1.5.3.3 Alkaline oxidative pretreatments Alkaline oxidative conditions are applied in pulp bleaching processes, although they could also be applied for pretreatment or fractionation of biomass. Different kinds of oxidative chemicals such as ozone, peroxides and peracetic acid, or oxy- gen can be used to modify lignocellulose in alkaline conditions. Hydrogen peroxide and peracetic acid are strong oxidative chemicals that have been applied in alka- line conditions at temperatures ranging from room temperature to 150°C (Ayeni et al., 2013; Saha and Cotta, 2007; Zhao et al., 2007). The efficiency of alkaline treatments can be improved in high-lignin materials when pressurized oxygen and catalysts are added (Chang et al., 2001; Hakola et al., 2010). In wet oxidation the lignocellulosic material is treated with water and high pressure oxygen or air (10– 12 bars) at elevated temperatures (120–200°C). The wet oxidation can also be carried out either in acidic or partly alkaline conditions in the presence of oxygen (Bjerre et al., 1996).
- 42. 1. Introduction 40 Similarly to alkaline treatment, alkaline oxidative conditions deacetylate hemi- cellulose and cause delignification, but the effect has been reported to be more intense in oxidative conditions (Kim and Holtzapple, 2006). Lignin is at least partly solubilized and degraded by oxidation reactions (Ayeni et al., 2013; Schmidt and Thomsen, 1998; Zhao et al., 2007). The heating value of lignin is decreased due to the oxidation. Hemicellulose is also partly solubilized and removed, which in- creases cellulose accessibility. Usually the oxidant is not selective and losses of hemicellulose and cellulose can occur. Inhibitors can be formed as soluble aro- matic compounds are produced by degradation of lignin. However, no furfural or hydroxymethylfurfural has been reported to be formed by peroxide or wet oxida- tion in alkaline conditions (Bjerre et al., 1996; Saha and Cotta, 2007). 1.6 Techno-economic considerations for sugar platform biorefineries Polysaccharides in the lignocellulosic raw materials can be hydrolysed to sugars and fermented to fuels and biochemicals in so-called sugar platform biorefineries. These biorefineries require raw materials that are processed by pretreatment, hydrolysis and fermentation before the product recovery. The requirements and the challenges of these processes are diverse. Raw materials should be utilized efficiently in the process, the costs of processing should be low, and products should be produced in high yields. To reach these targets, various economic and technical aspects need to be considered in the development of feasible processes. 1.6.1 Raw materials The availability of a cheap raw material in sufficient amounts is a very important issue for feasible biorefineries. Various raw materials such as wood residues, straws, corn stover, sugar cane bagasse, wood waste and switchgrass are re- garded as potential substrates and are used by the current commercial and demonstration plants for lignocellulosic biofuel production in Europe and the USA (Balan et al., 2013). Table 3 presents the annual production of selected agricultur- al and forest residues that are available for biorefineries. Costs related to raw materials are significant with respect to the process econ- omy of a sugar platform biorefinery. In ethanol production feedstock costs can be up to 40% of the total production costs (Gnansounou and Dauriat, 2010; Hame- linck et al., 2005; Table 4). The prices of feedstocks vary according to the type of feedstock, storage requirements, location, season, local supply-demand condi- tions and transportation needed. For forest residues, the transport costs are con- sidered to dominate the costs (de Wit and Faaij, 2010). The harvest of forest resi- dues within a 100–200 km radius of the end use appears to be economical and costs of 30–86 €/t dry matter including the transportation costs have been reported in Europe (Asikainen et al., 2008). For agricultural residues approximately 100 km radius has been considered economical (Kudakasseril Kurian et al., 2013; Lindh et
- 43. 1. Introduction 41 al., 2009). In Finland the price of straw was estimated to be 52 €/t including the costs of 50 km transportation (von Weymarn, 2007). Table 3. Annual production of lignocellulosic residues. Raw material Area Production (dry Mt/a) References Agricultural residues (corn stover, straw) Europe 217 Kim and Dale, 2004 USA, Canada 103–214 Kim and Dale, 2004; Mabee et al., 2011 World 1370 Kim and Dale, 2004 Sugar cane bagasse Brazil 84 Corrêa do Lago et al., 2012 World 180–210 Gudoshnikov, 2009; Kim and Dale, 2004 Wheat straw Europe 133 Kim and Dale, 2004 World 354–430 Kim and Dale, 2004; Talebnia et al., 2010 Forest residues USA, Canada 72 Mabee et al., 2011 Europe 90 Asikainen et al., 2008 1.6.2 Pretreatment Pretreatment of lignocellulosic biomass is a crucial step in a sugar platform biore- finery to modify the raw material into a hydrolysable form. Pretreatment also has a significant impact on the process economy. The capital costs of pretreatment consist of reactors and equipment required in a pretreatment process and in recy- cling of chemicals. The properties of treatment chemicals and the applied condi- tions affect the materials of construction needed. The proportion of pretreatment in the total capital costs varies between 2 and 27% depending on the pretreatment method (Table 4). Although the differences are significant, the total capital costs of lignocellulosic ethanol processes show only relatively small differences between pretreatments within each study (Eggeman and Elander, 2005; Kazi et al., 2010; Tao et al., 2011). The differences between different studies are more significant. The assumed raw material prices, enzyme costs and other process assumptions are proposed to be the main reason for differences (Kazi et al., 2010).
- 44. Table 4. Techno-economical evaluation of the most promising pretreatment methods. Pretreatment method Development stage a Applicability to different feedstocks Raw material used in economic evaluation Raw material cost (% of MESP) Pretreatment (% of total capital costs) MESP (EUR/l EtOH) b Reference Dilute acid Commercial Yes Corn stover 22 12 0.35 Eggeman and Elander, 2005 Corn stover 32 10 0.76 Kazi et al., 2010 Switchgrass 39 23 0.62 Tao et al., 2011 Corn stover 34 7 0.48 Humbird et al., 2011 Steam explosion Commercial Yes Switchgrass 34 19 0.66 Tao et al., 2011 Sugar cane bagasse nd nd 0.63–1.25 Macrelli et al., 2012 Spruce 34–37 nd 0.47–0.51 Wingren et al., 2008 Hot water Demonstration No Corn stover 24 2 0.44 Eggeman and Elander, 2005 Corn stover 33 2 1.02 Kazi et al., 2010 Switchgrass 39 11 0.74 Tao et al., 2011 Alkaline (Lime) Yes/No Corn stover 40 14 0.43 Eggeman and Elander, 2005 Switchgrass 38 27 0.71 Tao et al., 2011 42 1.Introduction
- 45. Pretreatment method Development stage a Applicability to different feedstocks Raw material used in economic evaluation Raw material cost (% of MESP) Pretreatment (% of total capital costs) MESP (EUR/l EtOH) b Reference Organosolv Demonstration Yes Softwood nd nd 1.58 Hagman et al., 2012 AFEX Laboratory No Corn stover 24 12 0.38 Eggeman and Elander, 2005 Switchgrass 39 16 0.62 Tao et al., 2011 Corn stover 34 8 0.82 Kazi et al., 2010 Aqueous ammonia Commercial No Corn stover 24 23 0.44 Eggeman and Elander, 2005 Switchgrass 38 23 0.92 Tao et al., 2011 a Advanced Ethanol Council, 2013; Balan et al., 2013 b MESP = Minimum ethanol selling price; prices were adjusted to 2013 EUR value using consumer price index data (US Bureau of Labor Statistics, 2013) and 1 EUR=1.33 US$ and 1 EUR = 8.7 SEK. AFEX = ammonia fibre explosion nd = no data 43 1.Introduction
- 46. 1. Introduction 44 Operating costs of pretreatment typically include costs of chemicals that are used in pretreatments and neutralization, and steam and other utilities. Besides the direct costs, pretreatment also impacts on the costs of the other process steps such as hydrolysis and fermentation, waste treatment and product separation. For example, the dry matter content in pretreatment affects the product concentration and separation. Similarly to total capital costs, economic evaluations showed that the obtained minimum ethanol selling prices (MESP) by the pretreatment methods had only relatively small differences within each study, whereas the differences between different economic evaluations are significant (Table 4). It appears that the low costs in pretreatment are counterbalanced by low product yields or in- creased costs in other steps such as hydrolysis and product recovery. Organosolv treatment, which is considered to have relatively high investment costs of reactors and solvent recycling, has not been intensively studied with respect to the econo- my of ethanol production. However, it has been estimated that the total capital cost for ethanol organosolv treatment is 1.4-fold and the MESP 2.6-fold compared to steam explosion, and high additional value from organosolv-lignin and other by- products was required to achieve a similar MESP as for steam explosion (Hagman et al., 2012). The costly pretreatment step is one factor that hinders the commercialization of lignocellulosic ethanol. Integration with other production facilities such as sugar- cane biorefineries, pulp and paper processes or repurposing of closing Kraft mills are potential alternatives to improve energy efficiency and to decrease the high investment costs related to cellulosic ethanol (Corrêa do Lago et al., 2012; Fornell et al., 2012; Phillips et al., 2013). The risk for investment might also decrease as a result of experience obtained with the commercial scale plants that will soon start production. 1.6.3 Process configurations for hydrolysis and fermentation Hydrolysis and fermentation are the key processing steps in a sugar platform biorefinery. There are several process configurations for enzyme production, hy- drolysis and fermentation: separate hydrolysis and fermentation (SHF), simultane- ous saccharification and fermentation (SSF) and consolidated bioprocessing (CBP). In SHF, hydrolysis and fermentation can be carried out in different condi- tions and reactors and there is no need to compromise between different optimal hydrolysis and fermentation conditions. Separate reactors have also been sug- gested to improve process control (Hamelinck et al., 2005). However, the accumu- lation of end products can reduce the efficiency of hydrolysis in SHF. Improved SHF can potentially be obtained by the use of higher temperatures in the hydroly- sis and by selecting -glucosidases with decreased product inhibition (Teugjas and Väljamäe, 2013). In simultaneous saccharification and fermentation the hydrolysis and fermenta- tion occur at the same time in the same reactor, thus decreasing the product inhi- bition of enzymes and reducing investment costs (Drissen et al., 2009; Wingren et
- 47. 1. Introduction 45 al., 2003). The limitation in SSF is that the optimal temperatures of enzymes in the hydrolysis are different from that of fermentation with conventional yeasts. Ther- mostable microorganisms have been suggested to enable operation in higher or varied temperatures, thus improving the hydrolysis and ethanol production in SSF (Kang et al., 2012). A prehydrolysis before SSF can provide a possibility to operate in elevated and optimal conditions in hydrolysis and to decrease viscosity before fermentation (Szijarto et al., 2011; Viikari et al., 2007). Gradual or stepwise addition of substrate i.e. fed-batch processing is also one option to decrease the viscosity, content of inhibitors, and enzyme inhibition (Liu et al., 2010; Zhang et al., 2010). In SHF and SSF, enzymes for the hydrolysis are produced separately, whereas in CBP at least some of the enzymes are produced by the fermenting organism. CBP combines the steps of enzyme production, polysaccharide hydrolysis, and fermentation of sugars into one unit operation performed by one or several organisms. The technology is still under development for production of cellulosic ethanol, but CBP is foreseen to provide significant savings in production costs (Lynd, 2005). Hydrolysis and fermentation at high dry matter is a requirement to achieve high product concentrations without further concentration steps and to decrease reactor volumes. High dry matter hydrolysis produces high concentrations of glucose and cellobiose, which inhibit cellulases (Gruno et al., 2004; Takagi, 1984). On the other hand, high dry matter and the high viscosity can cause problems in mixing and in mass and heat transfer (Pimenova and Hanley, 2003). Hydrolysis at high dry mat- ter content can be increased by efficient mixing (Palmqvist et al., 2011). Different kinds of reactors providing efficient mixing at high dry matter content have been developed (Jorgensen et al., 2007; Zhang et al., 2010). Enzyme costs are still important in the production of lignocellulosic ethanol, alt- hough the costs have been significantly decreased in the past few years. It has been estimated that enzyme costs can be 16% of ethanol production costs in a process using corn stover raw material and dilute acid pretreatment (Humbird et al., 2011). Decreased enzyme costs can be obtained by development of improved production strains producing enzymes more efficiently or with improved properties and composition of enzymes (Gusakov, 2011; Zhang et al., 2006). Cheaper raw materials in enzyme production or on-site production of enzymes are also possible alternatives to decrease enzyme costs (Humbird et al., 2011). Enzyme consump- tion can be decreased by efficient pretreatments, by improved enzymes and en- zyme mixtures working synergistically (see Section 1.3.4), by adding enzyme components such as LPMOs (see Section 1.3.2), by consolidated bioprocessing, or by enzyme recycling. Enzyme recycling has been studied for many years. The main approaches are recycling of enzymes attached to insoluble solids (Ramos et al., 1993; Weiss et al., 2013) or recycling of desorbed enzymes in the liquid phase (Tu et al., 2007; Wu et al., 2010). The main difficulties with respect to recycling are the inactivation and binding of cellulases to lignin (Rahikainen et al., 2011; Tu et al., 2009). En- zyme desorption can be improved by efficient delignifying pretreatments (Lee et al., 1995; Varnai et al., 2011b) and by the use of surfactants (Tu et al., 2007). In addition, it appears that the cellulases without CBM can hydrolyse as efficiently as
- 48. 1. Introduction 46 intact enzymes in high dry matter content, but with less binding to the substrate, thus making recycling possibly easier (Varnai et al., 2013). 1.6.4 Multi-product biorefineries Future biorefineries can be efficient production plants similar to oil refineries that utilize all the raw material components and produce various kinds of products with minimal or no waste. Typically in lignocellulosic biorefineries, sugars are used to produce cellulosic ethanol or other products and the residues are burned to gen- erate steam and electricity. For example in ethanol production the process costs are high (see MESP in Table 4), whereas the ethanol selling price is relatively low. Therefore the process should also produce other products in addition to ethanol. The combined production of biofuels and high-value products in a biorefinery would enhance the economy of biomass processing (Zhang, 2008). To be eco- nomically feasible, multi-product biorefineries should have well developed frac- tionation technology to separate biomass into valuable components or their inter- mediates. As integral parts of a biorefinery, efficient recovery and recycling of chemicals, water recirculation as well as waste water treatment are also needed. In addition to ethanol, various other products such as other alcohols, polymers, lactic acid, and glutamic acid can be derived from cellulose in either biochemical or chemical conversion processes (Menon and Rao, 2012). Hemicellulose can be fermented to ethanol, other alcohols or used for the production of ferulic acid or furfural. Hemicellulose can also be utilized in a polymeric form in different applica- tions. Although lignin is in many cases burned in biorefinery concepts, there are various other options for lignin. For example, lignin-derived compounds can be used in resins, as a substitute for polymeric materials, as a glue in composites, as dispergents, and for the production of syngas, hydrocarbons, formaldehyde, phe- nols, oxidised products or carbon fibre (Menon and Rao, 2012; Zhang, 2008). In addition to biomaterials, chemicals and fuels, biorefineries can produce energy and electricity to the grid. In the near future, the experience obtained with the commercial scale plants will provide essential knowledge about the bottlenecks in the processes and proper- ties of the process streams. As a result of this learning and experience, improved equipment will be developed. Together with cheap and adequate renewable ligno- cellulosic raw materials, improved fractionation technologies, energy and process integration, decreased enzyme costs, and more valuable products, future biorefin- eries can become efficient, profitable and sustainable production facilities.
- 49. 2. Aims of the present study 47 2. Aims of the present study The overall aim of the work was to develop technologies for future biorefineries by improving the enzymatic hydrolysis of various lignocellulosic materials, by devel- oping pretreatment methods, and by optimizing enzyme mixtures for the hydrolysis of pretreated raw materials. More specifically, the aims were: 1. To analyse the suitability of reed canary grass and barley straw – both potential lignocellulosic feedstocks in northern climates – for enzymatic hydrolysis and ethanol production in a lignocellulosic biorefinery. 2. To develop an improved alkaline oxidative pretreatment method that frac- tionates lignocellulose efficiently, is flexible with respect to different raw materials and produces material that can be hydrolysed efficiently by en- zymes. 3. To optimize enzyme mixture composition for the hydrolysis of pretreated materials in order to decrease enzyme dosage in hydrolysis.
- 50. 3. Materials and methods 48 3. Materials and methods A summary of the materials and methods used in the thesis is presented in this Section. Detailed descriptions can be found in the original publications I–IV. 3.1 Lignocellulosic substrates and pretreatments 3.1.1 Preparation of materials Various lignocellulosic raw materials were pretreated by hydrothermal treatment, steam explosion, alkaline oxidation or catalytic oxidation. A summary of the pre- treatment conditions applied for different raw materials is presented in Table 5. More detailed descriptions can be found in publications I–IV.
- 51. Table 5. Studied raw materials and pretreatments. Raw material Pretreatment Pretreatment conditions (Catalyst, temperature, reaction time, other parameters) Publication Reed canary grass (spring or autumn harvest) Steam explosion 2% SO2, 190°C, 5 min I Barley straw Steam explosion 2% SO2, 190–200°C, 5 min I Wheat straw Hydrothermal 3 g/l acetic acid, 190°C, 12 min IV Sugar cane bagasse Steam explosion 0.5% H2SO4, 200°C, 5 min III, IV Alkaline oxidation 5.4 mol Na2CO3/kg substrate (d.m.), 10 bar O2, 120°C, 20 h, reactor fill 10% of total volume III, IV Norway spruce Steam explosion 2% H2SO4, 205°C, 15 min or 2% SO2, 215°C, 5 min III Alkaline oxidation 5.4 mol alkali /kg substrate (d.m.), 10 bar O 2, 120–140°C, 4–20 h, alkali Na2CO3, KOH, NaOH or Ca(OH)2, reactor fill 10%–75%) II, III Catalytic oxidation 5.4 mol Na2CO3 /kg substrate (d.m.), 120–140°C, 1–20 h, reactor fill 10%–75% of total volume II, III Birch Steam explosion 0.5% H2SO4, 200°C, 5 min III Alkaline oxidation 5.4 mol Na2CO3/kg substrate (d.m.), 10 bar O2, 120°C, 20 h, reactor fill 10% of total volume III 49 3.Materialsandmethods
- 52. 3. Materials and methods 50 3.1.2 Analytical procedures for biomasses Several methods were applied to characterize the chemical composition of raw materials and pretreated materials (Table 6). Table 6. Analysis methods to characterize the raw materials, pretreated solids and dissolved material. Analysis Description of the method Publication Characterization of raw materials and pretreated solids Carbohydrate content and composition of monosaccharides Two step sulphuric acid hydrolysis (first step 70%, second step 4% H2SO4) of polysaccharides to monosaccharides followed by high performance anion exchange chromatography (HPAEC) of monosaccharides with pulse amperometric detection (PAD) I–IV Lignin content Gravimetric analysis of the residue remaining after the two step sulphuric acid hydrolysis (Klason lignin) combined to a UV-spectroscopic analysis of acid soluble lignin III Ash content Gravimetric analysis of the residue after burning at 550°C III Extractives Gravimetric analysis of the extract obtained by Soxhlet extraction with heptane and evaporation of the solvent III Characterization of material dissolved in pretreatments Carbohydrate content and composition of monosaccharides Dilute sulphuric acid (4%) or enzymatic hydrolysis of oligosaccharides to monosaccharides followed by HPAEC-PAD analysis I–III Carboxylic acid content and composition Capillary electrophoretic separation of samples and photodiode array UV detection II Lignin content UV spectroscopic detection at 280 nm using an absorptivity of 20 l/g/cm and gravimetric analysis by precipitation of lignin by acidification II Molecular weight distribution Size exclusion chromatography with UV and refractive index (RI) detection II
- 53. 3. Materials and methods 51 3.2 Enzymatic hydrolysis 3.2.1 Enzymes The studied commercial, pre-commercial and purified enzymes are presented in Table 7 and in Table 9. The protein content of the enzyme samples was measured using a Bio-Rad protein assay kit based on the Lowry method (Lowry et al., 1951). Bovine serum albumin was used as a standard. The enzyme activity assays that were used to characterize the enzymes are summarized in Table 8. Table 7. Commercial enzymes used in the hydrolysis of lignocellulosic raw materials. Commercial product name Manufacturer Main activities Protein content (mg/ml) Studied in publication Celluclast 1.5L Novozymes cellulases, 69 FPU/ml 125 I, III Novozym 188 Novozymes -glucosidase, 6360 nkat/ml 181 I, III Cellic Ctec2 Novozymes cellulases, 124 FPU/ml 183 III, IV Cellic Htec Novozymes xylanase, 98300 nkat/ml 36 III, IV Spezyme CP Genencor cellulases, 64 FPU/ml 111 I Econase CE Roal cellulases, 69 FPU/ml 110 I Table 8. Enzyme activity assays to characterize enzymes. Activity method Substrate Reference Total cellulase activity Filter paper Ghose, 1987 -Glucosidase activity p-nitrophenyl- - glucopyranoside Bailey and Linko, 1990 Endoxylanase activity Birch wood xylan Bailey et al., 1992
- 54. Table 9. Pre-commercial and purified enzymes used in the hydrolysis of lignocellulosic raw materials. Enzyme GH family Origin Abbreviation Purification Publication Cellobiohydrolase I (EC 3.2.1.176) Cel7A Trichoderma reesei TrCel7A (CBHI) Suurnäkki et al., 2000 I, IV Cel7A Thermoascus aurantiacus + CBM/TrCel7A TaCel7A (CBHI) Heat treated at 60°C, 2 h/ purified according to IV IV Cel7A Acremonium thermophilum AtCel7A (CBHI) Heat treated at 60°C, 2 h IV Cellobiohydrolase II (EC 3.2.1.91) Cel6A Trichoderma reesei TrCel6A (CBHII) Suurnäkki et al., 2000 I, IV Cel6A Chaetomium thermophilum CtCel6A (CBHII) Heat treated at 60°C, 2 h / purified according to IV IV Endoglucanases (EC 3.2.1.4) Cel7B Trichoderma reesei TrCel7B(EGI) Suurnäkki et al., 2000 I, IV Cel5A Trichoderma reesei TrCel5A (EGII) Suurnäkki et al., 2000 I, IV Cel5A Thermoascus aurantiacus + CBM/CtCel7A TaCel5A (EGII) Heat treated at 60°C, 2 h IV -glucosidase (EC (3.2.1.21) Cel3A Aspergillus niger AnCel3A( G) Sipos et al., 2010 IV Cel3A Thermoascus aurantiacus TaCel3A( G) Heat treated at 60°C, 2 h IV Xylanase (EC 3.2.1.8) Xyn10A Thermoascus aurantiacus TaXyn10 Heat treated at 60°C, 2 h IV Xyn11A Trichoderma reesei TrXyn11(XYLII) Tenkanen et al., 1992 I, IV Xyn11A Nonomurea flexuosa NfXyn11A Heat treated at 60°C, 2 h IV 52 3.Materialsandmethods
- 55. 3. Materials and methods 53 3.2.2 Hydrolysis experiments The hydrolysis experiments were typically carried out at 1% dry matter (d.m.) content, at 45°C, pH 5.0 in a 50 mM sodium acetate buffer for various time peri- ods. For thermostable enzymes higher temperatures, 52–55°C were applied. In addition, hydrolysis at higher d.m. content (8–12%) was studied (Publications I, IV). The cellulase dosage was 2–20 FPU/g d.m (Publications I, III) or 4–15 mg pro- tein/g d.m (Publications III, IV). -Glucosidase dosage was 100–200 nkat/g d.m. Hydrolysis reactions were stopped by boiling samples for 5 min (Publications I, III, IV) or by adding 10 M NaOH (Publication IV). The hydrolysates were diluted to 1% d.m. content (when necessary) and supernatants were collected by centrifugation. The release of soluble sugars in the enzymatic hydrolysis was monitored by analysing the reducing sugars by the DNS-method (Bernfeld, 1955) with glucose as a standard (Publications I, III, IV) or by high performance anion exchange chromatography (HPAEC) to detect the content of monosaccharides and oligo- saccharides in the hydrolysates (Publications I, III). The optimal enzyme mixtures were determined by statistically designed exper- iments using Modde software (Umetrics, Sweden) for hydrothermally pretreated wheat straw, alkaline oxidised bagasse and for steam exploded bagasse. The design of experiments is presented in Appendix 5 (Tables 1–4). After hydrolysis experiments the hydrolysis results were analysed and modelled by Modde as described in publication IV. 3.3 Ethanol fermentation The fermentation of pretreated lignocellulosic substrates was carried out as simul- taneous saccharification and fermentation at 10–12% d.m. content after 6–24 h prehydrolysis in oil lock shake flasks as described in publications I and III. Prehy- drolysis was carried out with a commercial enzyme mixture Celluclast + Novozym 188 (10 FPU/g+100 nkat/g) or Cellic Ctec2+Htec (13.5 mg/g+1.5 mg/g). Prehydro- lysed substrates were inoculated using 3.5 g/l of yeast strains VTT B-03339 or Red Star (LeSaffre). Fermentation was followed by measuring the mass loss due to formation of CO2 and analysing ethanol from the broth by HPLC as described in publications I and III.
- 56. 4. Results and discussion 54 4. Results and discussion 4.1 Potential Nordic raw materials for a sugar platform biorefinery The availability and the costs of raw materials are important factors in the feasibil- ity of sugar platform biorefineries. Switchgrass and residues from the production of corn (e.g. corn stover) belong to the most studied gramineous raw materials for sugar platform biorefineries. In northern climates the growth conditions significant- ly limit the selection of potential raw materials. Cultivation of many grasses is not feasible, or even possible due to climate conditions. On the other hand, a com- mercial scale production plant requires a constant supply of raw materials irre- spectively of the harvesting season. It is probable that several different raw mate- rials are required to meet the needs in different production seasons. Thus, new feedstocks especially suitable for northern climates are required. Reed canary grass (Phalaris arundinacea L.) is a perennial grass species that can be cultivated in different kinds of low value areas such as bogs after peat production, and in fields which are not needed for food production. The annual production can be 7–8 tons dry matter per hectare (Saijonkari-Pahkala, 2001). Reed canary grass has attracted considerable interest as an energy crop for the production of heat and electricity by combustion and as a cellulosic raw material for paper manufacturing (Hadders and Olsson, 1997; Saijonkari-Pahkala, 2001). Reed canary grass has also been proposed as a raw material for the production of ethanol (Belkacemi et al., 1997; Dien et al., 2006; Digman et al., 2010) and biogas (Lakaniemi et al., 2011). The cultivation of reed canary grass has been relatively limited and the cultivated area was approximately 15 000 ha in Finland in 2011 (TIKE, 2012), corresponding to an annual production of 0.1 Mt. Wheat straw and barley straw are important side streams from grain production in Europe and their production, 133 and 44 Mt/a, respectively, is significantly higher than the produc- tion of reed canary grass. Reed canary grass and barley straw were found to have a high carbohydrate content, which is essential for a raw material in a sugar platform biorefinery (Publi- cation I). The carbohydrate contents of the studied reed canary grasses were 56– 64%. Similar values have also been reported in the literature (Table 10; Finell et al.,
- 57. 4. Results and discussion 55 2011), and the value is comparable to the contents of wheat straw (57–59%) and barley straw (56–68%). The glucose and xylose yields of reed canary grass after steam explosion and enzymatic hydrolysis were 82–89% and 87–94%, respective- ly, whereas for barley straw the yields were slightly higher, 90% and 97%. The yields reported in the literature for wheat and barley straws are at a similar level. The fermentation yields from glucose were 82% and 74% for pretreated reed canary grass and barley straw, respectively. These results were similar to those reported in the literature for reed canary grass, barley and wheat straws. Due to the relatively high pentose content, high potential improvement in ethanol concen- tration could be obtained by using a yeast that could ferment pentoses efficiently. In Finland, reed canary grass is typically harvested in the spring for combustion purposes. When harvested in the spring, the water content of the biomass has decreased to a level enabling storage without additional drying. This so called delayed harvesting also supports the density and survival of the grass plantation (Saijonkari-Pahkala, 2001). On the other hand, dry matter losses as well as sugar losses occur during the winter time (Hadders and Olsson, 1997; Landström et al., 1996). The ash content of the biomass decreases, whereas the proportion of stem and fibre content increases during the winter (Pahkala and Pihala, 2000). Consist- ently with this, the cellulose content of reed canary grass harvested in the spring was significantly higher than that of the autumn-harvested crop (Publication I). On the other hand, the autumn-harvested crop contained fructose, which may have originated from water soluble fructan polysaccharides or from sucrose (Dien et al., 2006). The degree of hydrolysis of reed canary grass harvested in the spring was higher than that of the autumn-harvested crop, probably due to more efficient xylan removal in the pretreatment. Ageing in the field during winter might also have brought about chemical and physical changes improving the efficiency of the steam explosion. Results indicated that reed canary grass and barley straw are potential lignocel- lulosic feedstocks for a sugar platform biorefinery. The reed canary grass harvest- ed in the spring had a higher degree of hydrolysis than the autumn-harvested crop. Thus, in addition to the other advantages of the spring harvest, reed canary grass harvested in the spring is a favourable raw material for a sugar platform biorefinery. Other grasses and reeds, such as common reed or surplus forage, would be interesting options for future studies.
- 58. Table 10. Reed canary grass, wheat straw and barley straw as raw materials for sugar platform biorefineries. Raw material Carbohydrate content (% of d.m.) Yield (%) a Ethanol yield (%) b Reference Glucose Xylose Reed canary grass Autumn 56 82 94 nd Publication I Spring 64 89 87 82 Publication I 52–60 nd nd 83 Dien et al., 2006; Digman et al., 2010 Wheat straw 57–59 75 69 93c Østergaard Petersen et al., 2009; Thomsen et al., 2008 Barley straw 68 90 97 74 Publication I 56–60 92 67 82 Linde et al., 2007; Linde et al., 2006 a Yield in pretreatment and hydrolysis from raw material glucose and xylose b Ethanol yield calculated from glucose nd= no data available c high enzyme dosage of 30 FPU/g was applied compared to 10 FPU/g in publication I 56 4.Resultsanddiscussion
- 59. 4. Results and discussion 57 4.2 Development of alkaline oxidation pretreatment Various pretreatment methods have been developed to improve enzymatic hydrolysis of lignocellulosic raw materials. Economic evaluation of the most promising methods has shown that none of them are clearly superior (Table 4). Thus new methods still need to be developed. In this work, a new alkaline oxidation pretreatment was developed to efficiently fractionate lignocellulosic materials and to produce a car- bohydrate fraction with high enzymatic hydrolysability (Publications II, III). 4.2.1 Optimizing the treatment conditions for alkaline oxidation Alkaline oxidation of spruce was studied in various treatment conditions with tem- perature, amount of oxygen, treatment time, presence of catalyst and alkali source as variables (Publications II, III). The effects of the different treatment conditions on the fractionation of lignocellulose and on enzymatic hydrolysability are summa- rized in Table 11. In alkaline oxidation, a clearly lower temperature, 120–140°C was required as compared to steam explosion which was conducted at 200–215°C. On the other hand, the residence time in alkaline oxidation was relatively long, typically 4–20 h, compared to about 5–15 min in steam explosion. The oxidative conditions were provided by using 10 bar oxygen pressure and by a varying degree of reactor fill (10–75%). Alkaline conditions were achieved by addition of sodium carbonate to a concentration of 0.25 mol/l. The most significant difference compared to wet oxida- tion treatment was that the conditions were alkaline throughout the reaction, whereas wet oxidation is carried out with lower alkali dosages and the reactions have been reported to occur at least partly in acidic or neutral conditions (Klinke et al., 2002). However, the alkali dosage was not optimized in this study. Alkaline oxidation (AlkOx) was efficient in the fractionation of spruce, birch and sugar cane bagasse into a cellulose-rich solid fraction and a solubilized fraction (Publication III). In contrast to steam explosion, a state-of-the-art technology that was studied for comparison, alkaline oxidation dissolved lignin and part of the hemicellulose, whereas steam explosion dissolved mainly hemicellulose. Cellulose remained mainly in the solid fraction after treatment by both methods. The chemi- cal reactions in alkaline oxidation are described in more detail in Section 4.2.4.
- 60. 4. Results and discussion 58 Table 11. Effect of different parameters in the alkaline oxidation treatment of spruce on dissolution of lignin and hemicellulose and on enzymatic hydrolysability. (+) increasing, (-) decreasing, and (+/-) no significant effect. nd = no data. Change in conditions Dissolution of lignin and hemicellulose Formation of organic acids Enzymatic hydrolysability Reference Increased tempera- ture (from 120°C to 140°C) + nd + Publication III, unpublished data Increased degree of reactor fill (from 10% to 50%) - - +/- Publication II, unpublished data Increased tempera- ture with decreased reactor fill + + + Publication II, unpublished data No oxygen (Argon pressure) - nd - Publication III Decreased particle size + nd + Unpublished data Decreased time from 20 h to 4–5 h with chips - nd - Publication II, unpublished data Copper- phenanthroline catalyst + + + Publications II, III Alkali source (Na2CO3, NaOH, KOH, Ca(OH)2) +/- +/- +/- Publications II, III The cellulose in the alkaline oxidised materials was very easily hydrolysed to glu- cose: in pretreatment and hydrolysis by a commercial enzyme mixture (Celluclast 1.5L + Novozym188), 84%, 91% and 97% yield was obtained within 72 hours from spruce, birch and sugar cane bagasse, respectively, using an enzyme dosage of 10 FPU/g d.m.+100 nkat/g. After steam explosion (SE) the corresponding values were clearly lower for spruce (52%) and bagasse (78%), whereas with birch the yields were similar (93%). The applied alkaline oxidation conditions were similar for spruce, bagasse and birch. Thus, alkaline oxidation showed flexibility with respect to raw material. However, the conditions for sugar cane bagasse and birch were not optimized and the optimum might be milder due to the lower lignin con- tent compared to spruce.
- 61. 4. Results and discussion 59 4.2.2 Hydrolysis and fermentation of alkaline oxidised materials The alkaline oxidised fibre fraction could be efficiently fermented to ethanol after a short prehydrolysis with commercial enzymes (Figure 4; Publication III). Alkaline oxidised sugar cane bagasse and spruce were fermented efficiently in 12% d.m. content using a commercial yeast (RedStar, LeSaffre) able to utilize only C6 sug- ars. The ethanol yield from hexoses in the pretreated fibre fraction was 80% of the theoretical yield in 1–3 days fermentation. On the other hand, ethanol production from steam exploded and prehydrolysed bagasse and spruce in the corresponding conditions was slow and 39% and 32% ethanol yields from hexoses were obtained respectively after 6 days SSF. The highest ethanol concentration, 49 g/l, was obtained with alkaline oxidised spruce at 12% d.m. content of fibre. The ethanol concentration obtained from alkaline oxidised bagasse was clearly lower, 38 g/l, due to the lower content of hexoses in the substrate. The enzymatic hydrolysis limited significantly the fermentation of steam exploded materials. The most prob- able reason for this was non-productive adsorption and decreased accessibility of enzymes caused by residual lignin. Steam explosion has been shown to increase enzyme adsorption on lignin (Rahikainen et al., 2013). By contrast, hydrolysis of alkaline oxidised materials evidently did not limit the fermentation to the same extent as the hydrolysis of steam exploded material, and ethanol production from hexoses was efficient. Figure 4. SSF of alkaline oxidised (AlkOx) spruce and sugar cane bagasse and steam exploded (SE) spruce and bagasse at 12% d.m. content at 35°C. 6 hours prehydrolysis was carried out with Cellic Ctec2+Htec enzyme mixture (enzyme ratio was 90:10; total dosage 15mg/g) at 50°C, pH 5 before inoculation with a commercial yeast, Red Star. Ethanol yields were calculated from the measured mass loss during fermentation. 0 20 40 60 80 100 0 1 2 3 4 5 6 7 Ethanol(%oftheoretical) Time (d) AlkOx spruce AlkOx bagasse SE spruce SE bagasse
- 62. 4. Results and discussion 60 4.2.3 Effect of particle size in alkaline oxidation pretreatment Mechanical pretreatments such as chipping, cutting and grinding are often applied before chemical pretreatments to improve the mass transfer and pretreatment efficiency. Due to the high energy consumption of mechanical pretreatments, excessive particle size reduction should be avoided. When the efficiency of 4 hours alkaline oxidation was studied in the fractionation of spruce chips either with or without a catalyst, the treatment yielded a non-homogeneous material consist- ing of dark brown spruce chips in a matrix of yellow pulp (Publication II). Fractiona- tion and dissolution of lignin was less efficient than with sawdust. Therefore, it appeared that the mass transfer of alkali and oxygen in these larger particles was limited to the surfaces and the smallest particles. Particle size reduction was studied as a method to improve the efficiency of short alkaline oxidations (Figure 5, unpublished results). Spruce chips were ground coarsely in a hammer mill or powdered in a hammer mill followed by circu- lation (5 x) in a sieve plate press. After mechanical pretreatments, materials were alkaline oxidised at 120°C as described in publications II and III but 5 hours oxida- tion time was applied instead of 20 h oxidation. In addition, alkaline oxidation of sawdust was carried out for comparison. Figure 5. Effect of mechanical pretreatments and treatment temperature on alka- line oxidation of spruce. Carbohydrate composition of spruce raw material without alkaline oxidation (no treatment) and washed fibre fractions after alkaline oxidation at 120°C (or 140°C), for 5 hours. Unpublished results. Particle size reduction had a clear effect on delignification in alkaline oxidation, and on carbohydrate content and composition of the produced fibre fraction (Figure 5). Compared to the reference treatment, spruce chips processed at 120°C, all the treatments including reduced particle size produced material having higher cellulose content and evidently lower lignin content. Part of the hemicellu- lose, both galactoglucomannan and arabinoglucuronoxylan, was solubilised and 0 20 40 60 80 100 No treatment Chips, 120°C Hammer milling (coarse), 120 °C Sawdust, 120 °C Sieve plate grinding (fine), 120°C Chips, 140°C Content(%ofd.m.) Arabinose Xylose Galactose Mannose Glucose
- 63. 4. Results and discussion 61 the proportion of hemicellulose to total carbohydrate content in the treated solids decreased with decreasing particle size. Interestingly, the content of hemicellulose in the fibre fraction remained almost constant regardless of particle size. Powder- ing of wood by sieve plate grinding increased delignification and solubilisation of hemicellulose in alkaline oxidation, whereas with the other particle size reduction methods the impact was less pronounced. Figure 6. Effect of mechanical pretreatments and oxidation temperature on enzy- matic hydrolysis of washed fibre fraction after alkaline oxidation at 120°C (or 140°C), for 5 hours. Hydrolysis in test tubes at 45°C, 1% d.m. content with en- zymes Celluclast 1.5L (10 FPU/g d.m.) and Novozym 188 (100 nkat/g d.m.). Hy- drolysis products were analysed as reducing sugars. Unpublished results. Particle size reduction before alkaline oxidation also improved the enzymatic hy- drolysis of the obtained solid fraction (Figure 6). The degree of hydrolysis was increased by over 25% compared to the degree of hydrolysis obtained with spruce chips when using a treatment temperature of 120°C. The highest hydrolysis yield was obtained with the smallest particle size, i.e. after sieve plate grinding. A slight- ly lower degree of hydrolysis was obtained with hammer milled spruce and saw- dust with a particle size of approximately 2–5 mm. Consistently with this, grinding to a particle size below 1 mm was reported to give no additional improvement in lime treatment of corn stover (Chang et al., 1997). Although hammer milled spruce and sawdust had about the same particle size, delignification was slightly more efficient with sawdust, probably due to different shapes and surface areas of the particles obtained by the two methods. However, no significant difference was found in enzymatic hydrolysability. Improved accessibility of oxygen and alkali to the target compounds in alkaline oxidation treatment, and more homogeneous processability are the most probable reasons for increased pretreatment efficiency with reduced particle sizes. 0 20 40 60 80 100 0 24 48 72 Hydrolysisproducts(%oftheoretical) Hydrolysis time (h) Sieve plate grinding (fine), 120 °C Sawdust, 120 °C Chips, 140 °C Hammer milling (coarse), 120 °C Chips, 120 °C
- 64. 4. Results and discussion 62 4.2.4 Chemical reactions and modification of biomass in alkaline oxidation Alkaline oxidation combines the effects of alkaline and oxidative treatments, and several reactions take place simultaneously during the treatment. Hemicellulose and lignin are dissolved, oxidised and partially degraded, and carboxylic acids are formed. The effect of alkaline oxidation on the main chemical components was followed with spruce (Publication II). Polysaccharides can degrade in alkaline oxidative conditions through peeling reactions from chain ends and by random chain cleavages (Theander, 1980). In alkaline oxidation, either with or without a catalyst, cellulose remained mostly in the solid fraction throughout the 20-hour oxidation. Although cellulose is more resistant to degradation than hemicellulose, glucan losses did occur, ranging from 0 to 16% of raw material glucose (Publication II, Table 2). Taking into account the glucose present in galactoglucomannan, up to 11% cellulose losses occurred. Swelling, crystal structure change from cellulose I to cellulose II, decrease in the crystallinity of cellulose and improved hydrolysis have been observed as a result of alkaline pretreatments (Mittal et al., 2011). These changes can also contribute to high enzymatic hydrolysability of alkaline oxidised materials. On the other hand, extensive oxidation of reducing ends in cellulose has been reported to decrease significantly the susceptibility of the substrate to CBH activity (Xu et al., 2009a). Steam explosion resulted in high cellulose yield in pretreatment, but significantly lower enzymatic hydrolysability (Publication III, Tengborg et al., 1998). Hemicellulose was partly solubilized during alkaline oxidation of spruce. Signifi- cant dissolution of hemicellulose, both galactoglucomannan and arabinoxylan, was observed already after one hour of treatment (Publication II, Figure 5). Ap- proximately half of the hemicellulose was dissolved in the first four hours. Size exclusion chromatography (SEC) with RI detection showed that polysaccharides were mainly dissolved as small fragments of oligosaccharides with Mw ranging between 600 and 1600 (Publication II, Figure 3). In addition to solubilisation of hemicellulose, removal of acetyl substitutions has been reported with alkaline pretreatments; both of these effects can significantly enhance the enzymatic hy- drolysis (Selig et al., 2009). The carbohydrate yields after fractionation in various conditions showed that significant hemicellulose losses occurred during alkaline oxidation (Publication II, Table 2). Especially the yield of galactoglucomannan was low, 40–72% and 23–79% for mannose and galactose, respectively. The xylose yield varied between 66% and 92%. A larger proportion of galactoglucomannan than that of xylan was solubilized in alkaline oxidation of spruce, which probably made mannans more susceptible to oxidation. Oligosaccharides and monosac- charides can be oxidised to organic acids or even CO2 and water in alkaline oxida- tion, whereas no furfural is produced in alkaline conditions (Klinke et al., 2002). Steam explosion was also efficient in the dissolution of hemicellulose (Publication III). However, in acidic conditions degradation of pentoses and formation of fur- furals can occur, especially with increasing severity (Stenberg et al., 1998).
- 65. 4. Results and discussion 63 During alkaline oxidation of spruce, oxidation, degradation and dissolution of lignin was observed. The lignin content of the dissolved fraction ranged from 6– 20% of raw material d.m. after oxidations. In catalysed alkaline oxidation, one third of lignin in the raw material was dissolved already in one hour reaction time, and two thirds after four hours, but after that only minor changes occurred in the lignin content (Publication II, Table 4). Oxidative conditions have been shown to in- crease dissolution of lignin in lime treatments (Chang et al., 2001). The catalyst in alkaline oxidation, copper-phenanthroline complex, increased the degradation of lignin: precipitation yields of the dissolved lignin were higher for alkaline oxidation without catalyst than in the presence of catalyst, indicating increased degradation of lignin by the catalyst. Size exclusion chromatography with UV detection con- firmed that the solubilized lignin had a lower molecular weight distribution and was thus more degraded after 20 h alkaline oxidation in the presence of catalyst than without catalyst (Publication II, Table 3). Removal of lignin has been suggested to increase the access of enzymes to the remaining polysaccharides and to de- crease the non-productive binding of cellulases (Kumar et al., 2012). The degradation products, organic acids, were analysed from the dissolved fraction. After alkaline oxidation, the total content of organic acids varied from 10% to 22% of raw material d.m. Formic and acetic acids were the dominating organic acids. A similar composition of degradation products has been observed in wet oxidation of wheat straw (Bjerre et al., 1996). Organic acids can be formed in alkaline oxidative conditions by hydrolysis of acetyl groups from hemicellulose, and through oxidation of carbohydrates and lignin (Rovio et al., 2011; Sjöström, 1993). Almost 50% of the maximum content of acetic acid and formic acid was generated during the first hour of catalytic oxidation and the rest was formed dur- ing the following 3–7 hours. The formation of the other main carboxylic acids, including glycolic, oxalic, and 2,5-dihydroxy pentanoic acid, also occurred mainly during the first 4 hours. 4.2.5 Process conditions in alkaline oxidation The lignocellulosic raw materials were fractionated by alkaline oxidation. The compositions of different fractions obtained by alkaline oxidation of spruce at 120– 140°C for 4–20 h are presented in Table 12. High water and chemical consumption increases the costs of the pretreatment. Alkaline oxidations were carried out at relatively low d.m. contents of 5% using a liquid/wood ratio of 19–20. A significant amount of alkali, 0.5 g/g (Na2CO3/wood d.m.) was applied in alkaline oxidation. In addition to sodium carbonate, several other alkalis, Ca(OH)2, NaOH or KOH, were also shown to be possible alternatives (Publications II, III). The process should be optimized to minimize the amount of alkali required in the treatment. Efficient recovery and recycling of alkali are also needed to minimize the costs of chemicals in commercial processes.
- 66. 4. Results and discussion 64 Table 12. Raw materials, chemicals and the composition of product streams in alkaline oxidation of spruce (Publications II, III). Component Content (% of raw material d.m.) Raw materials Spruce 100 Water 1900–2000 Oxygen 8–12 Na2CO3 53 CuSO4 (in catalytic oxidation) 0.45 Phenanthroline (in catalytic oxidation) 0.59 Solid fraction Hexoses 40.5–56.8 Pentoses 2.0–5.9 Lignin (insoluble) 10.4 Ash 4.5 Extractives (heptane extraction) 0.5 Dissolved fraction Hexoses 0.4–3.2 Pentoses 0.5–3.1 Lignin (dissolved) 10.0–19.5 Organic acids 9.9–21.7 The function of a catalyst is to increase the rate of the reaction. Consistently with this, catalytically assisted alkaline oxidation by a copper-phenanthroline complex dissolved more carbohydrates than oxidation without a catalyst, especially those originating from galactoglucomannan and arabinoxylan (Publication II). Increased degradation of lignin was also observed. Thus, catalysed alkaline oxidation might enable shorter reaction time than the alkaline oxidation without the catalyst. On the other hand, the use of a catalyst increases chemical costs, and the used catalyst components, phenanthroline and CuSO4, are toxic, which emphasizes the need for recycling. The recycling would need additional equipment and process development. The role of oxygen is essential in alkaline oxidation. Oxygen pressure of 10 bars was applied and oxygen consumption, based on the measured pressure drop in alkaline oxidation, was 80–120 g O2/kg wood. The alkaline treatment using argon gas pressure instead of oxygen revealed that oxidative conditions enhanced radically both solubilisation in the pretreatment and enzymatic hydrolysability of the produced solid fraction (Publication III). Only 9% glucose yield (% of glucose in the fibre) in the enzymatic hydrolysis of the fibre fraction was obtained from pre- treatment of spruce in argon pressure compared to over 90% yield with material pretreated in oxygen pressure. In lime pretreatment, oxygen addition has been shown to increase the pretreatment efficiency with materials having high lignin content, such as poplar and softwood (Chang et al., 2001). Oxidation of lignin is the most probable reason for improved dissolution and delignification. The effect of the oxygen concentration on pretreatment efficiency was studied by filling the reactor to different levels of substrate-liquid suspension. The reactor fill i.e. liquid/gas ratio had a clear impact on the solubilisation and oxidation during the alkaline oxidation in the presence or absence of the catalyst. More carbohy-
- 67. 4. Results and discussion 65 drates and lignin were dissolved in 4 and 20 hour oxidations with a lower reactor fill (Publication II, Figure 1). These effects were also reflected as improved enzy- matic hydrolysis. The formation of acids was also significantly higher when a lower reactor fill (10% and 25% of total volume) was used compared to using higher reactor fill (50% and 75%) (Publication II, Figure 4). In addition to higher oxygen concentration, the lower reactor fill might have improved mixing of the suspension, decreased concentration gradients, and increased the solubilisation of oxygen throughout the reaction. From a process economy point of view, the lower reactor fill would mean larger reactors and subsequently increased capital costs. To scale- up the alkaline oxidation method, development of special reactors enabling effi- cient mixing and oxygen transfer would be required. Residence time of the process should be as short as possible in order to minimize process and investment costs. Alkaline oxidation could be shortened from 20 h to 5 h by particle size reduction. Alternatively, the higher temperature (140°C) in alkaline oxidation enhanced the efficiency of the pretreatment (Figure 5, Figure 6). However, both grinding and higher reaction temperature would increase the energy consumption. Preliminary techno-economic evaluation of process concept using alkaline oxidation pretreatment has been carried out outside this study (Biorefine programme report, 2012). The feasibility study indicated that the key cost elements for the concept were raw material costs and the capital costs related to alkaline oxidation pretreatment and energy production. Significant savings in capital costs of alkaline oxidation could be obtained by repurposing kraft mills and utilizing existing infrastructure. 4.3 Improved enzymatic hydrolysis by efficient pretreatment and optimization of enzyme mixtures Despite intensive development of cellulolytic and hemicellulolytic enzymes, enzyme cost is still an important factor in processes hydrolysing lignocellulose to monosaccha- rides. The expenses can be decreased by lowering the enzyme price or by reducing their consumption. The use of enzymes can be reduced by several ways as described in Section 1.6.3. In this study, two ways to decrease enzyme dosages, namely efficient pretreatments and optimized enzyme compositions, were studied. 4.3.1 Decrease in enzyme dosages by alkaline oxidation pretreatment Lignocellulosic raw materials are modified structurally and chemically by pretret- ment. An efficient pretreatment produces material with good enzymatic hydrolysa- bility and thus also enables decreased enzyme dosages or shorter hydrolysis time and affects process costs and feasibility. Enzymatic hydrolysis with lower enzyme dosages was studied using alkaline oxidised and steam exploded materials (Publication III). Cellulase dosage was varied, whereas the -glucosidase dosage was kept constant at 100 nkat/g d.m. Enzymatic hydrolysability of sugar cane bagasse and birch was good with both pretreatments using a dosage of 10 FPU/g d.m., which is a typical cellulase dos-
- 68. 4. Results and discussion 66 age used for hydrolysis of various materials. The hydrolysis yield was already 100% in 24 h hydrolysis of pretreated sugar cane bagasse, whereas 80–100% yield was reached with pretreated birch. The hydrolysis rate and the degree of hydrolysis of alkaline oxidised bagasse decreased with decreased enzyme dosag- es. Hydrolysis yields of 90% and 70% were obtained in 72 h with enzyme dosages of 4 and 2 FPU/g, respectively (Figure 7). On the other hand, the hydrolysis of steam exploded bagasse using the lower enzyme dosages stopped almost com- pletely after 4 hours and even lower hydrolysis yields of 70% and 40% were ob- tained with enzyme dosages of 4 FPU/g and 2 FPU/g, respectively. Lignin has been shown to decrease the accessibility of enzymes to polysaccharides and to cause unproductive binding of cellulases (Kumar et al., 2012; Palonen et al., 2004; Rahikainen et al., 2013; Várnai et al., 2010). Easily hydrolysable material, oligo- saccharides or small particles with high surface area probably increased the hy- drolysis rate of steam exploded bagasse in the first 4 hours of hydrolysis. High surface area has been shown to increase overall protein adsorption and to give a higher initial rate of hydrolysis (Piccolo et al., 2010). With steam exploded and alkaline oxidised birch, the hydrolysis levels were similar with similar dosage of enzymes for 24 hours but after that the hydrolysis rate of steam exploded birch decreased more steeply. With steam exploded spruce, enzymatic hydrolysability was low and was further decreased with lower enzyme dosages. By contrast, alkaline oxidised spruce had high enzymatic hydrolysability and the degree of hydrolysis was clearly decreased only when the enzyme dosage was lowered to 2 FPU/g. Although the pretreatment conditions used (205°C, 15 min) were not optimal for spruce, steam exploded spruce has often been found to be a challenging material to hydrolyse. It has been reported that the enzymatic hydrolysis of steam pretreated softwood can be limited even with enzyme dosages of 10–15 FPU/g without delignifying post-treatments (Kumar et al., 2010; Kumar et al., 2011; Tengborg et al., 1998; Várnai et al., 2010). The decreased hydrolysis of steam exploded spruce can be due to de- creased accessibility of enzymes to their substrates due to the high lignin content, or to unproductive binding of cellulases. The lignin in steam exploded spruce has been observed to be especially inhibitory to enzymes (Rahikainen et al., 2013). Denaturation of enzymes in prolonged contact with lignin at hydrolysis tempera- ture has also been observed (Rahikainen et al., 2011). Obviously, with steam exploded spruce having a high lignin content, these effects were more profound than with alkaline oxidised spruce. The results clearly showed that alkaline oxida- tion pretreatment enabled the use of lower enzyme dosages or shorter hydrolysis time compared to steam exploded materials. Further decrease in enzyme costs can be obtained by enzyme recycling. Cellu- lases have been shown to adsorb to substrate in the beginning of hydrolysis (Boussaid and Saddler, 1999; Varnai et al., 2011b). With materials having high lignin content, cellulases remain bound, which limits the recycling of enzymes. On the other hand, after delignifying pretreatments desorption of enzymes has been ob- served (Boussaid and Saddler, 1999; Varnai et al., 2011b). Increased desorption of enzymes in alkaline oxidised materials could make recycling of enzymes possible.
- 69. 4. Results and discussion 67 Figure 7. Effect of enzyme dosage and pretreatment type on enzymatic hydrolys- ability of steam exploded (SE) and alkaline oxidised (AlkOx) bagasse (A), birch (B), and spruce (C). Enzymatic hydrolysis at 1% d.m. consistency, 45°C, pH5, with Celluclast1.5L(2–10 FPU/g) and Novozym 188 (100 nkat/g). (Publication III) 0 20 40 60 80 100 0 24 48 72 Reducingsugars (%oftotalcarbohydrates) Hydrolysis time (h) BagasseA 0 20 40 60 80 100 0 24 48 72 Reducingsugars (%oftotalcarbohydrates) Hydrolysis time (h) BirchB 0 20 40 60 80 100 0 24 48 72 Reducingsugars (%oftotalcarbohydrates) Hydrolysis time (h) Spruce AlkOx 10 FPU/g AlkOx 4 FPU/g AlkOx 2 FPU/g SE 10 FPU/g SE 4 FPU/g SE 2 FPU/g C
- 70. 4. Results and discussion 68 4.3.2 Enzyme mixture optimization Optimal enzyme composition can provide maximal synergistic effect and thus an improved degree and rate of hydrolysis can be obtained with a minimal enzyme loading. Therefore the use of optimal enzyme compositions for a particular pre- treated raw material can minimise enzyme dosages and the costs of enzyme treatment. There are several options in designing optimal enzyme compositions for the hydrolysis of lignocellulosic raw material. In this thesis, supplementation of a commercial enzyme mixture with the rate limiting enzyme activities (Publication I), and development of optimized enzyme mixtures from monocomponent enzymes (Publication IV) were studied in the hydrolysis of pretreated materials. 4.3.2.1 Supplementation of commercial enzyme mixtures -Glucosidase activity is essential in the total hydrolysis of lignocellulosic sub- strates, although conventional Trichoderma reesei mixtures are known to contain insufficient -glucosidase activity. Supplementation of commercial T. reesei en- zyme mixtures with a commercial -glucosidase (Novozym 188) was studied in the hydrolysis of pretreated reed canary grass and barley straw. Hydrolysis by all three different conventional commercial cellulases showed clear improvement by the addition of commercial -glucosidase (Publication I, Table 3). In addition to improved production of glucose, the yield of xylose was also slightly enhanced by additional Novozym 188 due to some minor hemicellulase activities present in the commercial enzyme. It has been observed that the increased hydrolysis of cellu- lose synergistically enhances the hydrolysis of xylan in the complex lignocellulose material (Varnai et al., 2011a). The hydrolysis of side groups of xylan and mixed xylo-oligomers might also have been improved by the side activities in the - glucosidase preparation. Arabinose analysis of the hydrolysates supported this hypothesis, as detectable concentrations of arabinose were obtained only when - glucosidase was applied. Synergy between xylanases and cellulases in the hy- drolysis of pretreated raw materials has frequently been observed (Hu et al., 2011; Kumar and Wyman, 2009). The possibility to further enhance the hydrolytic performance of a commercial T. reesei cellulase mixture (Econase CE+Novozym 188) by the addition of poten- tial rate-limiting enzymes was studied in the hydrolysis of steam exploded reed canary grass by overdosing the mixture with the major cellulases and xylanase (Publication I). It has been reported that the optimal enzyme composition for total hydrolysis can be significantly different from that produced by T. reesei (Rosgaard et al., 2007; Zhou et al., 2009). The addition of Cel6A (CBHII) at a level of 5 mg/g cellulose improved the hydrolysis of the washed solid fraction of steam exploded reed canary grass most significantly, by 13% after 48 hours hydrolysis (Figure 8). This increase was observed in all time points, although standard deviations were relatively high in some points. The hydrolysis yield was also enhanced by the addition of Cel7B (EGI). In addition to endoglucanase activity, Tr Cel7B is also reported to have strong hemicellulolytic side activities (Bailey et al., 1993; Vlasen-
- 71. 4. Results and discussion 69 ko et al., 2010). Consistently with this, a high proportion of Cel6A and Cel7B has been observed in optimized mixtures for the hydrolysis of steam pretreated barley straw and corn stover (Rosgaard et al., 2007; Zhou et al., 2009). Supplementation with Cel7A (CBHI) and xylanase Xyn11A had a lower effect in 48 h hydrolysis, and supplementation with Cel5A (EGII) had practically no effect in the hydrolysis. Thus the amount of Cel5A was not considered to limit the rate or degree of hydrolysis of steam exploded reed canary grass. Figure 8. Effect of supplementation of a commercial enzyme mixture (Econase CE +Novozym 188, E+N) with purified T. reesei enzymes Cel7A, Cel6A, Cel7B, Cel5A, or Xyn11A in the hydrolysis of washed solid fraction of steam exploded reed canary grass (spring harvest). Enzyme dosages were for Econase CE 10 FPU/g cellulose, for Novozym 188 100 nkat/g cellulose, and for the purified enzymes 5 mg/g cellulose. Hydrolysis conditions were 1% cellulose content, 45°C, pH 5. Hydrolysis products were analysed as reducing sugars (Publication I). 4.3.2.2 Optimization of enzyme mixtures from monocomponent enzymes for the hydrolysis of pretreated raw materials Optimal enzyme mixtures were developed for the hydrolysis of alkaline oxidised and steam exploded sugar cane bagasse and hydrothermally pretreated wheat straw by statistically designed experiments (Publication IV). Most of the optimiza- tions of enzyme mixtures studied in the literature have been carried out using cellulases from Trichoderma sp. In this work, optimal enzyme mixtures were de- termined by using thermostable enzymes Cel7A from Acremonium thermophilum, Cel6A from Chaetomium thermophilum, Cel5A, Cel3A and Xyn10A from Thermo- ascus aurantiacus and the optimal mixture was compared to the best mixture obtained using Trichoderma reesei enzymes. The enzyme mixture compositions and the hydrolysis yields are presented in Table 13. In addition, the results of selected optimization studies in the literature are presented for comparison.
- 72. 4. Results and discussion 70 For the different pretreated raw materials and hydrolysis times the optimal en- zyme mixtures of thermostable enzymes contained 49–73% Cel7A (CBHI) of total protein, 10–30% Cel6A (CBHII), 6–16% Cel5A (EGII), 1–12% xylanase (Xyn10A), and 2–5% Cel3A ( G). The optimal mixture of T. reesei enzymes for the hydrolysis of pretreated wheat straw contained 39–42% Cel7A (CBHI), 32–34% Cel6A (CBHII), 20–26% Cel7B (EGI), 1–2% Cel5A (EGII), 1% xylanase (Xyn11A), and 1–2% Cel3A ( G). The results of the statistically designed experiments were mathematically modelled by the Modde program to describe the effect of each component on the hydrolysis efficiency. Contour plots of the models showed broad optimal areas (Publication IV, Figures 3 and 4) for the enzyme mixtures. Thus, relatively high variations in the proportions of enzyme mixture components were possible without significant impact on the degree of hydrolysis. The optimal enzyme mixtures were dominated by cellobiohydrolases, the pro- portion being 70–88% of total enzyme dosage in all the tested enzyme mixtures. This is significantly higher than the proportion analysed from the commercial en- zyme mixture Celluclast and higher than that naturally produced by T. reesei (Shoemaker et al., 1983; Sipos et al., 2010). High proportions of cellobiohydrolas- es have also been recommended in various other optimization studies carried out on steam exploded raw materials (Billard et al., 2012; Rosgaard et al., 2007; Zhou et al., 2009). The main component in all the enzyme mixtures was cellobiohydro- lase Cel7A. With thermostable enzymes the proportion of Cel7A was clearly high- er than that of Cel6A, whereas in T. reesei mixtures the role of Cel6A seemed to be more significant. It is possible that the ratio of Cel7A and Cel6A is not as im- portant as the total content of cellobiohydrolases (Cel7A+Cel6A), as the optimal ranges for cellobiohydrolases were relatively broad (Publication IV). The proportion of endoglucanases and xylanase showed more variation in the optimized mixtures. It appeared that hydrolysis of hydrothermally pretreated wheat straw and alkaline oxidised bagasse required a significant amount of xylanolytic activity, which was provided by either xylanase or by endoglucanase Cel7B. In T. reesei enzyme mixture Cel7B appeared to be able to replace both Cel5A and xylanase almost completely, whereas significantly higher proportions, 13–16% and 8–12% of Cel5A and xylanase, respectively, were present in the optimized ther- mostable enzyme mixture.
- 73. Table 13. Optimal enzyme mixtures of various pretreated lignocellulosic substrates (Publication IV). Raw material Pretreatment Content (% of total protein dosage) Hydrolysis yield (%) Ref. Cel7A (CBHI) Cel6A (CBHII) Cel7B (EGI) Cel5A (EGII) Xyn10A or Xyn11A Cel3A G) Other Sugar cane bagasse SE 57–73 10–24 - 9–11 1–4 3–5 - 84–98 a Sugar cane bagasse AlkOx 49–58 14–30 - 5–13 12 3 - 67–88 a Wheat straw hydrothermal 52–56 17–19 - 13–16 8–12 2–3 - 51–74 a Wheat straw hydrothermal 39–42 32–34 20–26 1–2 1 1–2 - 37–56 b Wheat straw SE 40 27 15 6 12 * 0 68 c Barley straw SE 27 47 27 0 - * * 56 d Barley straw Hot water 20 43 37 0 - * * 56 d Corn stover SE 27 35 21 - - 6 11 36 e Corn stover AFEX 29 19 35 - 14 * 3 51 f Corn stover AFEX 35 4 26 - 19 12 4 44 g Corn stover Alkaline 49 4 34 - 4 5 4 41 g Corn stover Alk. peroxide 43 4 30 - 11 8 4 58 g AlkOx = alkaline oxidation, SE = steam explosion, AFEX = ammonia fibre explosion * were present in the hydrolysis but were not in variables of optimization a 24–72 h hydrolysis with thermostable enzymes (this study). b 24–72 h hydrolysis with Trichoderma reesei and Aspergillus niger enzymes (this study). c 48 h hydrolysis with T. reesei and A. niger enzymes (Billard et al., 2012). d 24 h hydrolysis with T. reesei cellulases and Novozym 188 (Rosgaard et al., 2007). e 72 h hydrolysis with T. viride cellulases (Zhou et al., 2009) f 24 h hydrolysis with T. reesei, A. nidulans and A. niger enzymes (Gao et al., 2010a). g 48 h hydrolysis with T. reesei, T. longibrachiatum and A. niger enzymes (Banerjee et al., 2010b). 71 4.Resultsanddiscussion
- 74. 4. Results and discussion 72 On the basis of the studies carried out using thermostable enzymes some conclu- sions could be drawn about the effect of pretreatment and raw material on enzyme mixture composition. Firstly, more Acremonium thermophilum Cel7A (CBHI) was needed in the optimal mixture for steam exploded bagasse than for alkaline oxi- dised bagasse. Secondly, the hydrolysis of alkaline oxidised bagasse required at least three times more xylanase than the hydrolysis of steam exploded bagasse. The difference can be explained by the chemical composition of the materials. Bagasse pretreated by alkaline oxidation had a very high xylan content, 24%, whereas the xylan content of steam exploded bagasse was only 3%. Banerjee et al. (2010b) observed that more xylanase (Tr Xyn10) was needed after AFEX than after NaOH or alkaline peroxide pretreatments. Although no composition of pre- treated raw materials was presented in the study of Banerjee et al. , it is probable that AFEX treated material had a higher lignin content than NaOH and alkaline peroxide treated materials and also potentially a higher hemicellulose content. Therefore it appears that substrates with either high hemicellulose or high lignin content require more xylanase to increase the accessibility of substrates to cellu- lases. Thirdly, comparison of two raw materials, wheat straw and bagasse, pre- treated with relatively similar methods showed that more At Cel7A and a lower proportion of xylanase were needed with steam exploded bagasse than with hy- drothermally pretreated wheat straw, although the carbohydrate compositions were similar. The composition of the optimal enzyme mixture might be affected by the higher enzyme dosage and degree of hydrolysis with steam exploded bagasse than with wheat straw. Wheat straw substrate was used as dried sheet discs, whereas the more homogeneous steam exploded bagasse was pipetted as a never-dried slurry. Drying has been shown to decrease enzyme accessibility by closing of larger pores, the effect being more profound with substrates containing lignin (Esteghlalian et al., 2001; Luo and Zhu, 2011). It appears that drying- induced decrease in accessibility has increased the role of xylanases in improving the accessibility of cellulose to cellulase enzymes. Cell wall anatomy and micro- structure can have significant impacts on optimal enzyme proportions in addition to the content of lignin, cellulose and hemicellulose. The performance of the optimal enzyme mixtures of thermostable enzymes was evaluated by comparing the hydrolysis with that obtained using a mixture of Tricho- derma reesei enzymes and using commercial enzymes (Figure 9; Publication IV). The optimal mixture of thermostable enzymes produced 64% yield in 48 hours using an enzyme dosage of 6 mg/g, whereas the mixture of Trichoderma reesei enzymes produced only 51% hydrolysis yield. The hydrolysis yield obtained by the commercial enzyme mixture, Celluclast-Novozym 188 (10 mg/g+1000 nkat/g), was 68%. Novozym 188 preparation is reported to contain e.g. various cell wall de- grading glucanases, amyloglucosidases, and some xylanolytic activity (Banerjee et al., 2010b). The higher dosage and the accessory enzymes that were included in the commercial mixture improved the hydrolysis result. Thus the optimal en- zyme mixture of thermostable enzymes performed much better than the commer- cial enzyme mixture in the hydrolysis of hydrothermally pretreated wheat straw.
- 75. 4. Results and discussion 73 The optimization experiments of wheat straw substrate were carried out using dried wheat straw discs, as this made it possible to carry out a large number of small scale experiments with the same substrate. The never-dried wheat straw substrate was hydrolysed more easily than the dried substrate and the hydrolysis yield was 86% in 48 hours (Figure 9). Drying has been shown to decrease enzyme accessibility as discussed in the previous paragraph (Esteghlalian et al., 2001). However, the degree of hydrolysis was clearly decreased when the dry matter content was increased to 12%. It is probable that in the higher dry matter content the hydrolysis requires a different optimal enzyme mixture. Alkaline oxidation pretreated bagasse was hydrolysed very easily in high dry matter content of 8% and the degree of hydrolysis was 90–100% with both the optimized enzyme mixture and the commercial enzyme mixture (Figure 9). How- ever, slightly faster hydrolysis was obtained using the commercial Cellic enzyme mixture. With steam exploded bagasse the degree of hydrolysis was significantly lower than in the hydrolysis of alkaline oxidation pretreated bagasse at 8% dry matter content despite the higher enzyme dosage. The inhibitory effect of lignin is the most probable reason for this. The optimal enzyme mixture for the hydrolysis of steam exploded bagasse was slightly more efficient in the beginning of the hydrolysis but after 48 hours a similar degree of hydrolysis, 81–82%, was obtained with the commercial enzyme mixture (Publication IV). The thermostable enzyme mixture was composed of five enzyme component preparations. The accessory enzymes present in Cellic but not in the thermostable enzyme might have im- proved the rate and degree of hydrolysis. The accessory enzymes have been found to be more important in the later stages of hydrolysis (Banerjee et al., 2010c). Thus optimal enzyme mixtures could further be improved by including other enzymes, such as arabinofuranosidases or LPMOs, needed for efficient and complete hydrolysis of cell wall carbohydrates.
- 76. 4. Results and discussion 74 Figure 9. Hydrolysis of hydrothermally pretreated wheat straw, alkaline oxidised (AlkOx) sugar cane bagasse and steam exploded (SE) bagasse by an optimal enzyme mixture of T. reesei enzymes (TrMix), thermostable enzymes (thermo mix) and commercial enzymes for 24 h (A) and 48 h (B). The dry matter content was 1–12% and enzyme dosage 4–38 mg/g d.m. depending on the experiment. In the hydrolysis of AlkOx and SE bagasse the commercial enzyme mixture was Cellic Ctec2+Htec, 85%+15% of total protein and in hydrolysis of wheat straw a mixture of Celluclast +Novozym 188 (C+N,10 mg/g +1000 nkat/g d.m). Hydrolysis temperature was 45°C for TrMix and for C+N, and 52°C in the hydrolysis of wheat straw with thermostable enzymes. The hydrolysis temperature with pretreated bagasses was 52°C and 50°C, at 1% and 8% d.m. contents, respectively. Hydrol- ysis yield was measured as reducing sugars. 0 20 40 60 80 100 1 % 1 % 1 % 12 % 1 % 8 % 1 % 8 % 6 mg/g 38 mg/g 6 mg/g 6 mg/g 4 mg/g 10 mg/g 10 mg/g 15 mg/g Dried wheat straw Never-dried wheat straw AlkOx Bagasse SE Bagasse Hydrolysisyield(%oftheoretical) Optimal TrMix Optimal thermo mix Commercial enzymes (Cellic) Commercial enzymes (C+N) 0 20 40 60 80 100 1 % 1 % 1 % 12 % 1 % 8 % 1 % 8 % 6 mg/g 38 mg/g 6 mg/g 6 mg/g 4 mg/g 10 mg/g 10 mg/g 15 mg/g Dried wheat straw Never-dried wheat straw AlkOx Bagasse SE Bagasse Hydrolysisyield(%oftheoretical) Optimal TrMix Optimal thermo mix Commercial enzymes (cellic) Commercial enzymes (C+N)
- 77. 5. Conclusions and recommendations for future studies 75 5. Conclusions and recommendations for future studies Alternative raw materials for biorefineries, a novel alkaline oxidation pretreatment method and optimization of enzyme mixture compositions were studied in this work. Reed canary grass and barley straw were shown to have high carbohydrate content similarly to wheat straw, and they could thus be interesting alternatives or supplementary raw materials for the production of sugars in a biorefinery, espe- cially in a northern climate. Reed canary grass harvested in the spring had higher cellulose content, more xylan was removed from it by steam explosion and the pretreated fibre was hydrolysed more efficiently compared to the autumn harvest- ed material. The modification of the raw material in the field during winter might work as a partial pretreatment and bring out other advantages in raw material properties, such as decreased moisture and ash contents. Thus, spring was found to be a more suitable harvest time for the production of sugars from this material in a biorefinery. Although the cultivation of reed canary grass is currently low, it could be increased significantly e.g. in marginal land, without competing with food pro- duction. The other potential raw materials such as common reed and surplus forages would be interesting options for future studies. Alkaline oxidation was found to be an efficient pretreatment method, fractionating biomass into a carbohydrate-rich fibre fraction and a dissolved fraction containing most of the lignin. Alkaline oxidised fibre showed high enzymatic hydrolysability and the hydrolysis was also efficient at relatively high 12% dry matter content. Compared to the 52% glucose yield obtained by steam explosion pretreated spruce in pretreatment and hydrolysis, a significantly higher glucose yield of 84% was obtained after alkaline oxidation pretreatment. The efficiency of alkaline oxi- dation was also shown with different types of raw materials, such as spruce, birch and sugar cane bagasse. After alkaline oxidation, cellulose remained mainly in the solid fraction. Cellu- lose losses were 0–11%, depending on the treatment conditions. The losses were probably due to oxidation of cellulose. The structural changes of cellulose in alka- line oxidation, the oxidation of cellulose and changes in cellulose crystallinity re- quire further studies. Part of the hemicellulose, both galactoglucomannan and xylan, was solubilised and further oxidised to other products and therefore hemi- cellulose yields were relatively low. Up to 60% of mannan and 34% of xylan was
- 78. 5. Conclusions and recommendations for future studies 76 lost in the pretreatment. The hemicellulose losses due to oxidation should be minimized by further optimization of process conditions. In addition to carbohydrates, alkaline oxidation treatment dissolved and oxi- dised lignin. Alkaline oxidation produced a new type of lignin, which is oxidised and sulphur-free, unlike the lignin separated from black liquor of the Kraft pulping process. Sulphur-free oxidised lignin is a very interesting material and its proper- ties and suitability for different applications should be investigated. However, the heating value of lignin generated by alkaline oxidation is decreased due to the oxidative nature of the pretreatment. Organic acids were formed in alkaline oxida- tion as degradation products of both lignin and carbohydrates. Their formation should be minimized by optimizing process conditions. The recovery and econom- ical exploitation of organic acids should also be considered. Various process conditions were studied using spruce as raw material to im- prove the alkaline oxidation pretreatment. The pretreatment efficiency could be improved by reducing the particle size of the raw material, by increasing the treat- ment temperature from 120°C to 140°C, and by catalysing the reaction with a copper-phenanthroline catalyst. The alkaline treatment of spruce without oxygen pressure revealed that oxidative conditions enhanced radically both solubilisation in the pretreatment and the enzymatic hydrolysability of the produced solid frac- tion. In order to improve the feasibility of alkaline oxidation, optimized mechanical pretreatments and oxygen supply should further be developed. In addition, alkali dosages, solid to liquid ratios and treatment time and temperature should be opti- mized for different raw materials in order to obtain high enzymatic hydrolysability and yield. Advanced reactor systems providing efficient oxygen and mass transfer throughout the reaction would enable higher solid to liquid ratios and possibly also lower alkali dosages. Feasibility of the optimized alkaline oxidation process should be evaluated in future studies. Enzyme consumption and costs can be decreased by enhanced pretreatments. Especially with spruce, alkaline oxidation pretreatment enabled the use of de- creased enzyme dosages in the hydrolysis and thus significant reduction in en- zyme costs or hydrolysis time could be obtained. With steam exploded materials, high enzyme dosages were required to obtain high hydrolysis yield, most probably due to the inhibitory effect of high lignin content. The significant role of cellobiohydrolases in the hydrolysis of pretreated gra- mineous raw materials was demonstrated in experiments studying the supplemen- tation of commercial mixtures with purified enzymes as well as in optimization of mixtures of individual enzyme components. The results indicated that cellobiohy- drolase activity can be the limiting activity in the hydrolysis by commercial mix- tures. With thermostable enzymes the proportion of Cel7A in the optimal mixture was at least 50%, whereas with Trichoderma reesei enzymes almost equal amounts of Cel6A and Cel7A were required. The results also indicated that mate- rials having decreased accessibility to cellulose due to their high xylan content required higher xylanolytic activity in optimal mixtures. The structural collapse caused by drying of substrate might also have increased the need for high xylano- lytic activity in the case of pretreated wheat straw. It would be interesting in the
- 79. 5. Conclusions and recommendations for future studies 77 future to study the effects of the addition of accessory enzymes such as debranch- ing enzymes, different activities such as LPMOs and swollenins, or enzymes with improved properties, in enzyme mixtures. The effect of higher dry matter content as well as other hydrolysis conditions on enzyme mixture composition should be evaluated in future studies. The performance of optimal enzyme mixtures in hydrolysis was compared with that of the commercial enzymes Celluclast and Cellic. The results showed that significant improvement in the hydrolysis efficiency can be obtained by optimizing the enzyme mixtures. Optimized thermostable enzyme mixtures of five compo- nents hydrolysed pretreated materials as efficiently as commercial enzymes con- taining a whole spectrum of supplementary activities. The results indicated signifi- cant flexibility in the proportions of enzyme components leading to high hydrolysis yields. Thus, instead of using monocomponent enzyme mixtures, optimized ratios of commercial enzyme preparations are also one option to achieve efficient hy- drolysis of lignocellulosic materials with minimum enzyme consumption.
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- 109. 1 Supplementary material for Publication IV This data is related to Publication IV, Tables 3–4. Table 1. Design of experiments for optimization of thermostable enzyme mixtures for hydrolysis of steam exploded bagasse and hydrolysis yields obtained in exper- iments. Expe- riment Enzyme proportions Hydrolysis yield (% of d.m.) Cel7A (CBHI) Cel6A (CBHII) Cel5A (EGII) Xyn10A Cel3A G) 24h 48h 72h 1 0.55 0.1 0.1933 0.12 0.0367 48.55 55.98 58.59 2 0.643 0.1 0.171 0.057 0.029 51.99 56.69 60.78 3 0.25 0.4 0.2167 0.0833 0.05 48.81 52.49 55.42 4 0.7 0.1 0.03 0.12 0.05 51.17 54.91 59.26 5 0.55 0.1 0.2467 0.0933 0.01 48.75 54.42 58.51 6 0.67 0.1 0.21 0.01 0.01 50.98 56.61 59.71 7 0.25 0.4 0.3 0.0400 0.01 46.61 52.05 54.24 8 0.55 0.1 0.29 0.01 0.05 48.64 54.67 58.31 9 0.25 0.5733 0.1167 0.01 0.05 50.12 53.22 56.04 10 0.5125 0.3625 0.03 0.065 0.03 52.75 55.81 60.08 11 0.4465 0.2965 0.171 0.057 0.029 52.19 56.44 60.08 12 0.25 0.52 0.21 0.01 0.01 48.41 52.65 56.65 13 0.85 0.1 0.03 0.01 0.01 51.29 56.66 60.89 14 0.6233 0.2867 0.03 0.01 0.05 53.14 56.28 62.04 15 0.25 0.66 0.03 0.01 0.05 48.36 52.84 55.95 16 0.25 0.4 0.2067 0.12 0.0233 46.84 54.34 56.63 17 0.25 0.4 0.3 0.01 0.0400 46.70 53.82 55.18 18 0.4465 0.2965 0.171 0.057 0.029 50.50 57.20 60.41 19 0.4465 0.2965 0.171 0.057 0.029 50.70 57.39 60.63 20 0.81 0.1 0.03 0.01 0.05 49.33 55.95 60.52 21 0.74 0.1 0.03 0.12 0.01 48.63 56.06 59.60 22 0.25 0.59 0.03 0.12 0.01 47.37 53.43 57.82 23 0.25 0.55 0.03 0.12 0.05 47.15 52.46 56.49 24 0.45 0.5 0.03 0.01 0.01 49.69 54.57 60.13 25 0.57 0.1 0.3 0.01 0.02 48.94 55.40 58.40 26 0.25 0.7 0.03 0.01 0.01 45.92 51.99 56.52
- 110. 2 Table 2. Design of experiments for optimization of thermostable enzyme mixtures for hydrolysis of alkaline oxidised bagasse and hydrolysis yields obtained in exper- iments. Expe- riment Enzyme proportions Hydrolysis yield (% of d.m.) Cel7A (CBHI) Cel6A (CBHII) Cel5A (EGII) Xyn10A Cel3A G) 24h 48h 72h 1 0.44 0.1 0.3 0.15 0.01 50.77 69.67 70.63 2 0.25 0.3433 0.3 0.0567 0.05 40.2 49.56 64.84 3 0.73 0.1 0.01 0.15 0.01 61.1 75.79 78.98 4 0.25 0.7 0.01 0.03 0.01 40.23 56.90 61.07 5 0.25 0.4833 0.1067 0.15 0.01 54.86 78.20 70.85 6 0.25 0.4167 0.3 0.01 0.0233 20.84 31.33 46.08 7 0.4633 0.5067 0.01 0.01 0.01 32.31 42.94 62.72 8 0.4718 0.3218 0.1173 0.0627 0.0264 51.11 62.28 80.71 9 0.58 0.1 0.3 0.01 0.01 25.36 40.29 47.10 10 0.25 0.5833 0.1067 0.01 0.05 27.67 36.69 41.12 11 0.83 0.1 0.01 0.01 0.05 36.27 55.72 72.28 12 0.25 0.5533 0.01 0.15 0.0367 55.78 61.34 81.75 13 0.5267 0.3767 0.01 0.06 0.0267 56.56 74.04 77.96 14 0.25 0.5867 0.01 0.1033 0.05 52.4 60.99 67.62 15 0.25 0.3367 0.3 0.1033 0.01 47.22 62.06 61.66 16 0.54 0.1 0.3 0.01 0.05 27.68 46.02 47.17 17 0.4967 0.1 0.2033 0.15 0.05 50.81 79.86 74.24 18 0.4718 0.3218 0.1173 0.0627 0.0264 55.49 71.78 77.78 19 0.25 0.6933 0.01 0.01 0.0367 28.63 42.29 46.82 20 0.4718 0.3218 0.1173 0.0627 0.0264 47.77 72.41 78.18 21 0.676 0.1 0.128 0.068 0.028 51.85 70.50 75.09 22 0.25 0.52 0.21 0.01 0.01 24.77 44.42 47.34 23 0.85 0.1 0.03 0.01 0.01 40.75 55.67 56.39 24 0.69 0.1 0.01 0.15 0.05 62.39 84.62 72.59 25 0.6367 0.2933 0.01 0.01 0.05 40.01 51.53 62.38 26 0.25 0.25 0.3 0.15 0.05 43.55 73.87 71.67
- 111. 3 Table 3. Design of experiments for optimization of Trichoderma enzyme mixtures for hydrolysis of hydrothermally pretreated wheat straw and hydrolysis yields ob- tained in experiments. Exper- iment Enzyme proportions Hydrolysis yield (% of d.m.) Cel7A (CBHI) Cel6A (CBHII) Cel7B (EGI) Cel5A (EGII) Xyn11A Cel3A G) 24h 48h 72h 1 0.1 0.1 0.4433 0.3 0.0467 0.01 11.9 18.3 21.9 2 0.3567 0.1 0.1 0.3 0.12 0.0233 16.3 23.5 25.6 3 0.1 0.6 0.16 0.01 0.12 0.01 18.7 26.5 31.0 4 0.4033 0.1 0.1 0.3 0.0467 0.05 16.6 23.7 27.9 5 0.6 0.12 0.1 0.01 0.12 0.05 19.3 26.5 31.3 6 0.1 0.1 0.6 0.14 0.01 0.05 12.3 19.6 23.1 7 0.2663 0.2663 0.2662 0.1062 0.065 0.03 21.3 30.2 34.1 8 0.48 0.1 0.1 0.3 0.01 0.01 17.9 24.1 28.0 9 0.1 0.51 0.1 0.12 0.12 0.05 16.3 23.7 27.4 10 0.27 0.6 0.1 0.01 0.01 0.01 22.5 30.1 34.4 11 0.1 0.1333 0.6 0.01 0.12 0.0367 13.4 21.5 24.8 12 0.2663 0.2663 0.2662 0.1062 0.065 0.03 22.1 29.2 33.3 13 0.1 0.38 0.49 0.01 0.01 0.01 18.4 26.3 30.9 14 0.1 0.44 0.1 0.3 0.01 0.05 14.1 22.3 25.7 15 0.1567 0.6 0.1 0.01 0.0833 0.05 19.8 28.3 32.2 16 0.28 0.1 0.44 0.01 0.12 0.05 18.1 25.7 30.2 17 0.4767 0.3533 0.1 0.01 0.01 0.05 23.1 31.4 35.9 18 0.6 0.1 0.1 0.14 0.01 0.05 19.4 26.7 30.2 19 0.1 0.6 0.1 0.18 0.01 0.01 16.3 23.9 28.0 20 0.2663 0.2663 0.2662 0.1062 0.065 0.03 21.1 30.3 34.8 21 0.19 0.28 0.1 0.3 0.12 0.01 16.3 23.7 27.0 22 0.2663 0.2663 0.2662 0.1062 0.065 0.03 21.3 29.8 34.4 23 0.27 0.1 0.6 0.01 0.01 0.01 17.7 25.3 30.0 24 0.1 0.1 0.6 0.07 0.12 0.01 12.8 20.4 23.3 25 0.6 0.1 0.2433 0.01 0.01 0.0367 20.2 28.2 32.6 26 0.1 0.4067 0.1 0.3 0.0833 0.01 14.4 21.6 25.2 27 0.1 0.1 0.33 0.3 0.12 0.05 11.4 17.9 20.9 28 0.1 0.6 0.23 0.01 0.01 0.05 18.9 28.0 32.9 29 0.6 0.1 0.16 0.01 0.12 0.01 19.4 26.7 31.1 30 0.1 0.1933 0.6 0.01 0.0467 0.05 15.2 23.3 27.5 31 0.6 0.27 0.1 0.01 0.01 0.01 21.8 30.1 33.8 32 0.1 0.1 0.4667 0.3 0.01 0.0233 11.9 18.4 21.6 33 0.3621 0.2121 0.1668 0.1668 0.0626 0.0297 22.5 30.2 34.3 34 0.25 0.5467 0.03 0.03 0.12 0.0233 20.1 27.6 31.0 35 0.25 0.3733 0.3 0.03 0.01 0.0367 25.1 34.1 37.9 36 0.4 0.1 0.03 0.3 0.12 0.05 16.7 23.1 25.3
- 112. 4 Table 3 Continued. Exper- iment Enzyme proportions Hydrolysis yield (% of d.m.) Cel7A (CBHI) Cel6A (CBHII) Cel7B (EGI) Cel5A (EGII) Xyn11A Cel3A G) 24h 48h 72h 37 0.55 0.1 0.03 0.3 0.01 0.01 17.6 23.9 26.8 38 0.25 0.1 0.3 0.2067 0.12 0.0233 18.3 25.5 29.0 39 0.25 0.52 0.03 0.03 0.12 0.05 19.9 28.0 31.0 40 0.4233 0.1 0.1167 0.3 0.01 0.05 18.8 26.5 29.2 41 0.25 0.1 0.3 0.3 0.0400 0.01 17.2 24.7 28.3 42 0.6033 0.2767 0.03 0.03 0.01 0.05 21.1 28.6 31.6 43 0.44 0.48 0.03 0.03 0.01 0.01 21.2 29.4 32.0 50 0.4367 0.1 0.3 0.03 0.0833 0.05 21.7 30.5 35.1 51 0.3621 0.2121 0.1668 0.1668 0.0626 0.0297 22.3 29.5 35.2 52 0.25 0.1 0.1933 0.3 0.12 0.0367 17.4 23.7 27.9 53 0.71 0.1 0.03 0.03 0.12 0.01 19.1 25.7 29.8 54 0.25 0.29 0.03 0.3 0.12 0.01 17.6 24.1 27.6 55 0.3621 0.2121 0.1668 0.1668 0.0626 0.0297 22.8 30.8 35.2 56 0.25 0.3867 0.03 0.3 0.01 0.0233 17.9 25.1 29.6 57 0.25 0.2867 0.03 0.3 0.0833 0.05 17.2 24.1 27.2 58 0.25 0.2 0.3 0.08 0.12 0.05 21.7 29.2 35.5 59 0.58 0.1 0.12 0.03 0.12 0.05 20.9 28.7 33.2 60 0.25 0.29 0.3 0.03 0.12 0.01 23.2 31.5 37.5 61 0.82 0.1 0.03 0.03 0.01 0.01 19 25.2 29.9 62 0.78 0.1 0.03 0.03 0.01 0.05 18.9 25.7 29.2 63 0.55 0.1 0.3 0.03 0.01 0.01 22.4 30.7 35.9 64 0.58 0.1 0.03 0.12 0.12 0.05 18.7 25.3 28.9
- 113. 5 Table 4. Design of experiments for optimization of thermostable enzyme mixtures for hydrolysis of hydrothermally pretreated wheat straw and hydrolysis yields ob- tained in experiments. Experiment Proportion in the mixture Hydrolysis yield (% of d.m.) Cel7A (CBHI) Cel6A (CBHII) Cel5A (EGII) Xyn10A Cel3A G) 24h 48h 72h 1 0.5 0.1 0.27 0.12 0.01 33.19 42.6 49.5 2 0.5 0.3 0.03 0.12 0.05 31.31 40.3 47.91 3 0.5 0.1 0.265 0.085 0.05 32.98 42.02 48.64 4 0.5762 0.1762 0.1588 0.0588 0.03 35.12 44.03 50.19 5 0.5 0.14 0.3 0.05 0.01 33.56 41.95 49.37 6 0.5 0.315 0.165 0.01 0.01 32.89 41.69 48.57 7 0.5762 0.1762 0.1588 0.0588 0.03 35.42 43.9 50.44 8 0.58 0.1 0.3 0.01 0.01 31.5 40.53 47.52 9 0.622 0.1 0.182 0.066 0.03 33.69 41.11 50.45 10 0.5 0.45 0.03 0.01 0.01 30.09 38.62 44.94 11 0.6375 0.2375 0.03 0.065 0.03 32.45 41.5 47.42 12 0.5 0.41 0.03 0.01 0.05 30.02 37.48 43.46 13 0.5 0.275 0.165 0.01 0.05 31.28 40.72 49.07 14 0.74 0.1 0.03 0.12 0.01 31.39 41.01 46.66 15 0.7 0.1 0.03 0.12 0.05 31.09 39.23 46.86 16 0.5 0.1 0.25 0.12 0.03 33.39 41.79 49.66 17 0.54 0.1 0.3 0.01 0.05 29.88 40.09 44.75 18 0.5 0.34 0.03 0.12 0.01 31.52 42.73 48.22 19 0.655 0.255 0.03 0.01 0.05 30.68 40.96 45.31 20 0.85 0.1 0.03 0.01 0.01 30.6 38.38 45.34 21 0.5 0.16 0.3 0.01 0.03 31.34 41.02 46.7 22 0.5762 0.1762 0.1588 0.0588 0.03 33.84 44.37 51.21 23 0.715 0.1 0.165 0.01 0.01 32.17 42.6 47.74 24 0.81 0.1 0.03 0.01 0.05 29.29 38.77 44.79 25 0.675 0.275 0.03 0.01 0.01 31.15 40.34 47.7
- 115. PUBLICATION I Reed canary grass as a feedstock for 2nd generation bioethanol production In: Bioresource Technology 123, pp. 669–672. Copyright 2012 Elsevier. Reprinted with permission from the publisher.
- 117. I/1 Short Communication Reed canary grass as a feedstock for 2nd generation bioethanol production Anne Kallioinen a,⇑ , Jaana Uusitalo a , Katri Pahkala b , Markku Kontturi b , Liisa Viikari a,1 , Niklas von Weymarn a,2 , Matti Siika-aho a a VTT Technical Research Centre of Finland, P.O. Box 1000, FI-02044 VTT, Finland b MTT Agrifood Research Finland, Crop Production Research, FI-31600 Jokioinen, Finland h i g h l i g h t s " Hydrolysis of reed canary grass harvested in the spring and autumn was compared. " Additional b-glucosidase was shown to improve hydrolysis of cellulose and xylan. " Supplementary CBHII increased the hydrolysis level of RCG by 10%. " 82% ethanol yield was obtained on steam pretreated RCG. a r t i c l e i n f o Article history: Received 27 May 2012 Received in revised form 3 July 2012 Accepted 5 July 2012 Available online 16 July 2012 Keywords: Reed canary grass Barley straw Pretreatment Enzymatic hydrolysis Simultaneous saccharification and fermentation a b s t r a c t The enzymatic hydrolysis and fermentation of reed canary grass, harvested in the spring or autumn, and barley straw were studied. Steam pretreated materials were efficiently hydrolysed by commercial enzymes with a dosage of 10–20 FPU/g d.m. Reed canary grass harvested in the spring was hydrolysed more efficiently than the autumn-harvested reed canary grass. Additional b-glucosidase improved the release of glucose and xylose during the hydrolysis reaction. The hydrolysis rate and level of reed canary grass with a commercial Trichoderma reesei cellulase could be improved by supplementation of purified enzymes. The addition of CBH II improved the hydrolysis level by 10% in 48 hours’ hydrolysis. Efficient mixing was shown to be important for hydrolysis already at 10% dry matter consistency. The highest eth- anol concentration (20 g/l) and yield (82%) was obtained with reed canary grass at 10% d.m. consistency. Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction In recent years, growing attention has been devoted to the con- version of renewable raw materials into fuel ethanol. Key drivers behind this development include the objectives of counteracting the climate change, decreasing the dependency of imported oil, and finding new earning principles for agriculture and forestry. As an example of concrete policy measures, the EU has set an aim to replace by the year 2020 up to 10 energy-% of all road trans- port fuels consumed within the EU with energy of renewable basis. Similar policy measures are in place in many of the leading G20 countries. To achieve these goals production of bioethanol from materials rich in cellulose must be taken to industrial scale. Various herba- ceous crops and crop harvesting residues, such as corn stover (Kumar and Wyman, 2009), and switchgrass (Martin and Gross- man, 2012) have been studied as potential feedstock for production of ethanol or other biofuels. Worldwide, these materials are abun- dantly available. However, cultivation of many of these grasses and crops is not feasible or even possible in northern climate. In northern Europe and America, reed canary grass (Phalaris arundinacea L.) has aroused interest as an energy crop for produc- tion of electricity and heat by combustion (Jasinkas et al., 2008), and for production of ethanol (Digman et al., 2010). Reed canary grass is a rhizomatous, perennial grass species that can be culti- vated on the low value areas, such as bogs after peat production, and on fields, which are not needed for food production. In Finland, reed canary grass is typically harvested in the spring when the water content of the biomass has decreased to the level enabling storage without additional drying. The aim of the present work was to study the feasibility of reed canary grass as a feedstock for production of cellulosic ethanol. Steam explosion followed by enzymatic hydrolysis was chosen as the basic process option to produce the fermentable sugar solution. 0960-8524/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.biortech.2012.07.023 ⇑ Corresponding author. Tel.: +358 20 722 7187; fax: +358 20 722 7071. E-mail address: anne.kallioinen@vtt.fi (A. Kallioinen). 1 Present address: University of Helsinki, Department of Food and Environmental Sciences, P.O. Box 27, FI-00014 Helsinki, Finland. 2 Present address: Metsä Fibre Ltd, P.O. Box 30, FI-02020 Metsä, Finland. Bioresource Technology 123 (2012) 669–672 Contents lists available at SciVerse ScienceDirect Bioresource Technology journal homepage: www.elsevier.com/locate/biortech
- 118. I/2 Barley straw, which is also a material available in northern climate, was used as a reference material in the experiments. 2. Methods 2.1. Raw materials Reed canary grass (P. arundinacea L.) harvested in spring (April 2005) and autumn (August 2005), as well as barley (Hordeum vulgare L.) straw were obtained from the experimental fields of MTT Agrifood Research Finland in Jokioinen (60o 49N, 23o 28E). Raw materials were pretreated by steam explosion at Lund Univer- sity, Sweden at 190 °C for 5 min or 200 °C for 5 min (for barley straw only) after impregnation with 2% SO2 (Öhgren et al., 2005). The sugar compositions of the feedstocks were analysed after total acid hydrolysis (Puls et al., 1985). The resulting monosaccharides were analysed by high performance anion exchange chromatogra- phy (HPLC) (Tenkanen and Siika-aho, 2000). The composition of the material dissolved in the steam explosion was analysed by HPLC after a secondary enzymatic hydrolysis in order to hydrolyse possi- ble oligosaccharides to monosaccharides (Tenkanen et al., 1999). 2.2. Enzymes Celluclast 1.5L and Novozym 188 enzymes were provided by Novozymes and Econase CE by AB Enzymes. Spezyme CP was ob- tained from Genencor. Filter paper activity was measured by the method of Ghose (1987) and b-glucosidase activity was measured according to Bailey and Linko (1990). Protein content was analysed by the Lowry method (Lowry et al., 1951). Trichoderma reesei cellulases (CBH I/Cel 7A, CBH II/Cel 6A, EG I/ Cel 7B, and EG II/Cel 5A) were produced and purified as described by Suurnäkki et al. (2000). Xylanase (XYL II, i.e. Family 11 xylanase with pI 9) was purified as described by Tenkanen et al. (1992), but omitting the last gel filtration step. 2.3. Enzymatic hydrolysis and fermentation After the pretreatments the hydrolysis of washed solid fraction was carried out using commercial cellulases at a dosage of 10 FPU/g dry matter and b-glucosidase (Novozym 188) 100 nkat/g d.m. The hydrolysis experiments were carried out as triplicates in 50 mM sodium acetate buffer (pH 5) in magnetically stirred test tubes at 1% (w/w) d.m. consistency. The temperature was controlled at 45 °C. The reducing sugars in enzyme hydrolysates were monitored using the DNS method (Bernfeld, 1955) and the monosaccharides by HPLC (Tenkanen and Siika-aho, 2000). Effect of supplementation of commercial mixture (Econase CE) with the purified major cellulases of T. reesei was studied using the washed solid fraction of steam exploded reed canary grass (spring harvest). The dosages of the commercial cellulase mixture and b-glucosidase (Novozyme 188) were 10 FPU/g cellulose and 100 nkat/g cellulose, respectively. This mixture was supplemented with the purified enzymes with a dosage of 5 mg/g cellulose. The cellulases studied were CBH I (Cel7A), CBH II (Cel6A), EG I (Cel7B), EG II/(Cel5A) and xylanase (XYL II). Hydrolysis was carried out as triplicates in test tubes in 1% (w/w) cellulose content. The unwashed steam exploded raw materials were hydrolysed in Erlenmeyer flasks (liquid volume 60 ml) shaken at 100 rpm, or in stirred tank reactors (liquid volume 700 ml) equipped with a marine impeller enabling mixing at 400–1000 rpm at 10% d.m. consistency. Cellulase dosage was 20 FPU/g d.m. and b-glucosidase (Novozym 188) 200 nkat/g d.m. The experiments were carried out at 45 °C and pH 5. Hydrolyses in Erlenmeyer flasks were done as triplicates whereas hydrolyses in stirred tank reactor as single experiments. Fermentation of pretreated unwashed reed canary grass (spring harvest,190 °C, 5 min) and barley straw (200 °C, 5 min) were car- ried out in Erlenmeyer flasks equipped with oil locks at 50 ml working volume and 10% dry matter consistency. Autoclaved yeast nitrogen base (Difco, dissolved in 5 ml volume) was used as a nutrient in fermentation. The material was prehydrolysed for 24 h by using enzymes Celluclast (10 FPU/g dry matter) and b-glu- cosidase Novozym 188 (100 nkat/g) at 45 °C, pH 5 and then inocu- lated with 3.5 g/l yeast (yeast strain VTT-B-03339). For fermentation phase the temperature was reduced to 30 °C. Fer- mentation of barley straw was continued 12 days and reed canary grass for 15 days. Fermentation was followed by measuring the mass loss due to formation of CO2 and analysing ethanol from the broth by HPLC (Heinonen et al., 2012). 3. Results and discussion 3.1. Raw materials All raw materials consisted mainly of glucose, xylose and arab- inose, the monosaccharides present in cellulose and arabinoxylan (Table 1). Reed canary grass harvested in spring and in autumn had slightly different compositions. Generally, the proportion of stem and cellulosic fibre is higher in the spring than in the autumn (Pahkala and Pihala, 2000). Consistent with this, the content of cel- lulose in reed canary grass harvested in spring was significantly higher than in the autumn-harvested crop (Table 1). Expectedly, reed canary grass harvested in autumn contained fructose originat- ing from water soluble fructan polysaccharides or from sucrose. Table 1 The sugar composition (% of dry matter) of untreated raw materials and steam exploded (190 °C, 5 min, 2% SO2) washed solids. Sample Reed canary grass (autumn) Reed canary grass (spring) Barley straw Untreated raw material Steam exploded washed solids Untreated raw material Steam exploded washed solids Untreated raw material Steam exploded washed solids Rhamnose <0.1 <0.1 0.2 <0.1 <0.1 <0.1 Arabinose 3.2 0.6 2.3 0.4 2.6 0.7 Galactose 1.3 0.3 1.3 0.2 1.0 0.3 Glucose 36.0 63.3 48.5 66.9 48.6 63.5 Xylose 17.6 8.9 19.1 5.7 23.6 12.1 Mannose 0.2 <0.1 0.5 0.2 <0.1 <0.1 Fructose 3.8 <0.1 n.d. <0.1 n.d. <0.1 Monosaccharides tot. 62 73 72 73 76 77 As polysaccharides 56 66 64 66 68 69 n.d. = not determined. 670 A. Kallioinen et al. / Bioresource Technology 123 (2012) 669–672
- 119. I/3 Barley straw resembled reed canary grass in sugar compositions but had slightly higher xylose content. A relatively high proportion, 56–68% of the d.m. of reed canary grasses and barley straw were carbohydrates. 3.2. Effect of steam explosion During the pretreatment by steam explosion, a major part of arabinose, galactose, xylose, mannose, and fructose was solubi- lised, whereas glucose was enriched in the solid fraction (Tables 1 and 2). Thus, most cellulose remained in the solid fraction and the hemicellulosic material, mainly arabinoxylan, was efficiently solubilised. Water soluble carbohydrates, e.g. glucans, fructans, and sucrose, can explain the concentrations of glucose and fructose in the soluble fractions, especially in the autumn-harvested reed canary grass. The pretreatment caused, however, some losses of arabinose, xylose, and fructose, which are the sugars most suscep- tible for degradation. The low amount of rhamnose present in raw materials could not be detected after steam explosion. 3.3. Enzymatic hydrolysis with commercial enzymes Nearly theoretical amounts of reducing sugars were produced in small scale enzymatic hydrolysis with Celluclast 1.5L supple- mented by -glucosidase in 72 h hydrolysis (Table 3). The degree of hydrolysis of reed canary grass harvested in the spring was high- er than that of the autumn-harvested crop. This might be because a higher amount of xylan was removed by the pretreatment from the spring-harvested reed canary grass. Ageing during the winter in the field might also have brought about chemical and physical changes improving the efficiency of the steam explosion. The samples after the enzymatic hydrolysis were analysed by HPLC. In contrast to reducing sugar assay, release of glucose was lowest with Celluclast 1.5L and highest with Econase in hydrolysis of reed canary grasses, and with Spezyme in hydrolysis of barley straw. Based on HPLC analysis, the highest glucose yields obtained in the 72 h hydrolysis were 82%, 89% and 86% for reed canary grass harvested in autumn and in spring, and for barley straw, respec- tively. The total glucose yields after pretreatment and enzymatic hydrolysis were similar for spring harvested reed canary grass (89%) and barley straw (90%) but clearly lower for autumn-har- vested reed canary grass (82%). The amount of xylose released from reed canary grasses was similar with all commercial enzymes. However, clearly the highest amount of xylose was formed from barley straw by Econase, sup- plemented with b-glucosidase. Presumably, the higher xylanase activity in Econase enhanced the xylose release most significantly in the material having the highest xylan content. The total xylose yields were higher than the glucose yields: 94%, 87%, 97% for reed canary grass harvested in autumn, and in spring, and for barley straw, respectively. Part of the sugars might have remained as oli- gosaccharides after enzymatic hydrolysis especially with Cellu- clast, thus explaining the differences in the results of reducing sugars and HPLC assays. Addition of b-glucosidase improved the formation of glucose in hydrolysis of all materials, but the most significant improvement was found on reed canary grass (spring harvest) with Econase and with barley straw with all commercial enzymes (Table 3). The yield of xylose was also slightly enhanced by additional b-glu- cosidase. It has been observed that the increased hydrolysis of cellulose synergistically improves the hydrolysis of xylan in the Table 2 Sugar yields (% of original carbohydrates) in washed solid fraction and soluble fraction after steam explosion (190 °C, 5 min, 2% SO2). Sample Reed canary grass, autumn Reed canary grass, spring Barley straw Solid fraction Soluble fraction Solid fraction Soluble fraction Solid fraction Soluble fraction Rhamnose 0 0 0 0 0 0 Arabinose 13 75 13 72 19 82 Galactose 14 86 9 77 20 80 Glucose 94 6 97 3 92 4 Xylose 30 70 21 71 36 64 Mannose 0 99 19 81 0 100 Fructose 0 81 0 100 0 0 Total monosaccharides 66 32 72 25 71 27 Table 3 Reducing sugars, and glucose and xylose yields by HPLC after 72 h hydrolysis (1% d.m., 45 °C, pH 5) of steam exploded washed solid fraction of reed canary grass and barley straw with different commercial cellulases with or without b-glucosidase (Novozym 188). Raw material Enzymes Glucose (% of theoretical) Xylose (% of theoretical) Reducing sugars (% of theoretical) Reed canary grass, autumn Celluclast 63 63 69 Celluclast + Novozym 76 72 91 Econase 70 63 77 Econase + Novozym 82 71 84 Spezyme 72 64 74 Spezyme + Novozym 80 68 79 Reed canary grass, spring Celluclast 66 74 75 Celluclast + Novozym 78 76 103 Econase 68 67 82 Econase + Novozym 89 74 97 Spezyme 78 72 76 Spezyme + Novozym 88 77 89 Barley straw Celluclast 61 68 80 Celluclast + Novozym 78 69 92 Econase 63 73 83 Econase + Novozym 80 81 93 Spezyme 70 74 87 Spezyme + Novozym 87 72 93 Table 4 Hydrolysis of washed solid fraction of steam exploded reed canary grass (spring) by supplementing commercial cellulase mixture (Econase CE: 10 FPU/g cellulose + Nov- ozym 188 -glucosidase: 100 nkat/g cellulose) with purified enzymes; CBH I, CBH II, EG I, EG II, or XYL II (5 mg/g cellulose). Hydrolysis conditions were 1% cellulose content, 45 °C, pH 5. Hydrolysis products analysed as reducing sugars. Enzyme mixture Supplementation Hydrolysis products (% of theoretical) 0 h 6 h 24 h 48 h Econase + Novozym – 1.6 32 61 78 Econase + Novozym CBH I 1.6 37 68 83 Econase + Novozym CBH II 1.6 41 69 88 Econase + Novozym EG I 1.6 34 64 87 Econase + Novozym EG II 1.5 33 57 81 Econase + Novozym XYLII 1.5 37 63 85 Reference – 1.1 1.1 1.2 1.1 A. Kallioinen et al. / Bioresource Technology 123 (2012) 669–672 671
- 120. I/4 complex lignocellulose material (Varnai et al., 2011). The other side activities of the b-glucosidase preparation might also have enhanced the hydrolysis of side groups of xylan and mixed xylo- oligomers. Synergy between xylanases and cellulases in hydrolysis of pretreated corn stover has been frequently observed (Hu et al., 2011; Kumar and Wyman, 2009). The possibility to enhance the hydrolytic performance of a com- mercial T. reesei cellulase mixture (Econase CE) by addition of po- tential rate-limiting enzymes was studied by overdosing the mixture by the major cellulases and xylanase (Table 4). CBH II im- proved the hydrolysis of the washed solid fraction of reed canary grass most significantly, by 13% after 48 h hydrolysis. The hydroly- sis yield was also enhanced by EGI, probably due to its strong hemicellulolytic side activities (xylanase and xyloglucanase; Benko et al., 2008) and only slightly less by CBH I and xylanase. EGII had practically no effect in the hydrolysis and thus the amount of EGII was not considered to limit the hydrolysis rate or degree. 3.4. Enzymatic hydrolysis and fermentation at 10% dry matter consistency The hydrolysis yield, 75%, obtained in 10% consistency with unwashed steam exploded reed canary grass (spring harvest) was clearly lower than with washed materials at 1% consistency (results not shown). One major reason for low hydrolysis results at higher dry matter content presumably is the inefficient mixing by shaking, resulting in insufficient diffusion of enzymes and reac- tion products. By more efficient mixing in stirred tank reactor sig- nificantly higher hydrolysis yield of reducing sugars, 96%, and faster hydrolysis was obtained. It thus seems that improved mass transfer by efficient mixing increased the hydrolysis results. In addition to improved diffusion and access of enzymes to the substrate, the more even glucose and cellobiose concentrations ob- tained by efficient mixing probably decreased the end product inhibition of the enzymes. The HPLC analysis of the hydrolysates showed that both reed canary grass and barley straw were hydrolysed equally well with 95% hydrolysis yield. Both reed canary grass and barley straw were hydrolysed and fermented at 10% dry matter consistency. Neither of the materials was toxic to the yeast even at 10% dry matter consistency, and the formed sugars could be efficiently fermented to ethanol without a lag phase. The ethanol concentration obtained from reed canary grass was 20.4 g/l, and from barley straw 17.1 g/l. The ethanol yields on the two raw materials were 82% and 74% from the theo- retical yield from glucose, and 59% and 50% from both glucose and xylose, for reed canary grass and barley straw, respectively. Fer- mentation results showed that reed canary grass is a promising raw material for ethanol production. For this, improved hydrolysis techniques for high solids consistencies should be developed and the composition of enzymes should further be optimised to make the process more efficient. 4. Conclusions The enzymatic hydrolysis and fermentation of reed canary grass, harvested in the spring and autumn, and barley straw were studied. As expected, additional b-glucosidase improved the hydrolysis of cellulose and xylan in pretreated raw materials by the commercial enzymes studied. Supplementation of a commer- cial Trichoderma cellulase mixture with CBH II resulted in the high- est improvement of hydrolysis, when steam exploded reed canary grass was studied. High 95% hydrolysis yield was obtained with both the raw materials at 10% consistency. In fermentation the highest ethanol yield, 82% of theoretical based on glucose, was obtained on reed canary grass. Acknowledgements The financial support of the Finnish Funding Agency for Technology and Innovation (Tekes), ClimBus technology pro- gramme (Project 40333/05) is highly acknowledged. Authors thank Eila Leino and Ulla Vornamo for excellent technical assistance. References Bailey, M.J., Linko, M., 1990. Production of b-galactosidase by Aspergillus oryzae in submerged bioreactor cultivation. J. Biotechnol. 16, 57–66. Benko, Z., Siika-aho, M., Viikari, L., Réczey, K., 2008. Evaluation of the role of xyloglucanase in the enzymatic hydrolysis of lignocellulosic substrates. Enzyme Microb. Technol. 43, 109–114. Bernfeld, P., 1955. Amylases, a and b. In: Colowick, S.P., Kaplan, N.O. (Eds.), Methods of Enzymology, vol. 1. Academic press, New York, pp. 149–158. Digman, M., Shinners, K., Casler, M., Dien, B., Hatfield, R., Jung, H.-J., Muck, R., Weimer, P., 2010. Optimizing on-farm pretreatment of perennial grasses for fuel ethanol production. Bioresour. Technol. 101, 5305–5314. Ghose, T.K., 1987. 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Determination of hexenuronic acid in softwood kraft pulps by three different methods. J. Pulp Pap. Sci. 25, 306–311. Tenkanen, M., Puls, J., Poutanen, K., 1992. Two major xylanases of Trichoderma reesei. Enzyme Microb. Technol. 14, 566–574. Tenkanen, M., Siika-aho, M., 2000. An a-glucuronidase of Schitzophyllum commune acting on polymeric xylan. J. Biotechnol. 78 (2), 149–161. Varnai, A., Huikko, L., Pere, J., Siika-aho, M., Viikari, L., 2011. Synergistic action of xylanase and mannanase improves the total hydrolysis of softwood. Bioresour. Technol. 102, 9096–9104. 672 A. Kallioinen et al. / Bioresource Technology 123 (2012) 669–672
- 121. 5 PUBLICATION II Catalysed alkaline oxidation as a wood fractionation technique In: Bioresources 7, pp. 756–776. Copyright 2012 BioResources. Reprinted with permission from the publisher.
- 123. II/1 PEER-REVIEWED ARTICLE bioresources.com Rovio et al. (2012). “Oxidation for wood fractionation,” BioResources 7(1), 756-776. 756 CATALYSED ALKALINE OXIDATION AS A WOOD FRACTIONATION TECHNIQUE Stella Rovio,a, * Anne Kallioinen,a Tarja Tamminen,a Maija Hakola,b Markku Leskelä,b and Matti Siika-ahoa Alkaline oxidation (AlkOx) is an effective fractionation technique for lignocellulosic raw materials. The efficiency of the AlkOx treatment can further be enhanced by using a catalyst (CatOx). Both CatOx and AlkOx provide a fiber fraction containing readily hydrolysable carbohydrates that can be utilized in biotechnical processes and a liquid fraction containing solubilized lignin and reaction products from various biomass components. The effects of different fractionation conditions on yields and chemical composition of solubilized and insoluble fractions were investigated. Two temperatures and two reaction times were studied with and without a catalyst. The composition and content of carbohydrates in the fiber and liquid fractions were examined. The generation of aliphatic carboxylic acids as oxidation products was also investigated. The catalytically assisted oxidation was more efficient than the alkaline counterpart in dissolution of wood components under a four-hour treatment period resulting in higher dissolution of hemicelluloses. A longer reaction time of 20 hours leveled out the differences between the oxidation processes. Comparison of different bases showed that similar solubilisation of dry matter was obtained with NaOH, KOH, and Na2CO3. Oxidation in Na2CO3 caused higher dissolution of glucomannan and greater acid production. The dissolution of hemicellulose and lignin, and their oxidation to acids was most efficient in the first 4 hours of oxidation. Keywords: Catalysis; Alkaline oxidation; Chromatography; Capillary electrophoresis; Spruce; Pretreatment; Fractionation Contact information: a: VTT Technical Research Centre of Finland, VTT, P.O.Box 1000, FI-02044 VTT, Finland; b: University of Helsinki, P.O.BOX 55, FIN-00014 University of Helsinki, Finland; *Corresponding author: Stella. Rovio@vtt.fi INTRODUCTION The use of lignocellulosic biomass as a source of chemicals and fuels has been a subject of interest recently due to concerns about climate change and the need to find an alternative to fossil resources. Currently, the main transport biofuel products on the market are bioethanol, biodiesel, and biogas. Bioethanol has the largest market by far, although biodiesels are also expected to increase in popularity (Mosier et al. 2002). Biomass valorization concentrates mainly on cellulose, but an economically viable production necessitates the valorization of all components of lignocellulosic biomass. Approximately only 2% of the lignins available from the pulp and paper industry are used commercially, and the remainder is burned as a low value fuel: however, lignin has significant potential as an aromatic resource for bulk chemical and fuel production (Zakzeski et al. 2010; Hallac et al. 2009).
- 124. II/2 PEER-REVIEWED ARTICLE bioresources.com Rovio et al. (2012). “Oxidation for wood fractionation,” BioResources 7(1), 756-776. 757 Pretreatment is one of the most critical steps in converting lignocellulosic materials into bioethanol or other chemicals produced by fermentation. It decomposes the cell wall and separates the material into its components, partly degrading the biopolymers into smaller fragments. This kind of fractionation is needed to produce carbohydrate material readily hydrolysable to sugars by enzymes (Chen et al. 2006). Efficient pretreatment also reduces the cellulose crystallinity and increases the porosity of the material; however, fractionation should not cause remarkable carbohydrate yield losses (Sun and Cheng 2002). In addition, production of toxic or inhibiting compounds should be minimal, and the pretreatment should be technically and economically feasible (Kumar et al. 2009). Several pretreatments have been studied relative to degradation purposes (Zheng et al. 2009). Among these are steam explosion, organosolv pulping, acidic pretreatments, and wet oxidation Uncatalyzed steam-explosion, also named as autohydrolysis, is a technology in which only steam water is used to destroy the physical structure of lignocellulosic material. During the pretreatment, the hemicellulose is often hydrolyzed by organic acids such as acetic acids and other acids formed from acetyl or other functional groups, released from biomass. In addition, water itself also possesses certain acidic properties at high temperature, which further catalyze hemicellulose hydrolysis (Avellar and Glasser 1998; Taherzadeh and Karimi 2008). Steam-explosion is typically conducted at a temper- ature of 160 to 270 for several seconds to a few minutes before pretreated contents are discharged into a vessel for cooling. The action of steam explosion can be further enhanced by addition of catalyst such as SO2, H2SO4, and CO2. Catalyzation improves hemicellulose removal and generates less inhibiting compounds (Zheng et al. 2009). In the organosolv pretreatment process, organic or aqueous organic solvent with inorganic acid catalysts are used to dissolve the lignin and separate the fibres (Sun and Cheng 2002). Organosolv pulping can remove lignin from fibres selectively, resulting in a pulp of higher yield and viscosity and less condensed residual lignin (Lohrasebi and Paszner 2001). The benefit of the organosolv process is the possibility of recycling the cooking liquid, which decreases the process costs. In addition, it is also obligatory, since the solvent may be may be inhibitory to the growth of organisms, enzymatic hydrolysis, and fermentation (Sun and Cheng 2002). Dilute acid pretreatment is also a widely used pretreatment method. It is conducted typically using sulfuric acid at high temperature (160 to 200 °C) (Shuai et al 2010). Dilute acid treatment dissolves hemicellulose and partially hydrolyzes cellulose. Serious equipment corrosion problems and extensive condensation of lignin limit the usefulness of this pretreatment. In addition, dilute acid pretreatment can achieve satis- factory levels of cellulose saccharification for agricultural residues and some hardwood species, but it is not effective for softwoods. Sulfite pretreatment to overcome recalci- trance of lignocellulose (SPORL) has been recently developed for robust and efficient bioconversion of softwoods (Zhu et al. 2009). The process involves treatment of wood chips under acidic conditions using 8 to 10% bisulfite and 1.8 to 3.7% sulfuric acid at 180 °C for 30 min. By this pretreatment almost complete hemicellulose separation, partial delignification, and lignin sulfonation were achieved, offering improved recovery of fermentable sugar, when compared with dilute acid pretreatment. Moreover, a benefit of
- 125. II/3 PEER-REVIEWED ARTICLE bioresources.com Rovio et al. (2012). “Oxidation for wood fractionation,” BioResources 7(1), 756-776. 758 SPORL pretreatment is its applicability of variety lignocellulosic raw material (Tian et al. 2011). The above mentioned pretreatment methods are usually carried out under acidic conditions. Wet oxidation is a pretreatment that is performed under neutral environment. It involves the treatment of the biomass with water and air, or oxygen, at temperatures between 120 and 350°C and at elevated pressures (> 1 MPa) (McGinnis et al. 1983; Bjerre et al. 1996; Palonen et al. 2004). In more recent developments, wet oxidation has been combined with an alkaline- aided hydrolysis, utilizing either sodium carbonate or lime as the base (Bjerre et al. 1996; Chang et al. 2001). The alkaline wet oxidation is typically applied to biomasses having a low lignin content, such as wheat straw, clover, and ryegrass (Galbe and Zacchi 2002; Bjerre et al. 1996; Martin et al. 2008). This process has also been used as a pretreatment of softwood and hardwood in order to produce bioethanol and biogas (Palonen et al. 2004). The alkaline wet oxidation process has been found to convert many organic polymers to oxidized compounds, such as low molecular weight carboxylic acids, or even to CO2 and H2O (Taylor and Weygandt 1974; Klinke et al. 2002); even 65% of wheat straw lignin could be removed by wet oxidation. Enhancement of the alkaline oxidative fractionation by the addition of a catalyst is a new attractive approach (Korpi et al. 2004), especially in the case when pH of the reaction is strongly alkaline (Hakola et al. 2010). Alkaline oxidation can be catalyzed by a copper-phenanthroline complex, while copper(II) ions form oxygen-activating copper– diimine complexes e.g with 1,10-phenanthroline (phen) and its substituted derivatives (Korpi et al. 2006). According to Korpi et al. (2004) the in situ complexed Cu–phen is an active catalyst for the oxidation of lignin compounds and is thus a potential catalyst for the biomass pretreatment process. Softwoods are generally considered to be a difficult lignocellulosic raw material to hydrolyze to sugars for fermentation, primarily owing to the nature and amount of lignin. When compared to hardwoods, softwoods contain more hemicellulose, and they have lower xylose content but higher mannose content (Mabee et al. 2006). The goal of the study was to evaluate an alkaline pretreatment method for the production of bioethanol from softwood. The focus of the present paper is on the chemistry that takes place in alkaline oxidation and the effects of different process parameters, in particular the role of the catalyst in a laboratory scale batch reactor. Other process parameters studied were temperature, treatment time, reaction volume, liquid/gas ratio, and the alkali source. The decomposition of spruce raw material was monitored by analyzing the carbohydrate composition and yield of the fiber fraction and the compounds formed during the fractionation and dissolved in alkaline liquors. Soluble carbohydrates were analysed with anion exchange chromatography, dissolved lignin with UV measurements, and the formation of small aliphatic carboxylic acids with capillary electrophoresis (CE). The evaluation of the effects of pretreatments on enzymatic hydrolysis yield will be reported separately.
- 126. II/4 PEER-REVIEWED ARTICLE bioresources.com Rovio et al. (2012). “Oxidation for wood fractionation,” BioResources 7(1), 756-776. 759 EXPERIMENTAL Raw Materials Three kind of particle size were used, depending on the reactor volume. Industrial wood chips consisting mainly of Norway spruce (Picea abies) were obtained from a Finnish pulp mill. Part of the chips was ground into wood meal using a Wiley-mill equipped with a 5 mm sieve. Softwood saw dust was collected from a Finnish saw mill and had an average particle size of 2 mm x 2 mm x 10 mm. Oxidation Treatments Four different decomposition treatments were carried out. In the first and second series, catalyzed alkaline oxidation (CatOx) was compared with alkaline oxidation (AlkOx). Reaction volume, temperature, and time were variables in both series, as shown in Table 1. In addition, an excess of oxygen was fed into the reactor to kept pressure at a constant level in series I. Sodium carbonate was the alkali used in these experiments. In the third series, oxidations using different alkali sources (calcium hydroxide, sodium hydroxide, potassium hydroxide, and sodium carbonate) without a catalyst were compared. Detailed reaction conditions are listed in Table 1. The fourth series was also catalytically assisted, but the reaction volume was one liter and the experiment was performed in four parallel reactions with different reaction times (1, 4, 8, and 20 h). In the case of catalyzed oxidation, 0.84 mmol L-1 of CuSO4*5H2O and 1.66 mmol of L-1 phenanthroline were added to the reaction solution. The reagents were dissolved and added to the preheated autoclave (volume of 1 or 2 liters) equipped with a stirrer or added to a stirred tank reactor (volume of 40 liters). The substrate was added to the reactor, stirring was started, and the autoclave was closed and pressurized immediately with 10 bar of oxygen and then heated to the reaction temperature of either 120 °C or 140 °C. The liquid to wood ratio and oxygen loading as well as consistency are compiled in Table 1. In all experimental series, the reactors were cooled down after the reaction completion and the pressure was released. At temperatures below 50 °C, the reactor was opened and the solid material was washed and separated with vacuum filtration to remove any soluble material. The sugar compositions of the filtrates were then analysed. Part of the filtrate was acidified with 1 M HCl to a pH of 2.5. The precipitate (lignin and some carboxylic acids) was recovered by centrifugation (4000 rpm, 15 min), washed with water (adjusted to a pH of 2.5), centrifuged, and finally freeze-dried. Analytical Methods Carbohydrates The carbohydrate content and composition were determined as monosaccharides using acid hydrolysis and high performance anion exchange chromatography with pulse amperometric detection (HPAEC/PAD) (Willför et al. 2009). A CarboPac PA-1 column 4×250mm (Dionex, Sunnyvale, CA, USA) was coupled with a Dionex ICS-3000 series chromatograph equipped with a pulsed amperometric detector (Dionex ICS-3000). For monosaccharide analysis the system was equilibrated with 15 mM sodium hydroxide.
- 127. II/5 PEER-REVIEWED ARTICLE bioresources.com Rovio et al. (2012). “Oxidation for wood fractionation,” BioResources 7(1), 756-776. 760 Table 1. Conditions Used in the Decomposition Tests Series Sample code used in Figures and Tables Sample form Liquid–to– wood ratio (v/w) Alkali (conc. 0.25 M) Catalyst T (°C) Oxygen loading (g O2/g wood) Liquid volume / reactor volume (Liter) (% fill) Reaction time (hours) I AlkOx 120 °C 4 h 75 % Chips 20 Na2CO3 No 120 0.1(* 30 / 40 (75 %) 4 CatOx 120 °C 4 h 75 % Chips 20 Na2CO3 Yes 120 0.1 (* 30 / 40 (75 %) 4 AlkOx 140 °C 4 h 25 % Chips 20 Na2CO3 No 140 0.7(* 10 / 40 (25 %) 4 CatOx 140 °C 4 h 25 % Chips 20 Na2CO3 Yes 140 0.7 (* 10 / 40 (25 %) 4 II CatOx 120 °C 20 h 50 % Wood meal 20 Na2CO3 Yes 120 0.2 1 / 2 (50 %) 20 CatOx 120 °C 20 h 10 % Wood meal 20 Na2CO3 Yes 120 2.1 0.2 / 2 (10 %) 20 AlkOx 120 °C 20 h 50 % Wood meal 20 Na2CO3 No 120 0.2 1 / 2 (50 %) 20 AlkOx 120 °C 20 h 10 % Wood meal 19 Na2CO3 No 120 2.1 0.2 / 2 (10 %) 20 III AlkOx Ca(OH)2 120 °C 20 h 10 % Saw dust 20 Ca(OH)2 No 120 2.1 0.1 / 1 (10 %) 20 AlkOx NaOH 120 °C 20 h 10 % Saw dust 20 NaOH No 120 2.1 0.1 / 1 (10 %) 20 AlkOx KOH 120 °C 20 h 10 % Saw dust 20 KOH No 120 2.1 0.1 / 1 (10 %) 20 AlkOx Na2CO3 120 °C 20 h 10 % Saw dust 20 Na2CO3 No 120 2.1 0.1 / 1 (10 %) 20 IV 1-4 Saw dust 20 Na2CO3 Yes 120 2.1 1 / 2 (50 %) 1, 4, 8 or 20 * oxygen loading at start; pressure was kept constant by feeding excess O2.
- 128. II/6 PEER-REVIEWED ARTICLE bioresources.com Rovio et al. (2012). “Oxidation for wood fractionation,” BioResources 7(1), 756-776. 761 After sample injection, 15 mM sodium hydroxide flowed through the column for 2 min, and from 2 to 36 min 100% of ultra-pure water was run isocratically. A solution of 300 mM NaOH was added to the column effluent before the PAD cell at a flow rate of 0.4 mL min-1 . The column was washed with a solution of 100 mM NaOH and 300 mM sodium acetate for 3 min and then washed with 300 mM sodium hydroxide for 4 min. The flow rate was set to 1 mL min-1 , column temperature to 30°C, and sample temperature to 15°C. Injection volume was 25 µL. The resulting data were processed using Chromeleon software. Carboxylic acids Separations of the carboxylic acid standards and liquid fractions of the oxidation samples were performed using a P/ACE MDQ capillary electrophoresis instrument (Beckman-Coulter, Fullerton, CA, USA) equipped with a photodiode array UV detector. The background electrolyte (BGE) was fine tuned to enhance the resolution between closely migrating acids and for quantifying the C(5)-C(6) hydroxy acids (Rovio et al. 2010). Briefly, anionic compounds were analyzed using the BGE solution containing 20 mM 2,3-pyrazine dicarboxylic acid, 30 mM tricine, 2 mM BaCl2, 0.5 mM cetyltrimethyl- ammonium bromide, and 2 M urea at a pH of 8.06 (± 0.02, adjusted with triethanol- amine). Uncoated fused silica capillaries with a 50 µm I.D. and a length of 100/110 cm (effective length/total length) were employed in the analyses. The samples were injected at a pressure of 0.5 psi (34.5 mbar), and the injection time was set to 20 s. The capillary and samples were thermostatted to +15 °C. The detection wavelength was set to 281 nm using an indirect detection mode. Before the measurements, the new capillaries (from Teknolab Trollåsen, Norway) were conditioned by sequentially rinsing with 0.1 M sodium hydroxide, 0.1 M HCl, and ultra-pure water. The capillaries were rinsed with each solution for 20 min and then with an electrolyte solution for 5 min. Between analyses, the capillaries were rinsed with 0.1 M HCl for 3 minutes, 0.1 M NaOH for 1 min, and the electrolyte solution for 5 min, all under a pressure of 20 psi. Prior to CE analyses, the oxidation samples were diluted to a ratio of 1:10 (v/v) with ultra-pure water. Lignin content The content of the dissolved, aromatic lignin was measured from the soluble fraction UV spectroscopy at 280 nm using an absorptivity of 20 L g-1 cm-1 (Tamminen and Hortling 2001). Total organic carbon Total organic carbon (TOC, SFSEN 1484:1997) was determined at Kymen Laboratorio Oy (Finland). Size exclusion chromatography (SEC) Size exclusion chromatography (SEC) was performed in a Waters HPLC system. Two Ultrahydrogel (pore sizes of 250 Å and 120 Å, Waters Assoc. USA) columns were linked in series. The detection was carried out with an UV at 280 nm and refractive index (RI) detectors in series. Isocratic chromatography was performed using 0.1 M NaNO3 as the eluent, which was pumped through the columns at a rate of 0.5 mL/min. Each sample
- 129. II/7 PEER-REVIEWED ARTICLE bioresources.com Rovio et al. (2012). “Oxidation for wood fractionation,” BioResources 7(1), 756-776. 762 was diluted with a ratio of 1:10 (v/v) with eluent and filtered with 0.45 um disposable filters prior to the injection of 50 µL into the SEC system. The analysis temperature was 30 °C. The molecular weight distributions (MWD) and the average number and weight average molecular weights (Mn, Mw) of the lignin and polysaccharides were calculated using pullulan standards (5900-212000 Da). The system was controlled and data were analysed using Empower 2 software. RESULTS AND DISCUSSION General Composition of the Fractions Catalyzed alkaline oxidation produces fiber fractions with high enzymatic hydrolysability (Hakola et al. 2010). The outcome of both the alkaline and catalyzed reactions can be compared by examining the mass loss, i.e. the proportion of the dissolved lignin and hemicellulose from the original dry weight of the wood. During the decomposition of wood under oxygen-alkaline conditions, with or without a catalyst, the wood material was divided into two fractions: the solid cellulose rich fraction and the water soluble fraction. Figures 1a, 1c, and 1e present the percentage portions of both the solid and soluble fractions obtained using different reaction condi- tions. A more detailed distribution of the monosaccharides, small aliphatic carboxylic acids, and lignin in the soluble fraction is depicted in Figs. 1b, 1d, and 1f. Catalytically assisted decomposition of wood under oxygen-alkali conditions increased the degradation of spruce compared to its counterparts treated in alkali without catalyst (Figs. 1a and 1c). The resulting material from batch reactor was, however, inhomogeneous, containing also unreacted particles, and further scale-up of the technique is needed in order to evaluate its feasibility in full scale industrial process. A clear difference between non-catalyzed and catalyzed treatment can be observed in the 120°C experiment, where the portions of soluble fractions were 14% and 28%, respectively. By increasing the reaction temperature to 140°C and lowering the filling ratio of the reactor, the difference became smaller, but still the catalyzed reaction was more efficient. Increasing the reaction time to 20 h leveled out the dissolution efficiency between the AlkOx and CatOx treatments, while the proportions of fiber fraction and dissolved fraction became similar (Fig. 1c). The loading of the reactor had a small effect on fractionation. The lower filling ratio, 10% versus 50%, enhanced the dissolution, however. Over 35% of the dry matter was solubilized in the alkaline or catalytic oxidation when the loading was 10% or 25% of the reactor volume (Figs. 1a and 1c). The higher oxygen volume as a result of lower liquid volume likely enhanced the oxidation reaction and solubilisation. Lower liquid volumes might improve the mixing of the suspension, decrease the concentration gradients, and increase the solubilisation of oxygen through- out the reaction. These observations need to be taken into account when scaling up the method. The proportions of monosaccharide, carboxylic acid, and lignin in the soluble fractions followed the trend of the total soluble fraction (Fig. 1b). In the four-hour experiments, the proportions of chemical components in the liquid fraction were 6 to 12%, 10 to 20%, and 1 to 4% of the raw material dry weight for lignin, carboxylic acid,
- 130. II/8 PEER-REVIEWED ARTICLE bioresources.com Rovio et al. (2012). “Oxidation for wood fractionation,” BioResources 7(1), 756-776. 763 and monosaccharides, respectively. The increase in the reaction time from 4 h to 20 h had greater affect on the proportion of dissolved lignin and monosaccharides, while the proportions nearly doubled when compared to the shorter treatment (Fig. 1d). Catalyt- ically assisted treatment produced only more dissolved monosaccharides than the alkaline treatment, but its effect on lignin solubility was not as clear. Most likely, the dissolution of lignin was more affected by the increased reaction time coupled with the action of alkali. Quite similar solubilisation was obtained with the three bases with the exception of Ca(OH)2 (Fig. 1e). Among these three experimental series the highest yield of dissolved substances was 43%, which was achieved with 0.25 M Na2CO3. The cumuli- tive amount of the recognized dissolved compounds was about 30% (Fig. 1f), which means that 3 to 11% of the dissolved matter remained unidentified. The unexplained part may include other degradation products that cannot be determined with the applied methods, and unidentified carboxylic acids. In addition, among the uncalculated compounds is carbon dioxide, which is the final degradation product of carboxylic acids (Suzuki et al. 2006). The fractionation with calcium hydroxide was more complicated to study, since calcium hydroxide was not soluble in the applied concentration and conditions. This also explains the high yields of fiber fraction in the experiments (Fig. 1e). The unrealistic high result can be explained with undissolved Ca(OH)2, oxalic acid which precipitates as calcium oxalate, and undissolved lignin, which was not monitored among the identified compounds in the dissolved fraction (Fig. 1f). The differences in chemical compositions of the dissolved fractions of NaOH, KOH, and Na2CO3 in 20 hours of treatment were minimal. Carbohydrates The carbohydrate content of the spruce raw material was 66 and 65% for the chips and saw dust, respectively. A minor part (1 to 6% of the raw material dry weight) of the carbohydrates in the raw material was found in the soluble fractions, whereas most of the carbohydrates remained in the solid fraction after the fractionation (Fig. 2). The carbo- hydrate content of the solid fraction after fractionation was between 40 and 60% of the raw material dry weight, and the respective carbohydrate yield was between 66 and 92% of the raw material carbohydrates. The solubilized fraction contained only oligosac- charides and no monosaccharides. Most likely, the reducing ends of the soluble carbohydrates were oxidized under experimental conditions, as the soluble fraction gave no response to reducing sugars assay (data not shown). In general, the absence of monosaccharides and the low concentration of solubilized oligosaccharides may be explained by their further oxidation into carboxylic acids. As the total carbohydrate yield varied between 72% and 99% of the raw material carbohydrate content, it is obvious that carbohydrate yield losses occurred. The composition of the dissolved carbohydrate material is basically similar to that reported earlier for wet oxidized spruce (Palonen et al. 2004), i.e. xylose, mannose, arabinose, galactose, and glucose, illustrating that mainly the hemicelluloses (arabino- xylan and galactoglucomannan) dissolve while the cellulose remains in the fiber fraction (Fig. 2).
- 131. II/9 PEER-REVIEWED ARTICLE bioresources.com Rovio et al. (2012). “Oxidation for wood fractionation,” BioResources 7(1), 756-776. 764 Fig. 1. Percent distribution of fibre fraction and soluble fraction after oxidation experiments in series I (A), II (C), and III (E). Portions of soluble carbohydrates, lignin, and small carboxylic acids found in dissolved fraction in series I (B), II (D), and III (F). The details of fractionation conditions of the samples are described in Table 1.
- 132. II/10 PEER-REVIEWED ARTICLE bioresources.com Rovio et al. (2012). “Oxidation for wood fractionation,” BioResources 7(1), 756-776. 765 In addition, the figures show that under more severe conditions, i.e. with a catalyst, 20 hours of reaction time, and a higher temperature of 140 °C, there was a higher carbohydrate content in the liquid fraction. The shorter reaction time (4 h), lower temperature (120 °C), and Na2CO3 as an alkali all resulted in lower carbohydrate dissolution. The oxidations were carried out with either 0.25 M Na2CO2, NaOH, KOH, or Ca(OH)2, producing highly alkaline conditions (pH 12) early in the reaction. During oxidation part of the alkali was consumed and after 20 hours the pH was usually between 7.5 and 8.5. The dissolution of arabinoxylan was higher than that of galactoglucomannan in all experiments, but an especially clear difference was seen between the CatOx and AlkOx treatments (Figs. 2a and 2c). In addition, the comparison of alkali sources showed that the dissolution of galactoglucomannan was enhanced when Ca(OH)2, KOH, or NaOH was used. The lower mannose content after oxidation with Na2CO3, however, can be explained by the degradation of galactoglucomannan, derived monosaccharides, and more pronounced acid production (Figs. 1f and 2e). The monosaccharide content and composition of the fiber fraction were also determined (Figs 2b, d, f). As expected, the majority of this fraction consisted of cellulose, but portions of 17-22%, 14-19%, and 12- 18% of hemicelluloses, galactoglucomannan and arabinoxylan, were also observed in series I-III, respectively. Table 2 compiles the results of the carbohydrate yields in the solid and liquid fractions. The total monosaccharides yield reveals clearly how much carbohydrate was lost, i.e. it gives information about the magnitude of the degradation of carbohydrates in the acids. This range varied between 3 and 28 percent, with the least degradation occurring with AlkOx, at 120°C, for 20 h, and a filling ratio of 50%, and the most degradation occurring with AlkOx, Na2CO3, at 120°C, for 20 h, and a filling ratio of 10%, indicating that the high filling ratio of the reactor hindered the oxidation reactions. Lignin Alkaline SEC is a common relative method for determining the molecular weight of lignins, as reviewed by Baumberger et al. (2007). The separation was calibrated here with pullulan standards in accordance with quoted literature, which can be considered as adequate for comparative purposes. SEC analyses of the dissolved fractions of series I and II reveal that the dissolved fraction contains both lignin and carbohydrate-based oligomeric substances having rather low molecular weights monitored with UV and RI detectors (Table 3). The four-hour reaction time with AlkOx treatment and a filling ratio of 75 % was not sufficient to dissolve lignin or hemicelluloses completely, and it was supposed that only small molecular weight lignin or oligosaccharides were dissolved; therefore, the molecular weight results obtained from this experiment are not comparable with its CatOx counterpart. The CatOx fractionation produced smaller lignin and polysaccharide oligomers than the AlkOx treatment during the 20 hour oxidation. This difference is not as clear as in the four-hour fractionations, except in the case of 140 °C. In addition, the polydispersities were lower in the CatOx samples than in the AlkOx samples in the 20 hour treatments. This indicates relatively narrow molecular weight distributions of dissolved substances.
- 133. II/11 PEER-REVIEWED ARTICLE bioresources.com Rovio et al. (2012). “Oxidation for wood fractionation,” BioResources 7(1), 756-776. 766 Table 2. Carbohydrate Fraction Yield and Total Yield SAMPLE Fraction GLU MAN XYL GAL ARA Total mono- saccharides AlkOx 120°C 4 h 75% solid 103 59 86 47 77 92 liquid 0.1 1 6 11 14 1 total 103 60 92 58 90 93 CatOx 120°C 4 h 75% solid 89 56 61 30 49 78 liquid 0.7 6 20 21 30 4 total 89 62 81 50 79 82 AlkOx 140°C 4 h 25% solid 99 41 71 29 50 83 liquid 0.2 1 12 14 26 2 total 99 42 83 43 76 86 CatOx 140°C 4 h 25% solid 100 48 54 20 38 83 liquid 1.1 7 28 21 37 6 total 101 55 82 42 74 89 CatOx 120°C 20 h 50% solid 105 61 46 11 24 88 liquid 0.9 11 40 37 52 8 total 106 72 86 49 75 96 CatOx 120°C 20 h 10% solid 106 55 41 6 19 87 liquid 1.1 14 42 37 52 9 total 107 69 83 43 71 96 AlkOx 120°C 20 h 50% solid 109 61 64 14 37 93 liquid 0.3 3 24 21 41 4 total 110 64 88 36 78 97 AlkOx 120°C 20 h 10% solid 102 49 57 8 30 85 liquid 0.3 3 25 25 47 5 total 103 52 82 33 78 90 AlkOx Ca(OH)2 120°C 20 h 10% solid 93 26 31 5 20 70 liquid 2.0 15 36 18 51 9 total 95 40 66 23 71 78 AlkOx NaOH 120°C 20 h 10% solid 84 35 36 12 24 67 liquid 1.0 11 31 38 59 8 total 85 46 67 50 83 74 AlkOx KOH 120°C 20 h 10% solid 88 41 36 40 26 70 liquid 1.4 15 31 40 53 15 total 89 56 68 79 78 85 AlkOx Na2CO3 120°C 20 h 10% solid 83 36 40 11 27 66 liquid 0.5 5 25 26 56 5 total 84 41 66 37 83 72 * Yield is expressed as % of the raw material carbohydrate content
- 134. II/12 PEER-REVIEWED ARTICLE bioresources.com Rovio et al. (2012). “Oxidation for wood fractionation,” BioResources 7(1), 756-776. 767 Fig. 2. Monosaccharide composition in the soluble fraction (A, C, and E) and the fibre (B, D, and F) fraction after acid hydrolysis in series I - III. The carbohydrate composition of spruce chips, wood meal, and saw dust are also presented in Figs. B, D, and F, respectively. The composition was determined by HPAEC-PAD after acid hydrolysis and presented as % (w/w) of monosaccharides in the original raw material. The details of the fractionation conditions are shown in Table 1. Comparison of the SEC chromatograms obtained with Ultrahydrogel columns using UV and RI detectors reveals that polysaccharides are dissolved as small fragments of oligosaccharides with a Mw range of 600 to 1600. The AlkOx treatment favored the dissolution of larger fragments (Fig. 3). On the other hand, CatOx fractionation produced even larger oligosaccharide fragments with a Mw of 1600 to 1900, monitored with the RI detector. Moreover, for the 20 h experiments, the CatOx treatment dissolved more lignin with a Mw between 3900 and 4000 than the corresponding AlkOx treatments, as seen in the chromatograms monitored with the UV detector.
- 135. II/13 PEER-REVIEWED ARTICLE bioresources.com Rovio et al. (2012). “Oxidation for wood fractionation,” BioResources 7(1), 756-776. 768 Comparison of the precipitation yields of lignin in series I and II indicates that the yields were larger in the AlkOx samples than in the CatOx samples for both the 4 h and 20 h fractionations. At longer fractionations these correlated well with the amount of dissolved lignin as presented in Fig. 1d. The lowest yields, 17.6 % and 13 %, were obtained in the CatOx samples in which the filling ratios were 25 % and 10 %, respect- tively. This indicates that the co-operation of catalyst and oxygen promoted lignin degradation. Moreover, the lower yield correlates with the higher amount of oxalic acid, which is the oxidation product of lignin (Fig. 4). Fig. 3. SEC chromatograms of series I (left) and II (right) obtained from both the RI and UV detectors. The details of the fractionation conditions are described in Table 1. Acids The acidic pretreatment of lignocellulosic materials forms a variety of degradation products, which have an inhibiting effect on both the enzymatic action and the fermen- tation by yeast or bacteria. Among those compounds are phenols, aromatic aldehydes, soluble lignin compounds, and organic acids, such as formic and acetic acids (Sassner et al. 2006; Mosier et al. 2005; Chen et al. 2006; Hendriks and Zeeman 2009). The AlkOx and CatOx processes produce considerable amounts of the mentioned aliphatic carboxylic acids; therefore it is justified to study the nature and concentrations of the acidic compounds formed during the fractionations. Both the AlkOx and CatOx treatments of spruce chips showed a release of lignin along with hemicelluloses. The acids were formed via the degradation of the phenolic structures of lignin and the degradation of hemicelluloses. The reactions are similar to those reactions proposed to occur in conventional oxygen delignification (Kuitunen et al. 2011; Kadla et al. 1999; Gierer 1986). The hemicelluloses are degraded through peeling reactions and chain cleavage.
- 136. II/14 PEER-REVIEWED ARTICLE bioresources.com Rovio et al. (2012). “Oxidation for wood fractionation,” BioResources 7(1), 756-776. 769 Table 3. Average Molecular Weights (Mn, Mw), Polydispersities (Mw/Mn), and Precipitation Yields. * Sample Mw (g mol-1 ) Mn (g mol-1 ) PD (Mw/Mn) (g mol-1 ) Mw (g mol-1 ) Mn (g mol-1) PD (Mw/Mn) (g mol-1 ) Yield of precipitation (%) NaNO3 /UV pullulan std NaNO3/ RI pullulan std AlkOx 120°C 4 h 75% 4 000 3 000 1.4 2 300 1 100 2.1 20.3 CatOx 120°C 4 h 75% 10 600 2 600 2.1 4 200 1 400 3 20.4 AlkOx 140°C 4 h 25% 7 100 3 300 2.1 3 900 1 300 3 33.9 CatOx 140°C 4 h 25% 4 100 2 600 1.6 3 300 1 300 2.5 17.6 CatOx 120°C 20 h 50% 5 700 2 600 2.2 5 200 1 600 3.3 26.7 CatOx 120°C 20 h 10% 4 500 2 500 1.8 4 100 1 600 2.6 13.0 AlkOx 120°C 20 h 50% 9 000 3 300 2.7 6 300 1 600 3.9 38.7 AlkOx 120°C 20 h 10% 9 700 3 500 2.8 6 100 1 600 3.8 40.2 * Precipitation yield was calculated using an absorptivity value of 20 L g-1 cm-1 . The reactions of both lignin and hemicellulose with molecular oxygen, perhydroxyl/superoxide radicals (HO2 * /O2 *- ), hydroperoxide anions (HOO- ), and hydrox- yl radical/oxide anion radicals (HO* /O*- ) cause degradation of those macromolecules and the formation of small aliphatic carboxylic acids (Gellerstedt et al. 1980; Young and Gierer 1976; Bailey and Dence 1969; Kuitunen et al. 2011). Capillary zone electrophoresis (CZE) with indirect UV detection was used for the simultaneous determination of inorganic anions and C(1)-C(6) aliphatic carboxylic acids liberated in the lignocellulosic processed samples (Rovio et al. 2010). From the analytical point of view, the separation of the aliphatic carboxylic acids is a challenging task, since they have similar mass-to-charge ratios and similar molecular structures. The optimized method was applied for determining the carboxylic acids in the soluble fractions of the oxidation treatments. Both volatile and non-volatile carboxylic acids were detected. The volatile acids include formic and acetic acids, while the non-volatile acids include dicarboxylic acids and hydroxy acids. The dominating acids in the experiments were formic, acetic, oxalic, and glycolic acid both in alkali-oxidized and catalytically-oxidized samples. Moreover, malonic, fumaric, succinic, malic, lactic, 3-hydroxypropionic, 2-OH-butyric, and 2.5-di- OH-pentanoic acids were found in smaller quantities (Fig. 4). In general, the acid profiles were similar in all experiments with only small variations in the concentrations of
- 137. II/15 PEER-REVIEWED ARTICLE bioresources.com Rovio et al. (2012). “Oxidation for wood fractionation,” BioResources 7(1), 756-776. 770 individual acids. Most likely, the reason for this is the alkaline oxidative environment which promotes similar reactions despite the nature of the alkali. The most crucial reaction parameters with respect to the formation of the acids were the presence of a catalyst, reactor filling, and reaction temperature. The catalyt- ically assisted oxidation produced more acid regardless of the reaction temperature. Catalyzation had the greatest affect in the formation of formic acid and oxalic acid. Oxalic acid has been generally recognized as a degradation product of lignin (Kuitunen et al. 2011; Bailey and Dence 1969). The total acid formation was enhanced both in the AlkOx and CatOx experi- ments at a temperature of 140 °C. The acid production in both treatments was balanced with a reaction time of 20 hours; however, the lower reactor filling (10% instead of 50%) had a positive effect on wood degradation and oxidation of dissolved lignin and hemicelluloses, leading to the increase in acid formation from ca. 15% to 20% (Fig. 4). Comparison of bases showed similar acid productions, but slightly more oxalic and minor acids were formed in samples treated with Na2CO3, NaOH, and KOH than with Ca(OH)2, which in turn favors the generation of formic and acetic acids. The composition of the major acids was similar to what was observed in the wet oxidation of wheat straw (Bjerre et al.1996) and the mixture of clover and ryegrass (Martín et al. 2008). In addition, Samuelson and Sjöberg (1976) studied spent liquor obtained from oxygen bicarbonate cooking of birch and found the same major acids: formic, acetic, oxalic and glycolic acids. Shuai et al. (2010) observed that dilute acid and SPORL pretreatments produced formic acid levels of 7.4 and 1.9 g L-1 , respectively, whereas the amount of formed acetic acid were 5.3 and 2.7 g L-1 , respectively. Moreover, Larsson et al. (1999) reported, that steam explosion of acid treated spruce chips produced 1.6 g L-1 of formic acid and 2.4 g L-1 of acetic acid. It should be noted that acidic pretreatments produce levulinic acid, which is a potent inhibiting compound too (Palmqvist and Hahn-Hägerdal, 2000). In our study the concentrations of formic acid and acetic acid varied within the ranges 0.8 to 2.5 g L-1 and 0.9 to 3.3 g L-1 , respectively. Fig. 4. Distribution of four main aliphatic acids and sum of minor acids found in the soluble fraction in series I - III after oxidation. Acid composition was determined by capillary electrophoresis as described in Chapter Methods and presented as % acid from the original raw material. The details of fractionation conditions are described in Table 1.
- 138. II/16 PEER-REVIEWED ARTICLE bioresources.com Rovio et al. (2012). “Oxidation for wood fractionation,” BioResources 7(1), 756-776. 771 Kinetics of Catalyzed Oxygen Delignification The progress of the reaction was monitored by analyzing the dissolved material as a function of time. Analysis of the samples of the CatOx reaction mixture at predetermined intervals showed increased dissolution of hemicelluloses after one hour of oxidation (Fig. 5, center). After four hours of treatment the carbohydrates were solubilized more slowly. The content of carbohydrates in the fiber fraction originating from hemicellulose de- creased by half of its original concentration during 20 hours of the reaction. The cellulose was not dissolved significantly, as the concentration of glucose decreased by only five per cent (Fig. 5, left). Almost half of the acetic acid and formic acid was generated during the first hour, and such generation continued over the course of the oxidation. The formation of glycolic, oxalic, and 2,5-di-OH-pentanoic acids was more vigorous during the first four hours and became slower thereafter (Fig. 5, right). The dissolved lignin content was estimated based on UV analysis at 280 nm. For aromatic lignin, an absorptivity of 20 L g-1 cm-1 was used. In addition, the absorptivities of the isolated (unpurified) lignins were determined. Table 4 shows the yields of dissolved lignin based on the UV results. The content of dissolved aromatic lignin followed a similar dissolution pattern as the hemicelluloses, while the majority of the dissolution of lignin occurred during the first four hours of the reaction, and only minor changes were seen between 4 and 20 hours. About half of the dissolved lignin was precipitated by the acidification, and this proportion was not affected by the stage of the reaction. The precipitation yields in the kinetic series were larger than those of the AlkOx and CatOx samples (Table 3). The formation of carboxylic acids was supposed to occur via both the oxidation of dissolved hemicelluloses and the degradation of aromatic lignin structures. It was presumed that the oxidation of lignin affected the absorptivity of the dissolved lignin. Surprisingly, a decrease in the absorptivity was not observed, but rather a slight increase. Part of the fiber components may have dissolved in the form of degradation products and were not detected by the methods applied; therefore, the total content of dissolved organic material was reported as total organic carbon (TOC), and the contributions of lignin and carbohydrates to TOC were calculated using carbon contents of 0.60, 0.44, and 0.40 for lignin, carbohydrates, and carboxylic acids, respectively (Laine and Tamminen 2002). Table 4. Precipitated Lignin, Dissolved Lignin, Precipitation Yield, and Absorptivity Time (h) Precipitated lignin, mg g-1 Dissolved lignin, mg g-1 Precipitation yield, % Absorptivity of lignin precipitate, L g-1 cm-1 1 44,5 100.4 44 17.5 4 87.3 185.6 47 20.1 8 78.0 195.2 40 22.6 20 90.1 194.0 46 22.1 *Weight is based on dry wood. Precipitation yield was calculated using an absorptivity value of 20. Absorptivity of the precipitate was at 280 nm.
- 139. II/17 PEER-REVIEWED ARTICLE bioresources.com Rovio et al. (2012). “Oxidation for wood fractionation,” BioResources 7(1), 756-776. 772 Fig. 5. The development of concentrations of different monosaccharides under 20 hours of CatOx oxidation in the fiber fraction (left) and in the soluble fraction (center). Formation of carboxylic acids under the same conditions is presented in the right figure. Details of fractionation conditions of the samples are described in Table 1, samples IV 1-4. As seen in Fig. 6, lignin accounted for more than half the TOC, whereas the carbohydrates had only a small contribution. The contribution of carboxylic acids was approximately three times than that of the carbohydrates. About 20 to 25 percent of the TOC was composed of unidentified material. Fig. 6. Total dissolved organic carbon and its distribution between lignin, carbohydrates, and other organic material. Details of fractionation conditions of the samples are described in Table 1, samples IV 1-4.
- 140. II/18 PEER-REVIEWED ARTICLE bioresources.com Rovio et al. (2012). “Oxidation for wood fractionation,” BioResources 7(1), 756-776. 773 CONCLUSIONS Alkaline oxidation can be catalyzed by a copper-phenanthroline complex to enhance the fractionation of lignocellulosic material and decrease the reaction time. This catalyzed oxidation (CatOx) process was studied in a laboratory scale reactor in order to understand the chemistry taking place, in particular the role of the catalyst, using Spruce wood as the raw material. 1. During the catalytic pretreatment the wood chips lost their macroscopic structure, and more than 28% of the dry wood material was solubilized during the 20 hours catalytic pretreatment. Both catalytic and alkaline oxidations resulted in a cellulose-rich fiber material. The solid material contained over 83% of the original total carbohydrates after four hours of catalytic oxidation at 140 °C, which was a similar value to that of a non-catalyzed counterpart. An extended pretreatment time did not alter the total yields of the fiber fraction, being 87 to 88% in CatOx and 85 to 93% in AlkOx samples. The applicability of the fiber fraction for bioethanol production was shown in earlier studies (Hakola et al. 2010). 2. The effectiveness of the catalyst in the dissolution of lignocellulosic material was also observed by analyzing the composition of the dissolution fraction. The closer examination of dissolved compounds revealed that the catalytically assisted oxidation was more efficient than its alkaline counterpart in the dissolution of carbohydrates, especially of those originating from glucomannan and arabino- xylan, which can be verified by analyzing the proportions of mannose and xylose in the dissolved fraction. In addition, the CatOx process was more efficient with a low reactor filling ratio. 3. The catalyst increased both the degradation of dissolved monosaccharides and the degradation of lignin to small aliphatic carboxylic acids. This observation was more pronounced with reactor filling ratios below 25%. The acid profile was quite similar in the presence and absence of the catalyst, although some variations in the concentration of individual acids were observed. Comparison of the four different bases showed that NaOH, KOH, and Na2CO3 were comparable in dissolution efficiencies. ACKNOWLEDGMENTS Financial support from the Academy of Finland (project CaDeWo, 124500) and the Tekes BioRefine program (project SugarTech) are gratefully acknowledged. Tiina Riekkola is thanked for helping with oxidation treatments. The authors thank Olli Jauhiainen, and Ulla Vornamo for excellent technical assistance. Dr. Tiina Liitiä and Eila Turunen are thanked for performing SEC analysis.
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- 145. PUBLICATION III A novel alkaline oxidation pre-treatment for spruce, birch and sugar cane bagasse In: Bioresource Technology 140, pp. 414–420. Copyright 2013 Elsevier. Reprinted with permission from the publisher.
- 147. III/1 A novel alkaline oxidation pretreatment for spruce, birch and sugar cane bagasse Anne Kallioinen a,⇑ , Maija Hakola b , Tiina Riekkola b , Timo Repo b , Markku Leskelä b , Niklas von Weymarn a,1 , Matti Siika-aho a a VTT Technical Research Centre of Finland, P.O. Box 1000, FI-02044 VTT, Finland b University of Helsinki, Laboratory of Inorganic Chemistry, A. I. Virtasen Aukio 1, P.O. Box 55, FI-00014 University of Helsinki, Finland h i g h l i g h t s Alkaline oxidation is an efficient pretreatment method for various raw materials. Clearly lower enzyme dosages could be used as compared to steam exploded materials. An ethanol yield of 80% could be obtained with both bagasse and spruce in 1–3 days. a r t i c l e i n f o Article history: Received 30 January 2013 Received in revised form 25 April 2013 Accepted 25 April 2013 Available online 6 May 2013 Keywords: Lignocellulose Pretreatment Enzymatic hydrolysis Alkaline oxidation Ethanol a b s t r a c t Alkaline oxidation pretreatment was developed for spruce, birch and sugar cane bagasse. The reaction was carried out in alkaline water solution under 10 bar oxygen pressure and at mild reaction temperature of 120–140 °C. Most of the lignin was solubilised by the alkaline oxidation pretreatment and an easily hydrolysable carbohydrate fraction was obtained. After 72 h hydrolysis with a 10 FPU/g enzyme dosage, glucose yields of 80%, 91%, and 97%, for spruce, birch and bagasse, respectively, were achieved. The enzyme dosage could be decreased to 4 FPU/g without a major effect in terms of the hydrolysis perfor- mance. Compared to steam explosion alkaline oxidation was found to be significantly better in the con- ditions tested, especially for the pretreatment of spruce. In hydrolysis and fermentation at 12% d.m. consistency an ethanol yield of 80% could be obtained with both bagasse and spruce in 1–3 days. Ó 2013 Elsevier Ltd. All rights reserved. 1. Introduction Targets to decrease greenhouse gas emissions and dependency of fossil fuels, amongst others, have increased the interest for fuel ethanol in the industrialised world. Currently ethanol is mainly produced from the sugars derived from sucrose or starch, which are also important constituents of food and feed. A more sustain- able way to produce ethanol would be to use cellulose as the main carbohydrate raw material. The most promising cellulosic raw materials, in terms of industrial scale, include the various side- streams generated when sugar cane, sugar beet, corn, grains (e.g. wheat) and wood are processed, dedicated energy crops as well as industrial and municipal wastes. However, cellulose is typically tightly bound to hemicelluloses and lignin forming a matrix struc- ture that is very difficult to break down. Consequently, producing monomeric sugars from cellulose and hemicellulose at high yields is far more difficult than deriving fermentable sugars from sucrose- or starch-containing crops, e.g. sugar cane and corn. The ethanol production concept relevant for this study consists typically of three main processing steps: Firstly, the native, poly- meric carbohydrates are hydrolysed by acid or enzymatic treat- ment into their respective monomer sugars. Secondly, the monomer sugars are fermented to ethanol, and thirdly, ethanol is separated and purified (dehydrated). As such, enzymatic hydrolysis is inefficient without first making the lignocellulosic biomass ma- trix more accessible to enzymes by a so-called ‘pretreatment’. Pre- treatments can affect biomass in many different ways. Desirable outcomes of pretreatment include liberation of cellulose from the matrix, decrease in crystallinity of cellulose and increase in acces- sible surface area and pore size of cellulose (Gong et al., 1999; Sun and Cheng, 2002). An effective pretreatment should, at the same time, avoid degradation or loss of carbohydrates, avoid formation of inhibitory by-products for subsequent hydrolysis and fermenta- tion and be cost-effective. Various pretreatment methods have been recently reviewed by Mosier et al. (2005), Galbe and Zacchi (2007), Yang and Wyman (2008), Hendriks and Zeeman (2009) and Alvira et al. (2010). Over 0960-8524/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.biortech.2013.04.098 ⇑ Corresponding author. Tel.: +358 20 722 7187; fax: +358 20 722 7071. E-mail address: anne.kallioinen@vtt.fi (A. Kallioinen). 1 Present address: Metsä Fibre Ltd., P.O. Box 30, FI-02020 Metsä, Finland. Bioresource Technology 140 (2013) 414–420 Contents lists available at SciVerse ScienceDirect Bioresource Technology journal homepage: www.elsevier.com/locate/biortech
- 148. III/2 the last few decades, several pretreatment methods, especially thermochemical methods, have been shown to be promising for a variety of feedstocks. A comprehensive comparison study of sev- eral state-of-the-art pretreatment technologies has been reported by the CAFI consortium (Wyman et al., 2005; Eggeman and Eland- er, 2005; Kumar et al., 2009; Wyman et al., 2011). The 2005 CAFI study concluded that all five methods studied, i.e. dilute acid, hot water, ammonia fiber explosion (AFEX), ammonia recycle percola- tion (ARP) and lime, were effective in corn stover pretreatment and produced similar overall sugar yields (Wyman et al., 2005). On the other hand, methods applying alkaline pH provided a possibility to decrease the enzyme dosage. Alkaline pretreatments have one advantage compared to acidic pretreatments and hot-water pretreatments, i.e., alkalis readily re- move lignin and xylan side chains, resulting in a dramatic increase in enzymatic saccharification (Sun and Cheng, 2002; Chang and Holtzapple, 2000; Nlewem and Thrash, 2010). In addition, alkaline pretreatments remove acetyl substitutions on the hemicellulose. The addition of oxygen to the reaction mixture greatly improves the delignification of biomass, especially with highly lignified materials (Chang et al., 2001). Particle size reduction is often ap- plied before alkaline treatment of woody materials like poplar (Sierra et al., 2009; Chang et al., 2001) or wet oxidation of spruce (Palonen et al., 2004). In this study, a novel wet oxidation pretreatment at high al- kali concentration (‘alkaline oxidation’) was developed for pre- treatment of spruce, bagasse and birch and the effects of the pretreatment on the chemical composition and enzymatic hydro- lysis were evaluated. The pretreated materials were compared in terms of hydrolysis and fermentation efficiency to those pro- duced using either catalytic oxidation or steam explosion pretreatments. 2. Methods 2.1. Raw materials Norway spruce (Picea abies) chips were obtained from UPM- Kymmene Kaipola plant and birch (Betula sp.) chips from Kuusan- niemi pulp mill. Sugar cane bagasse from the harvest of 2007 was obtained from Illovo Sugar, Malawi. Bagasse was manually cut 5 cm length. Norway spruce saw dust (2 mm  2 mm  10 mm) was provided by VTT Jyväskylä, Finland and was only used in sep- arately specified experiments. 2.2. Pretreatments Alkaline oxidation (AlkOx) of spruce chips, bagasse, or birch chips was carried out by the method of Hakola et al. (2010) using 0.25 M Na2CO3 but without the Cu-phenanthroline catalyst at 120 °C for 20 h. 10 bar oxygen pressure was used in all the experiments. For the comparison of AlkOx and catalytic oxida- tion (CatOx) spruce saw dust was preteated at 120 °C or 140 °C for 5 or 20 h with or without the Cu-catalyst. To study the effect of different alkaline agents in AlkOx, 0.25 M NaOH, KOH or Ca(OH)2 were applied instead of Na2CO3 with spruce saw dust as a raw material. At the end of the reaction the solid material was separated by vacuum filtration. Steam explosion (SE) was carried out at different conditions depending on the raw material. Raw material was immersed into dilute sulphuric acid (0.5% or 2% w/w) to get final acid dosages of 0.4% (w/w acid per dry biomass) for spruce, 0.14% w/w for birch and 0.5% w/w for bagasse, kept at room temperature for 30 min, and the excess acid was drained. Materials were stored overnight at +4 °C before SE treatments. SE was carried out in 400 g (dry) batches in a 10 L vessel. The material was heated by steam to 205 °C with spruce and 200 °C with birch and ba- gasse. The temperature was kept constant for 15 min for spruce and 5 min for birch and bagasse before opening a valve for sud- den pressure drop and explosive release of the material. Opti- mally pretreated steam exploded spruce (2% SO2, 215 °C for 5 min) was obtained from Lund University and used in the fer- mentation experiments. 2.3. Enzymes Celluclast 1.5 L, Novozym 188, Cellic Ctec2, and Cellic Htec en- zymes were kindly provided by Novozymes. Filter paper activity was measured by the IUPAC-method (Ghose, 1987) and b-glucosi- dase activity was measured according to Bailey and Linko (1990). Protein content of the enzymes was analysed by the Bio-Rad anal- ysis kit using Lowry et al. (1951) method. 2.4. Enzymatic hydrolysis The solid fraction obtained by filtration was washed with dis- tilled water and hot tap water ($50 °C) to remove any soluble material. The sugar compositions of the filtrates and the washing waters were analysed. The remaining solid fractions were enzy- matically hydrolysed in order to evaluate hydrolysability of the samples after different pretreatment procedures. The hydrolysis of the washed solid fraction was carried out using commercial cellulase Celluclast 1.5 L FG (10 FPU/g dry mat- ter) and b-glucosidase Novozym 188 (100 nkat/g dry matter). The hydrolysis experiments were done in 50 mM sodium acetate buffer (pH 5) in test tubes with magnetic stirring at 1% (w/w) dry matter concentration. The temperature was controlled at 45 °C by water bath. Hydrolyses were done as triplicates. The effect of enzyme dosage was studied by hydrolysing the washed solid fractions with Celluclast 1.5 L FG using dosages 10 FPU/g d.m., 4 FPU/g and 2 FPU/g. In this experiment, Novozym 188 b-glucosidase dosage was kept as constant 100 nkat/g d.m. 2.5. SSF The simultaneous saccharification and fermentation (SSF) of washed pretreated materials was tested in oil-lock shake flasks at 12% d.m. consistency in 40 mL working volume. Yeast Nitrogen Base (4 mL of 10x stock solution) was used as a nutrient and pH was adjusted to pH 5 by adding 200 mM sodium citrate buffer. A 6 h prehydrolysis with enzyme dosage 15 mg/g d.m. of commercial enzymes (90:10 mixture of Cellic Ctec2 and Htec) was carried out at 50 °C before inoculation with commercial Red Star yeast (3.5 g/L, Le Saffre). Fermentation was carried out at 35 °C for 3–6 days. 2.6. Assays The sugar compositions of the feedstocks were analysed after total acid hydrolysis (Puls et al., 1985). The resulting monosaccha- rides were analysed by high performance anion exchange chroma- tography (HPAEC-PAD) using a CarboPac PA-1 column in a Dionex DX 500 series chromatograph equipped with pulse amperometric detection (Tenkanen and Siika-aho, 2000). The reducing sugars re- leased during pretreatment and in enzymatic hydrolysis were monitored using the DNS method (Bernfeld, 1955). The composi- tion of the material dissolved in the SE was analysed by HPAEC- PAD after acid hydrolysis to hydrolyse possible oligosaccharides to monosaccharides. For acid hydrolysis, 0.5 mL of 70% sulphuric acid was added to 10 mL of sample and autoclaved for 1 h at 120 °C. After cooling to room temperature, sample was diluted with water to 25 mL and analysed according to Tenkanen and Sii- A. Kallioinen et al. / Bioresource Technology 140 (2013) 414–420 415
- 149. III/3 ka-aho (2000). The monosaccharides in the enzyme hydrolysates were analysed by HPAEC-PAD. Ash content was analysed from wet samples in a muffle oven by heating samples stepwise first to 103 °C for 7 h to evaporate water and then to 550 °C for 16 h to ash the samples. After that, the ashed samples were cooled and the residue weighted. To analyse the content of extractives and lignin, air dried sam- ples were extracted with heptane in a Soxhlet extraction system. The heptane extract was dried and the weight of the residue was measured to get gravimetric extractive content. The lignin content was analysed from the extracted samples by acid hydrolysis with 70% sulphuric acid. Klason lignin was obtained by analysing the acid insoluble residue after drying and soluble lignin by analysing the UV absorbance with wavelength of 203 nm. Extinction coeffi- cients of 128 L/g/cm and 110 L/g/cm were used for spruce and birch, respectively. Acid-soluble lignin was not analysed from ba- gasse before or after the pretreatments as its extinction coefficient was unknown. Fermentation was followed by measuring the mass loss due to formation of CO2 and at the end of fermentation analysing ethanol from the broth by HPLC using Aminex HPX-87H column (Bio Rad) with 2.5 mM H2SO4 as eluent and flow rate 0.3 mL/ min. The column was maintained at 55 °C. Peaks were detected using a Waters 410 differential refractometer and a Waters 2487 dual wavelength UV (210 nm) detector. 3. Results and discussion 3.1. Raw material characterisation Carbohydrate content of raw material is important in the pro- duction of chemicals via the sugar route as the maximal potential product concentration is dependent on it. The other compounds, lignin, ash and extractives affect the processability of the raw material and can be recovered as side products of the process. The raw materials (spruce chips and saw dust, birch chips and su- gar cane bagasse) were characterized regarding their chemical composition (Table 1). All the selected raw materials consisted mainly of carbohy- drates and lignin. Glucan (mainly cellulose) content of spruce, birch and bagasse was high, about 40%. Bagasse had the highest hemicellulose content (24%). In bagasse and birch hemicellulose consisted mainly of xylan whereas in spruce the major hemicel- lulose was galactoglucomannan. The obtained results are in accordance with the fact that hardwood xylan contains usually methyl glucuronic acid subunits but in grasses glucuronoarabin- oxylan is prevailing (Harris and Stone, 2008). The highest lignin content, 28.5%, was analysed from spruce saw dust. In addition to carbohydrates and lignin, bagasse had significant ash content. The sum of the analysed compounds was below 100%, especially with birch. One reason is that the acetyl content and monosac- charides from pectic polysaccharides were not analysed. Galact- uronic acid, the main component in pectin, could not be analysed as it was partly degraded during acid hydrolysis used in the carbohydrate analysis. Galacturonic acid content in spruce has been reported to be about 1.5% (Bertaud and Holmbom, 2004). Some degradation of pentoses arabinose and xylose, might also occur during the analytical acid hydrolysis. The ob- tained hemicellulose and lignin contents of birch were also somewhat lower than the respective contents, 28% and 30%, ana- lysed by Mirahmadi et al. (2010). 3.2. Effects of alkaline oxidation and steam explosion pretreatments During AlkOx and SE the raw materials were partially solubi- lised. The carbohydrate content of the soluble fractions and the chemical composition of the solid fractions were analysed (Fig. 1). Accordingly in AlkOx most of the lignin was solubilised and most of the hemicellulose remained in the solid fraction. By contrast, in SE hemicellulose was partly solubilised whereas lignin remained in the solid fraction. In both pretreatments cellulose re- mained almost totally in the fibre fraction. To study the effects of AlkOx and SE on the performance of hydrolytic enzymes the solid fractions were enzymatically hydro- lysed. After 72 h enzymatic hydrolysis the monosaccharides were analysed from the hydrolysates. Fig. 2 presents the glucose and xy- lose yields after the pretreatments and after the enzymatic hydro- lysis. As mannan content in spruce is high, mannose yields are also presented. The hydrolysis yields of the monosaccharides were cal- culated by dividing the monosaccharides solubilised in enzymatic hydrolysis with the total amount of monosaccharides in the fibre fraction remaining after pretreatment. The overall yields of the monosaccharides were calculated by dividing the sum of the monosaccharides solubilized in the pretreatment and in the enzy- matic hydrolysis with the content of monosaccharides in the raw material. The cellulose from AlkOx was easily hydrolysed to monosaccha- rides in 72 h resulting the overall glucose yields of 84%, 91%, and 97%, for spruce, birch and bagasse respectively. Most of the cellu- lose in the solid fraction of SE bagasse and birch could also be enzy- matically hydrolysed giving the overall glucose yields of 78% and 93%, respectively. The major difference between AlkOx and SE could be seen in the enzymatic hydrolysability of pretreated spruce. While after AlkOx the solid fraction was easily hydrolysed, only 52% overall glucose yield was achieved with SE spruce. Tengborg et al. (2001) have determined the optimal conditions for SE of spruce to be 225 °C, 5 min, 0.5% H2SO4 and obtained 70% overall yield of fermentable sugars with 15 FPU/g in 96 h when using small size chips (2–10 mm) as raw material. In this study SE conditions were 205 °C, 15 min, 0.4% H2SO4. Higher tempera- tures could not be reached with the applied reactor and the lower reaction temperature was compensated with increased reaction time. The modification of the lignocellulosic matrix was evidently not adequate and unnecessary degradation of hemicelluloses also occurred, which can be seen from the results (Fig. 2). However, Al- kOx produced much higher glucose yield as 95% of the glucose in solid fraction was hydrolysed by enzymes in 72 h and 84% overall glucose yield was obtained with enzyme dosage 10 FPU/g. Alkaline pretreatments remove lignin, xylan side chains and acetyl groups efficiently from the raw material and thus clearly enhancing the enzymatic saccharification. Wet oxidations are usu- Table 1 Chemical composition of raw materials (% of d.m). Chemical component Spruce (chips) Spruce (saw dust) Birch Bagasse Glucan 40.9 38.5 40.3 42.0 Xylan 5.1 5.0 16.9 21.5 Mannan 10.1 11.7 1.7 0.2 Galactan 1.9 1.9 0.6 0.6 Arabinan 1.0 1.3 0.3 1.8 Rhamnan 0.1 0.1 0.3 0.1 Total polysaccharides 59 58 60 66 Methyl glucuronic acid 0.9 na 1.2 0.3 Glucuronic acid 0.1 na 0.1 0.1 Lignin 27.7 28.5 20.3 22.9b Extractivesa 1.2 2.1 1.4 0.7 Ash 0.3 na 0.4 5.3 TOTAL 90 89 84 96 na = not analysed. a Extracted with heptane. b Only Klason lignin analysed from bagasse. 416 A. Kallioinen et al. / Bioresource Technology 140 (2013) 414–420
- 150. III/4 ally carried out in partly acidic conditions, which is the main differ- ence to alkaline oxidation. The earlier reported wet oxidation pre- treatment of spruce (Palonen et al., 2004) was applied to ground wood and 79% carbohydrate yield of theoretical in 72 h hydrolysis with enzyme dosage 30 FPU/g was obtained. Highest yield was ob- tained in acidic pretreatment conditions. In our study we showed that 87% of carbohydrates in the AlkOx material could be hydroly- sed in 72 h with enzyme dosage of 10 FPU/g without grinding the raw material before oxidation. Pretreatment of birch has not been widely studied. Mirahmadi et al. (2010) studied the alkaline pretreatment of birch with 7% NaOH at 100 °C for 2 h. Glucose yield in the enzymatic hydrolysis for 96 h was 82% with cellulase dosage of 20 FPU/g. Both AlkOx and SE produced clearly higher enzymatic hydrolysability of birch. Pretreatment of sugar cane bagasse with SE and wet oxidation have been studied for several years. By far, the highest reported overall glucose yield by SE has been 92% with pretreatment conditions 190 °C, 5 min and with SO2-impregnation and enzy- matic hydrolysis for 72 h (Carrasco et al., 2010). The optimized wet oxidation conditions for sugar cane bagasse at 195 °C for 15 min at pH 10 followed by enzymatic hydrolysis of solid fraction for 48 h with 25 FPU/g dosage produced 79% glucose yield in hydrolysis and 74% overall glucose yield (Martin et al., 2006). Lime pretreatment has been reported to reach up to 88% overall glucose yield (Rabelo et al., 2008). In our study, as high as 97% overall glu- cose yield was obtained with AlkOx and enzymatic hydrolysis of cellulose in fibre fraction was complete. SE of bagasse at 200 °C for 5 min also produced very high enzymatic hydrolysability, and 93% of the cellulose was hydrolysed in 72 h hydrolysis. The overall glucose yield was 78% in pretreatment and hydrolysis. Hemicelluloses i.e. xylan and galactoglucomannan are suscepti- ble to be solubilised and degraded during pretreatments. A fraction of hemicellulose was solubilised as free monosaccharides and olig- omers during both SE and AlkOx of all raw materials (Fig. 2). The xylose yields were higher in AlkOx than in SE pretreated spruce and bagasse though a significant part, 30–50%, was degraded dur- ing both pretreatments. In contrast, the xylose from birch was al- most totally recovered after the pretreatments. Overall mannose yields were low for both SE and AlkOx pre- treated spruce. Similarly to xylan, galactoglucomannan was solubi- lised in SE and only a low amount remained in the solid fraction. In AlkOx higher proportion of mannose remained in the solid fraction than in SE but nearly similar amount was lost during both the pre- treatments. Mannose in the solid fraction of AlkOx spruce was only partially released by enzymatic hydrolysis. The hydrolysis of galac- toglucomannan presumably stopped at oligosaccharides, which the Celluclast and Novozym enzyme preparations were not able to hydrolyse further to monosaccharides. AlkOx was shown to be an effective pretreatment method for all the applied raw materials. When compared to SE, higher overall yields of glucose and xylose could be obtained from spruce and ba- gasse. With birch there was no difference in the overall yield. 0 20 40 60 80 100 Content(%ofdryweight) Ash Extractives Lignin Other hemicellulose Xylan Glucan Fig. 1. Composition of the raw materials and solid fractions after steam explosion (SE) and after alkaline oxidation (% of d.m.). 0 20 40 60 80 100 glucose xylose mannose glucose xylose mannose glucose xylose glucose xylose glucose xylose glucose xylose AlkOx spruce SE spruce AlkOx birch SE birch AlkOx bagasse SE bagasse Glucose,xyloseandmannoseyield (%ofrawmaterialglu,xylorman) Unhydrolysed fibre Yield in enzyme hydrolysis Yield in pretreatment Fig. 2. Glucose and xylose monosaccharide yields of spruce, birch and bagasse after pretreatment with alkaline oxidation, after steam explosion and after enzymatic hydrolysis for 72 h with 10 FPU/g Celluclast and 100 nkat/g Novozym 188. A. Kallioinen et al. / Bioresource Technology 140 (2013) 414–420 417
- 151. III/5 3.3. Comparison of catalytic oxidation and alkaline oxidation In addition to AlkOx studied here, we have previously intro- duced a catalytic oxidation pretreatment method based on in situ generated copper-phenanthroline catalyst in alkaline conditions (Hakola et al., 2010). The reaction conditions were similar to AlkOx, the major difference of the treatments being the addition of the catalyst. As in AlkOx, also in CatOx the share of carbohydrates in the so- lid fraction was significantly increased and a considerable part of the lignin was solubilised. The carbohydrate content of solid frac- tion was nearly similar, 93% and 96%, after AlkOx and CatOx, respectively. Similarly, the fibre fraction obtained after CatOx was easily hydrolysed by enzymes into monosaccharides. How- ever, material treated for 20 h with a catalyst had a slightly lower enzymatic hydrolysability (glucose yield in hydrolysis 92%) than the respective treatment without a catalyst (98%). It is possible that the CatOx for 20 h is too harsh and makes the structure of cellulose less favourable to enzymatic hydrolysis or causes the formation of inhibiting compounds. The use of the catalyst in the oxidation shortened the reaction time and allowed lower reaction temperatures. Catalytic oxidation of spruce saw dust for only 5 h at 120 °C gave 95% glucose yield in enzymatic hydrolysis. AlkOx for 5 h at 140 °C or 20 h at 120 °C re- sulted in similar glucose yields. Recycling the catalyst would lower the additional costs but, on the other hand, would require addi- tional equipment and the process has not been developed to a commercial scale. 3.4. Effect of oxygen and different alkaline agents on alkaline oxidation With AlkOx the role of oxygen was found to be essential. At ref- erence conditions, i.e. heat treatment at 120 °C for 20 h with argon pressure in the conditions resembling soda cooking, only 26% of the initial dry biomass was solubilised during the pretreatment and 9% glucose yield in enzymatic hydrolysis was obtained, whereas with oxygen present almost half of the biomass was sol- ubilised. The oxidation of lignin, and decrease in its molecular weight (Rovio et al., 2012) are probably the main reasons for the increased delignification. The challenges of adding the catalyst into the process are the in- creased costs and the mandatory toxicity evaluation. Different alkaline solutions, NaOH, KOH, Ca(OH)2 and Na2CO3, were com- pared in AlkOx of spruce. Carbohydrate yields after AlkOx with dif- ferent alkalis and after enzymatic hydrolysis for 48 h were analysed. The different alkalis produced quite similar carbohydrate yields (Fig. 3). Overall glucose yields were high: 96–97% of raw material glucose. Only with Ca(OH)2 pretreatment cellulose had a clearly lower enzymatic hydrolysability than with other alkalis. Nearly identical xylose yields were obtained with all alkalis whereas in mannose yields more variation was observed. As in ear- lier experiments with spruce, mannose yields were very low, only 13–24%. The lowest mannose yield was obtained with Na2CO3. Oxi- dation of mannose during the pretreatment was evidently high as the yield loss was 50–60%. Higher mannose content was found from liquid fraction after pretreatment with Ca(OH)2 and KOH than with the other alkalis. A considerable fraction of mannose as galac- toglucomannan in the solid fraction, 19–30%, was not hydrolysed to monosaccharides. Galactoglucomannan probably partly re- mained in the solid fraction after enzymatic hydrolysis or the hydrolysis was stopped to oligomers. With KOH pretreatment the enzymatic hydrolysis of mannose in the fibre was most incom- plete. Concerning the glucose and xylose yields, a great variety of different alkalis could be applied in AlkOx without decreasing the efficiency of pretreatment. 3.5. Effect of enzyme dosage on enzymatic hydrolysability of pretreated raw materials The use of cellulases in total hydrolysis is relatively expensive and the enzymes or the enzyme production costs are important contributors to the overall ethanol production cost (Zhang et al., 2006; Galbe and Zacchi, 2007). The expenses can be decreased by lowering the enzyme price or by reducing their consumption. One way to minimize the enzyme usage is to enhance the pretreat- ment so that the same hydrolysis level and rate can be obtained with a lower enzyme dosage. The possibility of decreasing the en- zyme dosage was studied with AlkOx and SE materials (Fig. 4). Enzymatic hydrolysability of bagasse and birch was high with both pretreatments with 10 FPU/g dosage as studied in 1% consis- tency in small laboratory scale. When enzyme dosage was de- creased to 4 and 2 FPU/g, hydrolysis level after 24 h was decreased for AlkOx bagasse. With SE bagasse, the hydrolysis with lower dosages was similar to 10 FPU/g in the first 4 h, but after 4 h hydrolysis the reaction was slowed down and the hydrolysis level of SE bagasse was decreased more steeply compared to AlkOx materials. Probably a part of the SE bagasse was very easily hydro- lysable and caused the higher hydrolysis levels. SE might also have 0 20 40 60 80 100 Glucose Xylose Mannose Glucose Xylose Mannose Glucose Xylose Mannose Glucose Xylose Mannose Ca(OH)2, 0.25M, 120°C 120°C 120°C 120°C NaOH, 0.25M, KOH, 0.25M, Na2CO3, 0.25M, Glucose,xyloseandmannoseyield (%ofrawmaterialglu,xylorman) Unhydrolysed fibre Yield in enzyme hydrolysis Yield in pretreatment Fig. 3. Glucose and xylose monosaccharide yields after alkaline oxidation pretreatment with Ca(OH)2, NaOH, KOH, or Na2CO3 and after enzymatic hydrolysis for 48 h with 10 FPU/g Celluclast and 100 nkat/g Novozym 188. Spruce saw dust was used as a raw material. 418 A. Kallioinen et al. / Bioresource Technology 140 (2013) 414–420
- 152. III/6 caused a higher surface area, which has been shown to increase overall protein adsorption and to give a higher initial rate of hydro- lysis (Piccolo et al., 2010). With SE and AlkOx birch, the hydrolysis levels were similar with similar dosages for 24 h but after that the hydrolysis rate of SE birch slowed down more steeply. With SE spruce, enzymatic hydrolysability was low and further decreased with lower enzyme dosages. Clearly, the SE pretreat- ment was suboptimal for this raw material. On the contrary, AlkOx spruce had high enzymatic hydrolysability and the hydrolysis level was clearly decreased only when the enzyme dosage was lowered to 2 FPU/g. The higher lignin content in SE material adsorbed en- zymes more efficiently during the hydrolysis whereas the AlkOx materials having lower lignin content were less liable to adsorp- tion. It has also been shown that prolonged contact with lignin at hydrolysis temperature may lead to denaturation of enzyme pro- teins (Rahikainen et al., 2011). According to Varnai et al. (2011) cellulases adsorbed quickly at the early stages of hydrolysis and remained bound throughout the hydrolysis. With CatOx spruce, the bound enzymes started to des- orb, whereas with SE spruce no desorption occurred. In addition to decreased adsorption of enzymes to lignin, removal of lignin has probably increased enzymatic hydrolysability by other ways. Lig- nin removal has been shown to correlate to enhanced enzymatic hydrolysability by removing the lignin barrier for enzymes, increasing the accessible surface area and porosity of substrate (Nlewem and Thrash, 2010; Chang and Holtzapple, 2000). Standard deviations of the triplicate hydrolyses were quite high in some cases, probably due to inhomogeneous materials contain- ing e.g. sticks and fines. Hydrolysis level was analysed by reducing sugars assay with DNS reagent (Bernfeld, 1955). The method gave in some cases hydrolysis degrees above 100% probably due to other reducing or coloured compounds released during the hydrolysis and due to inhomogeneity of solid fraction. Monosaccharide anal- ysis from the 72 h hydrolysates by HPAEC-PAD verified the high hydrolysis results (Table 2). 3.6. Ethanol production from alkaline oxidised materials Alkali recovery after pretreatment by separation and washing the fibre is essential part of AlkOx process concept. For reliable comparison SE materials were also washed. With washed material the amount of the inhibitors was low and thus no lag phase was observed in the beginning of SSF. Washed AlkOx bagasse and spruce fermented well in 12% consistency using commercial yeast able to utilize only C6 sugars (Fig. 5). A pre-hydrolysis followed by 1–3 days SSF gave 80% ethanol yield from hexoses. Ethanol produc- tion from SE bagasse and spruce was slow and 39% and 32% etha- nol yields from hexoses were obtained respectively in 6 days SSF. The maximal ethanol concentration, 49 g/L, was obtained with Al- kOx spruce. The ethanol concentration obtained by AlkOx bagasse was somewhat lower, 38 g/L, due to a lower content of hexoses in the substrate. After fermentation the monosaccharide contents were also analysed to get information about the extent of the hydrolysis. The remaining monosaccharides after fermentation of 0 20 40 60 80 100 0 24 48 72 Reducingsugars (%oftotalcarbohydrates) Hydrolysis time (h) BagasseA 0 20 40 60 80 100 0 24 48 72 Reducingsugars (%oftotalcarbohydrates) Hydrolysis time (h) BirchB 0 20 40 60 80 100 0 24 48 72 Reducingsugars (%oftotalcarbohydrates) Hydrolysis time (h) Spruce AlkOx 10 FPU/g AlkOx 4 FPU/g AlkOx 2 FPU/g SE 10 FPU/g SE 4 FPU/g SE 2 FPU/g C Fig. 4. Effect of enzyme dosage and pretreatment type on enzymatic hydrolysabil- ity of steam exploded and alkaline oxidised bagasse (A), birch (B), and spruce (C). Enzymatic hydrolysis at 1% d.m. consistency, 45 °C, pH 5, with Celluclast 1.5 L (2– 10 FPU/g) and Novozyme 188 (100 nkat/g). Table 2 Overall monosaccharide yield (sum of all monosaccharides analysed by HPAEC-PAD) in enzymatic hydrolysis of pretreated washed fibre fractions after 72 h hydrolysis time with different enzyme dosages. Results presented as % of total monosaccharides in solid fraction after acid hydrolysis. 10 FPU/g 4 FPU/g 2 FPU/g SE bagasse 104 75 47 AlkOx bagasse 102 94 70 SE birch 98 74 45 AlkOx birch 101 96 67 SE spruce 44 28 18 AlkOx spruce 87 84 57 0 20 40 60 80 100 0 1 2 3 4 5 6 7 Ethanol(%oftheoretical) Time (d) AlkOx spruce AlkOx bagasse SE spruce SE bagasse Fig. 5. Fermentation of alkaline oxidised spruce and bagasse and steam exploded spruce and bagasse at 12% d.m. consistency at 35 °C. 6 h prehydrolysis was carried out with Cellic Ctec2 + Htec enzyme mixture (enzyme ratio was 90:10; total dosage 15 mg/g) at 50 °C, pH 5 before inoculation with a commercial yeast Red Star. Ethanol yields calculated from the measured mass loss during fermentation. A. Kallioinen et al. / Bioresource Technology 140 (2013) 414–420 419
- 153. III/7 the materials were mainly xylose which is in line with the fact that the applied yeast could not ferment pentose sugars. The steam ex- ploded materials after fermentation contained only very low amounts of mono-, cello- and xylooligosaccharides, which indi- cates that the slow enzymatic hydrolysis with the used dosage (15 mg/g) of pretreated materials was limiting the fermentation. Enzymatic hydrolysis of steam exploded materials did not proceed during fermentation presumably due to inhibition of enzymes, high dry matter content, and suboptimal temperature for the hydrolysis. On the contrary, hydrolysis of AlkOx materials was not limiting the fermentation to the same extent than hydrolysis of SE material and the fermentation of produced hexoses was efficient. 4. Conclusions Alkaline oxidation is an effective pretreatment method for dif- ferent lignocellulosic raw materials. Compared to steam explosion higher glucose and xylose yields were obtained from spruce and bagasse. 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- 155. PUBLICATION IV Mixtures of thermostable enzymes show high performance in biomass saccharification Appl. Biochem. Biotechnol. In press. Reprinted with permission from the publisher.
- 157. IV/1 Mixtures of Thermostable Enzymes Show High Performance in Biomass Saccharification Anne Kallioinen Terhi Puranen Matti Siika-aho Received: 29 January 2014 /Accepted: 25 March 2014 # Springer Science+Business Media New York 2014 Abstract Optimal enzyme mixtures of six Trichoderma reesei enzymes and five thermostable enzyme components were developed for the hydrolysis of hydrothermally pretreated wheat straw, alkaline oxidised sugar cane bagasse and steam-exploded bagasse by statistically designed experiments. Preliminary studies to narrow down the optimization parameters showed that a cellobiohydrolase/endoglucanase (CBH/EG) ratio of 4:1 or higher of thermo- stable enzymes gave the maximal CBH-EG synergy in the hydrolysis of hydrothermally pretreated wheat straw. The composition of optimal enzyme mixtures depended clearly on the substrate and on the enzyme system studied. The optimal enzyme mixture of thermostable enzymes was dominated by Cel7A and required a relatively high amount of xylanase, whereas with T. reesei enzymes, the high proportion of Cel7B appeared to provide the required xylanase activity. The main effect of the pretreatment method was that the required proportion of xylanase was higher and the proportion of Cel7A lower in the optimized mixture for hydrolysis of alkaline oxidised bagasse than steam-exploded bagasse. In prolonged hydroly- ses, less Cel7A was generally required in the optimal mixture. Five-component mixtures of thermostable enzymes showed comparable hydrolysis yields to those of commercial enzyme mixtures. Keywords Biomass sugars . Enzyme mixture optimization . Total hydrolysis . Thermostable enzymes . Trichoderma reesei . Cellulase Introduction Lignocellulosic materials have great potential as a renewable feedstock for the production of fuels and chemicals. The enzymatic hydrolysis of lignocellulosic materials requires multiple activities acting cooperatively to degrade cellulose and hemicellulose to fermentable mono- saccharides. At least three categories of enzymes are necessary to convert cellulose into soluble Appl Biochem Biotechnol DOI 10.1007/s12010-014-0893-3 A. Kallioinen (*) :M. Siika-aho VTT Technical Research Centre of Finland, P.O. Box 1000, 02044 VTT, Espoo, Finland e-mail: anne.kallioinen@vtt.fi T. Puranen Roal Ltd, Tykkimäentie 15, 05200 Rajamäki, Finland
- 158. IV/2 sugars. These include endoglucanase (EG), which hydrolyses the internal β-1,4-glucosidic bonds randomly in the cellulose chain; cellobiohydrolase (CBH), which moves processively along the cellulose chain and cleaves off cellobiose units from the ends of the chain; and β- glucosidase (βG), which converts cellobiose and soluble cellodextrins into glucose. Individual cellulases have only limited hydrolytic effects, whereas a mixture of cellulases can exhibit a synergistic action in which the hydrolytic activity of the cellulase mixture is greater than the sum of the activities of the individual enzymes [1]. Enzymes hydrolysing hemicellulose are also required for complete hydrolysis of lignocellulosic biomass [2, 3] and they act synergis- tically with cellulases [4-7]. The enzyme cost is important for the economy of biofuel processes. Optimized enzyme mixtures can provide maximal synergistic effect, and thus, more efficient hydrolysis can be obtained with minimized enzyme dosages. Several studies concerning optimal enzyme mixtures have been published with various lignocellulosic substrates such as pretreated barley straw [8], pretreated wheat straw [9], pretreated corn stover [10-14], pretreated switchgrass, Miscanthus, distillers grains, poplar wood [2] and pretreated douglas fir [15]. The optimization of enzyme mixtures has been performed for a core set of cellulases [14], cellulases and xylanases [13], for multi- component mixtures [2] containing several accessory enzymes and for mixtures of commercial enzymes [16, 17]. Banerjee et al. [2] reported hitherto the only compar- ison of the effect of raw material and pretreatment on optimal enzyme mixtures. Optimized enzyme mixtures of Trichoderma reesei enzymes were shown to be dom- inated by CBHI and EGI for most of the raw materials and pretreatments, whereas the proportions of β-glucosidase and xylanase varied considerably depending on the raw material and pretreatment. According to their results, hydrolysis of NaOH-pretreated corn stover and switchgrass required less xylanase than ammonium fibre explosion (AFEX) and alkaline peroxide-pretreated materials. The high proportion of xylanase was generally compensated by lower Cel7A (CBHI) and Cel7B (EGI) contents. Wide variation was also observed depending on how many and which enzyme components were included in the optimization. Therefore, it is important to include all the most essential enzymes in the experimental design, although this will make the design more complex. The optimal enzyme mixtures have also been shown to be dependent on the total enzyme loading [13] and hydrolysis time [9]. Thermostable enzymes have several advantages in the hydrolysis of lignocellulose, such as the potential to operate in higher hydrolysis temperatures, reduction of the enzyme amount needed due to higher specific activity in elevated temperature and higher stability enabling longer hydrolysis times [18]. Higher hydrolysis temperature de- creases the viscosity, enabling higher substrate concentrations and potentially increas- ing product yields and consequently reducing processing and capital costs. A pre- hydrolysis that liquefies the high viscosity slurry is one suggested application for thermo- stable enzymes [18, 19]. The aim of this study was to develop optimal enzyme mixtures of thermostable enzymes for hydrolysis of three different pretreated lignocellulosic raw materials: hydrothermally pretreated wheat straw and alkaline oxidised or steam-exploded bagasse. The effects of raw material type and different pretreatments on the optimal mixture were investigated. The work was carried out in two stages: first two-component synergistic systems were used to narrow down the ranges for optimization, and secondly, optimal mixtures were developed using statistically designed multi-component optimization protocols. Mixtures of thermostable enzymes were also compared with mixtures of T. reesei enzymes and commercial enzyme products. Appl Biochem Biotechnol
- 159. IV/3 Materials and Methods Raw Materials Hydrothermally pretreated wheat straw was provided by Inbicon. Pretreatment was carried out at the Inbicon pilot plant (Skærbæk, Denmark) by the method described in Petersen et al. [20]. Straw was cut into pieces up to 6–10 cm long and pre-soaked in 3 g l−1 acetic acid at 80 °C for 10 min. After that, the feedstock was fed to the pretreatment plant at a rate of 50 kg h−1 and steamed in the hydrothermal reactor at 190 °C for 12 min. The pretreatment was conducted at a water/straw ratio of 5:1 and the pretreated straw was pressed to a higher final dry matter (d.m.) content of 33 %. For hydrolysis experiments, handsheets were prepared from pretreated wheat straw after homogenization by cold dispersion. Laboratory handsheets (target density 120 g/m3 ) were formed in a sheet mould producing 165 mm×165 mm sheets. The sheets were pressed at 490 kPa for 4 min and dried at 60 °C for 2 h. The d.m. content of pretreated wheat straw sheets was 93 % after drying. Handsheet disks with approximately 6 mm diameter were cut from the sheets using a paper punch. Alkaline oxidised (AlkOx) bagasse was prepared according to Kallioinen et al. [21]. Sugar cane bagasse was pretreated with 0.25 M Na2CO3 at 120 °C for 20 h at 5 % d.m. content. Material from ten 20 g batches was washed and mixed. Sticks remaining in the material (approx. 1 % of d.m.) were removed to provide a homogeneous material. The d.m. content of AlkOx bagasse was 30 %. Steam explosion (SE) of bagasse was carried out at 200 °C for 5 min [21]. Sugar cane bagasse was immersed into dilute sulphuric acid (0.5 % w/w) at room temperature for 30 min, the excess acid was drained and the material was stored overnight at 4 °C before SE treatments. After SE, the material was washed with water. The d.m. content of SE-pretreated bagasse was 32 %. The monosaccharide sugar compositions of the washed insoluble fractions of the pretreated feedstocks after total acid hydrolysis [22] were analysed by high performance anion exchange chromatography (HPAEC-PAD) using a CarboPac PA-1 column in a Dionex DX 500 series chromatograph equipped with pulse amperometric detection [23]. The carbo- hydrate contents of water-washed pretreated wheat straw and bagasse are presented in Table 1. Enzymes T. reesei cellulases Cel7A (CBHI), Cel6A (CBHII), Cel7B (EGI) and Cel5A (EGII) were produced and purified as described by Suurnäkki et al. [24]. Xylanase (XYL II, i.e. family 11 xylanase with pI 9) was purified by the method of Tenkanen et al. [25], but omitting the last gel filtration step. Aspergillus niger β-glucosidase was purified according to Sipos et al. [26]. Table 1 Carbohydrate composi- tion of washed insoluble solids of hydrothermally pretreated wheat straw and alkaline oxidised and steam-exploded bagasse (% of d.m.), used in the experiments Pretreated wheat straw AlkOx bagasse Steam-exploded bagasse Glucose 65.4 67.9 59.3 Xylose 3.6 23.9 3.0 Mannose 0.2 0.14 0.1 Galactose 0.06 0.1 0.1 Arabinose 0.05 1.3 0.4 Monosaccharides total 69 93 63 As polysaccharides 62 83 57 Appl Biochem Biotechnol
- 160. IV/4 The thermostable glycosyl hydrolases were kindly provided by Roal Oy (Rajamäki, Finland): three CBHs originating from strains of Acremonium thermophilum, Chaetomium thermophilum and Thermoascus aurantiacus; one EG originating from T. aurantiacus; two xylanases originating from strains of T. aurantiacus and Nonomuraea flexuosa; and a βG from T. aurantiacus. The enzymes (Table 2) were heterologously produced in a genetically modified T. reesei strain, deficient in the four major native cellulases (Δcbh1/cel7A, Δcbh2/cel6A, Δegl1/cel7B, Δegl2/cel5A), under the control of the strong cbh1/cel7A promoter of the host fungus [27]. The TaCel7A (CBHI) construct contained the cellulose-binding module (CBM) from T. reesei Cel7A (CBHI), and TaCel5A (EGII) contained the CBM from C. thermophilum Cel7A (CBHI). Thermostable enzymes were heat-treated at 60 °C, pH 6 for 2 h to inactivate the thermolabile side activities. For mixture 1, the CBHI component TaCel7A+CBM was purified by one-step anion exchange chromatography, using columns and chromatography media available from GE Healthcare. The preparation was first desalted and equilibrated with 0.02 M sodium phosphate buffer (pH 6) using a Sephadex G-25C column. After that, the sample was loaded on a DEAE Sepharose FF column, equilibrated with 0.02 M sodium phosphate at pH 6 and eluted using a gradient of 0–0.2 M sodium chloride. Protein fractions with TaCel7A+CBM eluting in ca. 0.07–0.11 M NaCl were pooled and equilibrated with 0.02 M sodium acetate buffer (pH 5.0) using a Sephadex G-25C column. The CBHII component of mixture 1 (CtCel6A) was expressed from the cbh1/Cel7A promoter in the T. reesei strain VTT D-00775. The expression construct was transformed into T. reesei as described in [28] and replaced the native cbh1/cel7A gene. The CtCel6A enzyme was produced for purification in shake flask cultures in Trichoderma minimal medium [28] supplemented with 4 % lactose and 2 % distiller’s spent grain extract at Table 2 Components of the enzyme mixtures. Thermostable mixture 1 and Trichoderma mixture 1 were used in preliminary optimization studies and thermostable mixture 2 and Trichoderma mixture 2 in final optimization studies Enzyme Abbreviation Used in mixture Purification Trichoderma reesei enzyme mixtures Trichoderma reesei Cel7A TrCel7A (CBHI) Mix 1, 2 [24] Trichoderma reesei Cel6A TrCel6A (CBHII) Mix 1, 2 [24] Trichoderma reesei Cel7B TrCel7B (EGI) Mix 2 [24] Trichoderma reesei Cel5A TrCel5A (EGII) Mix 1, 2 [24] Trichoderma reesei Xyn11A TrXyn11A (XYLII) Mix 1, 2 [25] Aspergillus niger Cel3A AnCel3A (βG) Mix 1, 2 [26] Thermostable enzyme mixtures Thermoascus aurantiacus Cel7A+CBM/TrCBHI TaCel7A (CBHI) Mix 1 Heat-treated+purified as described Acremonium thermophilum Cel7A AtCel7A (CBHI) Mix 2 Heat-treated at 60 °C, 2 h Chaetomium thermophilum Cel6A CtCel6A (CBHII) Mix 1 Heat-treated+purified as described Chaetomium thermophilum Cel6A CtCel6A (CBHII) Mix 2 Heat-treated at 60 °C, 2 h Thermoascus aurantiacus Cel5A+CBM/CtCBHI TaCel5A (EGII) Mix 1, 2 Heat-treated at 60 °C, 2 h Nonomurea flexuosa NfXyn11A Mix 1 Heat-treated at 60 °C, 2 h Thermoascus aurantiacus TaXyn10 Mix 2 Heat-treated at 60 °C, 2 h Thermoascus aurantiacus TaCel3A (βG) Mix 1, 2 Heat-treated at 60 °C, 2 h Appl Biochem Biotechnol
- 161. IV/5 200 rpm and 28 °C. The culture filtrate with CtCel6A was desalted and equilibrated with 0.01 M sodium phosphate buffer at pH 7.04 using a Sephadex G-25C column. After that, the sample was loaded on a DEAE Sepharose FF column and equilibrated with 0.01 M sodium phosphate (pH7.04), and the fractions with unbound protein were collected. The purification was continued using an affinity column as described by van Tilbeurgh et al. [29], eluting the CtCel6A protein with lactose. The affinity run was repeated twice, and the fractions with CtCel6A were concentrated by ultrafiltra- tion and at the same time washed to change the buffer to 50 mM sodium acetate, pH 5. The thermostable enzymes and T. reesei enzymes used are presented in Table 2. The protein contents of enzyme components were analysed using a Bio-Rad analysis kit based on the Lowry method [30]. Purity of the used enzyme components was analysed by SDS-PAGE using 12 % Tris-HCl gel (Bio-Rad) and pre-stained SDS- PAGE Standards (Bio-Rad) (Fig. 1). Xylanase activity was determined by the method of Bailey et al. [31], and β-glucosidase activity was measured according to Bailey and Linko [32]. Commercial enzymes Celluclast 1.5 L, Novozym188, Cellic Ctec2 and Cellic Htec were obtained from Novozymes. Hydrolysis Experiments The hydrolysis experiments were performed at 1 % d.m. content in 0.05 M sodium acetate buffer, pH 5.0, containing 0.02 % NaN3. The hydrolysis was carried out using a microassay procedure with 0.5 g of reaction mixture in 96-well plates. For hydrolysis of pretreated wheat straw substrate, two substrate disks (5 mg of dry matter) were dispensed to each well. With SE and AlkOx bagasse, the washed insoluble fraction of pretreated material was homogenized and diluted with buffer and the slurry was pipetted into the wells. In hydrolysis, the microwell plates were closed with adhesive foil and incubated in a mixer (Thermo micromixer Mxi4t, FinePCR, South Korea) shaken at 1,000 rpm at 45 °C (Trichoderma mixture), 55 °C (Thermostable enzymes, mixture 1) or 52 °C (Thermostable enzymes, mixture 2). The lower temperatures of 45 °C and 52 °C were selected for T. reesei and thermostable enzymes, respectively, to minimize the effect of enzyme inactivation on mixture optimization during prolonged incubation times. It has been shown that at higher temperatures, the activity of cellulases can decrease significantly in 48 h in materials contain- ing lignin [33]. After 24, 48 and 72 h, a plate containing four replicate samples for each test point was withdrawn from the incubator, and the reaction was terminated by adding 10 μl of 10 M NaOH. The supernatants were collected for analysis after centrifugation. The hydrolysis was monitored by measuring reducing sugars by the dinitrosalicylic acid (DNS) method using glucose as a standard [34]. Preliminary Evaluation of the Ratios of the Main Cellulases In the studies concerning the optimal ratio of CBHI and CBHII, the thermostable or T. reesei enzymes, with components described in Table 2 (mixtures 1), were dosed on the basis of protein content with a total dosage of 10 mg g−1 d.m. excluding β-glucosidases, which were dosed at 1,000 nkat g−1 d.m. The amount of CBHs (CBHI and CBHII combined) was 75 % of protein content (7.5 mg g−1 d.m.). The share of CBHII of total CBHs (CBHI+CBHII) was varied in the range 0–100 %. Xylanase dosage was 2,500 nkat g−1 d.m. (0.59 mg g−1 d.m. for thermostable xylanase and 0.26 mg g−1 d.m. for Trichoderma xylanase). The rest of the protein was EG (1.91 mg g−1 d.m. for the thermostable mixture and 2.24 mg g−1 d.m. for the T. reesei mixture). Appl Biochem Biotechnol
- 162. IV/6 The effect of the ratio of CBHs and EG in the thermostable cellulase enzyme mixtures was studied similar to that of total dosage (except βG) of 10 mg g−1 d.m. but varying the CBH/EG proportions in the range of 0–100 %. The CBHI/CBHII ratio in the mixtures was 80:20 and 60:40 for thermostable enzymes and Trichoderma enzymes, respectively. Xylanase dosage was 2,500 nkat g−1 d.m. (0.59 and 0.26 mg g−1 d.m for thermostable and Trichoderma enzymes, Fig. 1 SDS-PAGE analysis of the thermostable and T. reesei enzymes. The Bio-Rad ready-made gel (12 % Tris- HCl gel 161-1156) was run at 200 Vand 100 mA for 35 min using Tris-glycine-SDS buffer. The gel was stained with Coomassie solution for 1 h and destained twice, for 15 min and then overnight. Pre-stained SDS-PAGE standards solution (Bio-Rad 161-0318) was used to estimate the molecular weights. The protein dosage of thermostable enzymes was 5 μg/well and of T. reesei enzymes: CBHI (lane 1)=5 μg; CBHII=5 μg; EGI=4 μg; EGII=5 μg; Xyl=4 μg; AnβG=4 μg; and CBHI (lane 7)=9 μg/well Appl Biochem Biotechnol
- 163. IV/7 respectively) and the rest of the protein was CBHs and EG. βG was dosed as described above. The commercial T. reesei enzyme mixture Celluclast 1.5 L was used as a reference with a dosage of 10 mg g−1 d.m., supplemented by commercial β-glucosidase Novozym 188 (1,000 nkat g−1 d.m., 28 mg g−1 ). Determination of Optimal Enzyme Mixtures The optimal enzyme mixtures were studied by statistically designed experiments using Modde software (Umetrics, Sweden). The thermostable enzymes and T. reesei enzymes used are presented in Table 2 (mixtures 2). The factors (CBHs, EGs, xylanase and β-glucosidase) and the constraints of the factors, giving the highest and lowest allowed amounts for the share of each enzyme in the mixture, are presented in Table 3. With wheat straw, the constraints were set tighter than with the other raw materials on the basis of information obtained in the previous two-component studies (Fig. 1). For T. reesei, the first set of constraints in the hydrolysis of pretreated wheat straw gave optimum values near the boundaries of the studied range, and therefore, a new set of experiments was designed with different constraints. The results of these two sets of experiments were combined in the model. Computer-generated designs were formed using a quadratic regression model and by D-optimal design that generated the best subset of experiments from a candidate set. The optimization experiments with five or six enzyme components consisted of 25–26 and 32 exper- iments, respectively. The centre points, in the middle of the studied range, were carried out as four replicates in order to evaluate the reproducibility of the experi- ments. The thermostable enzyme mixtures or T. reesei mixtures were dosed on the basis of protein content at 4, 6 or 10 mg g−1 dry matter for AlkOx bagasse, pretreated wheat straw and SE bagasse, respectively. The experiments were otherwise carried out as described above. After hydrolysis, the data of reducing sugar formation was analysed, and the model was evaluated using the partial least squares (PLS) regression technique by the Modde program. The Q2 values were optimized by removing the less significant terms in the model. Evaluation of Optimal Enzyme Mixtures in Hydrolysis Evaluation of the optimization was made at 1, 8 or 12 % d.m. content for washed insoluble fractions of pretreated substrates using thermostable enzymes. Optimized enzyme mixtures from 72 h hydrolysis time were selected for each substrate, and the enzyme dosages were 6, 10 Table 3 Constraints, i.e. lower and upper levels for the enzyme components in models Component Wheat straw, Trichoderma mix Wheat straw, thermostable enzyme mix AlkOx bagasse SE bagasse βG 0.01–0.05 0.01–0.05 0.01–0.05 0.01–0.05 Xyl 0.01–0.12 0.01–0.12 0.01–0.15 0.01–0.12 EGII/Cel5A 0.03–0.3 0.03–0.3 0.01–0.3 0.01–0.3 EGI/Cel7B 0.03–0.3; 0.1–0.6 – – – CBHII/Cel6A 0.1–0.67; 0.1–0.6 0.1–0.45 0.1–0.7 0.1–0.7 CBHI/Cel7A 0.25–0.82; 0.1–0.6 0.5–0.85 0.25–0.85 0.25–0.85 Number of experiments 32+32 25 26 26 Appl Biochem Biotechnol
- 164. IV/8 or 15 mg g−1 (protein/d.m.) for wheat straw, AlkOx bagasse and SE bagasse, respectively. Hydrolyses were carried out at 50 °C in stirred test tubes in 50 mM sodium acetate buffer at pH 5. For comparison, a mixture of commercial Cellic Ctec2 and Htec enzymes (85:15) was applied using the same total protein dosages. An adequate amount of buffer was added to make the weight of the hydrolysis mixture 1.0 g (8 and 12 % d.m. content) or 3.0 g (1 %) after enzyme addition. Hydrolysis reactions were stopped by boiling for 10 min. The suspensions from experiments carried out at 8 and 12 % d.m. content were diluted before analyses with water to 1 % d.m. content. Results and Discussion Preliminary Evaluation of the Ratios of the Main Cellulase Components in the Hydrolysis of Wheat Straw In order to focus the actual detailed mixture optimization and to obtain information concerning the synergy of the major cellulolytic activities on industrial substrates, the effects of the ratios of CBH and EG enzymes on the degree of hydrolysis were first studied. The optimal ratio of Cel7A (CBHI) and Cel6A (CBHII) enzymes was studied in a small-scale system on pretreated wheat straw substrate at 45 °C for T. reesei and 55 °C for thermostable enzymes. The amount of cellobiohydrolases in the mixture was first kept constant (75 % of protein dosage, except βG) and the ratio of the two cellobiohydrolases Cel7A (CBHI) and Cel6A (CBHII) was varied. The dosages of the additional enzymes xylanase and β- glucosidase were kept relatively high in order to assure that they would not be the limiting factors in hydrolysis. CBH enzymes showed clear synergy in all cases, the optimal CBHI/CBHII ratio being between 80:20 and 60:40 for thermostable enzymes (Fig. 2a). The enhancing effect of CBHI- CBHII synergy on hydrolysis with constant protein dosage was rather low, 5–10 %, as compared to hydrolysis with the CBHI component only. The mixture without CBHI was inefficient. For T. reesei enzymes, the optimal ratio of CBHI and CBHII was around 60:40 (Fig. 2b). When comparing the thermostable enzyme mixture and the T. reesei enzyme mixture at 45 °C with the same protein loading (10 mg g−1 ), the efficiency of the thermostable enzyme mixture at 45 °C was clearly better than that of the mixture composed of the corresponding T. reesei enzymes. Only 30 % hydrolysis was obtained in 72 h with T. reesei enzymes compared to almost 70 % hydrolysis yield with the thermostable mixture. It is possible that the Trichoderma enzyme mixture lacked one or more synergetic components essential for efficient operation, e.g. another endoglucanase component. The optimal ratio of the endoglucanase component Cel5A (EGII) and CBH enzymes was studied using CBHI/CBHII ratios of 80:20 and 60:40 for thermostable enzymes and T. reesei enzymes, respectively, and varying the share of EGII in the mixture. Hydrolysis was most effective with thermostable enzymes when the ratio of CBH and EG was above 80:20 (Fig. 2c). At 45 °C, the result was similar (data not shown). At 55 °C, it was seen that even ca. 5 % of EGII/Cel5A in this mixture gave close to optimal synergetic effect (Fig. 2c). The highest synergy was observed in the range of CBH/EG ratios from 80:20 to 95:5. Thus, the corresponding proportions for CBHI/CBHII/EGII were 64–76:16–19:5–20. With T. reesei enzyme mixture, the optimal ratio of CBH and EG was 90:10 (Fig. 2d) and that of CBHI/ CBHII/EGII was 54:36:10. Appl Biochem Biotechnol
- 165. IV/9 Use of a Statistical Method to Develop Optimal Mixtures of Enzymes Conventional optimization changing only one component at a time was used in preliminary evaluation of optimal ratios and the synergy between cellulases. For the complete optimization, however, all the major enzyme components should be studied simultaneously. Information obtained in the preliminary evaluation was applied for determination of the constraint for optimization of enzyme mixtures in the hydrolysis of wheat straw. By the statistical design of the experiments, the number of experiments could be minimized and the effect of all components could be taken fully into account. After the experiments, the models were evaluated by Modde using different parameters describing the variation of the replicates, how well the experimental data fit in the model and how well the model predicts the new values. The hydrolysis experiments for 48 and 72 h produced very high reproducibility values of 92–99 %, based on variation of the results obtained by replicate experiments in centre points Fig. 2 The effect of the ratio of thermostable TaCel7A (CBHI) and CtCel6A (CBHII) at 55 °C (a) and T. reesei CBHI and CBHII at 45 °C (b) on hydrolysis of pretreated wheat straw. The effect of the ratio of thermostable cellobiohydrolases (TaCel7A+CtCel6A) and endoglucanase TaCel5A (EGII) (c) and T. reesei cellobiohydrolases (CBHI+CBHII) and endoglucanase TrCel5A (EGII) (d) on hydrolysis of pretreated wheat straw. For experi- mental details, see the text Appl Biochem Biotechnol
- 166. IV/10 compared to overall variability. In hydrolyses for 24 h, more variation was observed and the reproducibility was 72–98 %. The model fit (R2) describes how well the model fits the experimental data. R2 was over 0.81 for all the models and thus clearly above the limit (0.5) given by Modde for an acceptable model (Table 4). The predictability value, Q2, indicates how well the model predicts new data. Q2 was over 0.5 for most of the models (0.52–0.79). The only predictability that was below the 0.5 limit (0.43) was the model for hydrolysis of alkaline oxidised bagasse in 72 h. When examining the hydrolysis yields after 48 and 72 h (Table 4), it became clear that the continuation of hydrolysis from 48 to 72 h with AlkOx bagasse did not significantly increase the degree of hydrolysis, which might be the reason for the poorer model quality. As the constraints for the variables in AlkOx bagasse optimization were broader, it is also evident that a less accurate model would be obtained. The hydrolysis experiments were carried out with relatively low enzyme dosages (4– 10 mg g−1 ). The reason for the low dosages was the high hydrolysability of the substrates by the mix of thermostable enzymes used. Higher enzyme dosages were also tested, but they resulted in a situation in which the hydrolysis reached almost the maximum in 24 h and the low difference between the mixtures caused very low quality models (data not shown). As the analysis method, the reducing sugar assay, does not distinguish between different hydrolysis products (different monosaccharides, oligosaccharides), different kinds of product mixtures can produce the same apparent effect in the reducing sugar assay. This risk was higher with AlkOx bagasse having clearly higher xylan content. As the enzyme mixture used lacked an essential enzyme for hydrolysis of xylan to monosaccharides (β-xylosidase), it is probable that the conversion of xylan to monosaccharides was not complete, and thus, glucose was the main hydrolysis product detected by the reducing sugars assay. Composition of Optimal Enzyme Mixtures For the different pretreated raw materials and hydrolysis times, the optimal enzyme mixtures of thermostable enzymes contained 49–73 % of Cel7A (CBHI), 10–30 % of Cel6A (CBHII), 6– 16 % of Cel5A (EGII), 1–12 % of xylanase (Xyn10A) and 2–5 % of βG/Cel3A (Table 4). The optimal enzyme mixture of T. reesei enzymes for hydrolysis of pretreated wheat straw contained 39–42 % of Cel7A (CBHI), 32–34 % of Cel6A (CBHII), 20–26 % of Cel7B (EGI), 1–2 % of Cel5A (EGII), 1 % of xylanase (Xyn11A) and 1–2 % of Cel3A (βG) (Table 4). T. reesei enzymes showed relatively narrow optimal areas, whereas for thermostable enzymes, the optimal area within the studied proportions was very broad (Figs. 3 and 4). This can partly be explained by the fact that the constraints for T. reesei enzymes were set broader than for thermostable enzymes. The results obtained with both thermostable enzymes and T. reesei enzymes showed that in a longer hydrolysis of pretreated wheat straw, the optimal area was narrower, whereas a similar trend was not seen with pretreated bagasses. In the hydrolysis of complex lignocellulosic substrates, various synergistic reactions occur and the structure and composition of the substrate may change as the hydrolysis proceeds. One enzyme mixture can hydrolyse efficiently in the beginning but then the hydrolysis might be blocked and another enzyme mixture could be more efficient in the later stages. Effect of Pretreatment and Raw Material on Optimal Enzyme Mixtures A few optimizations have been carried out for lignocellulosic substrates resembling wheat straw and sugar cane bagasse (Table 5) using Trichoderma enzymes. In addition to different raw materials and pretreatments, the studies have been carried out in different conditions applying different enzyme dosages, hydrolysis durations, with wet or dried substrate, and Appl Biochem Biotechnol
- 168. IV/12 Trichoderma enzymes 72 h48 h24 h Thermostable enzymes Fig. 3 Effect of the ratio of Trichoderma reesei Cel7A (CBHI), Cel6A (CBHII) and Cel7B (EGI) and thermostable AtCel7A (CBHI), CtCel6A (CBHII) and TaCel5A (EGII) on the hydrolysis yield of pretreated wheat straw. Response contour plots predicting yield after 24, 48 and 72 h of hydrolysis. For the T. reesei mixture, the constant proportion of Cel5A=Xyn11=Cel3A=0.1 was used; for thermostable enzymes, the proportions TaXyn10=0.12 and TaCel3A=0.02. Hydrolysis yields are presented as percentage of d.m. content AlkOx bagasse h84h42 72 h SE bagasse Fig. 4 Effect of the ratio of thermostable AtCel7A (CBHI), CtCel6A (CBHII) and TaCel5A (EGII) on the hydrolysis yield of steam-exploded (SE) and alkaline oxidised (AlkOx) bagasse. Response contour plots predicting yields after 24, 48 and 72 h of hydrolysis. For SE bagasse, the constant proportions TaXyn10=0.03 and TaCel3A=0.03 were used; for AlkOx bagasse, TaXyn10=0.12 and TaCel3A=0.03. Hydrolysis yields are presented as percentage of d.m. content Appl Biochem Biotechnol
- 169. IV/13 different variables and constrains. The optimal enzyme composition of Trichoderma enzymes for hydrolysis of hydrothermally pretreated wheat straw resembled relatively well the results obtained by Billard et al. [9] with steam-exploded wheat straw. Compared to the study with steam-exploded wheat straw, the proportions of Cel7A (CBHI) and Cel6A (CBHII) and the sum of endoglucanases were similar to those obtained with hydrothermally pretreated wheat straw, whereas the ratio of Cel7B (EGI) and Cel5A (EGII) and the proportion of xylanase were more significantly different (Table 4). As TrCel7B has significant xylanolytic activity [35, 36], it appears that part of the requirement for xylanase activity was satisfied by a high proportion of Cel7B. Compared to Trichoderma enzyme mixtures, the results obtained using thermostable enzymes with all the lignocellulosic substrates were significantly different with respect to the main cellulases (Table 4). The general finding was that the thermostable enzyme mixtures required much more CBHI than any other optimal enzyme mixtures containing Trichoderma enzymes. Although the enzymes used in thermostable enzyme mixtures were not purified and the SDS-PAGE showed minor additional bands, the heat treatment almost totally inactivated the xylanolytic and endoglucanase activities present before the heat treatment (data not shown). Thus, the difference in the proportions is due to the different properties of thermo- stable enzymes compared to T. reesei enzymes. Because the experimental setup and differences in different studies appeared to affect significantly the optimal mixtures, general trends of the effects of different raw materials and pretreatments on the mixture composition were difficult to identify. However, some conclusions could be drawn about the effect of pretreatment and raw material on optimal mixtures of thermostable enzymes. Firstly, more AtCel7A (CBHI) was needed in the optimal Table 5 Optimized enzyme mixtures for hydrolysis of various pretreated lignocellulosic materials Raw material Pretreatment Cel7A Cel6A Cel7B Cel5A Xylanase βG Other Hydrolysis yield (%) Reference Sugar cane bagasse AlkOx 49–58 14–30 – 5–13 12 3 – 67–88a, b This study Sugar cane bagasse SE 57–73 10–24 – 9–11 1–4 3-5 – 84–98a, b This study Wheat straw Hydrothermal 52–56 17–19 – 13–16 8–12 2-3 – 51–74a, b This study Wheat straw Hydrothermal 39–42 32–34 20–26 1–2 1 1-2 – 37–56a This study Wheat straw SE 40 27 15 6 12 –c 0 68d [9] Barley straw SE 27 47 27 0 – –c –c 56e [8] Barley straw Hot water 20 43 37 0 – –c –c 56e [8] Corn stover SE 27 35 21 – – 6 11 36f [14] Corn stover AFEX 29 19 35 – 14 –c 3 51e [13] Corn stover AFEX 35 4 26 – 19 12 4 44d [2] Corn stover Alkaline 49 4 34 – 4 5 4 41d [2] Corn stover Alk. peroxide 43 4 30 – 11 8 4 58d [2] SE steam explosion, AlkOx alkaline oxidation a Twenty-four- to 72-h hydrolysis b Thermostable enzymes c Present in the hydrolysis but not in variables of optimization d Forty-eight-hour hydrolysis e Twenty-four-hour hydrolysis f Seventy-two-hour hydrolysis Appl Biochem Biotechnol
- 170. IV/14 mixture for SE bagasse than for AlkOx bagasse. Secondly, hydrolysis of AlkOx bagasse required at least three times more xylanase than hydrolysis of SE bagasse. The difference can partly be explained by the chemical composition of the materials. Bagasse pretreated by alkaline oxidation had a very high xylan content (24 %), whereas the xylan content of SE bagasse was only 3 %. Interestingly, Banerjee et al. [2] found that more xylanase (TrXyn10) was needed after AFEX than NaOH and alkaline peroxide pretreatments. The role of xylanase in improving the accessibility of cellulose to cellulases can be more significant in materials with high hemicellulose (alkaline oxidised bagasse) and lignin contents (AFEX-treated material) [4]. Thirdly, comparison of two raw materials pretreated with relatively similar methods, wheat straw and bagasse, showed that more AtCel7A (CBHI) and a lower amount of xylanase were needed with SE bagasse than with hydrothermally pretreated wheat straw, although the carbohydrate compositions were similar. Higher enzyme dosage and degree of hydrolysis with pretreated bagasse might have affected the composition of the optimal enzyme mixture. Wheat straw substrate was also used as dried sheet disks, whereas the more homo- geneous SE bagasse could be pipetted as a slurry and was used without drying. It is possible that drying also decreased the accessibility of enzymes and thus increased the role of xylanases. It appears that cell wall anatomy and microstructure can have a significant impact on optimal enzyme proportions in addition to the content of lignin, cellulose and hemicellulose. The Role of Cellobiohydrolases The total amount of CBHs varied between 70 and 88 % of the protein in all the optimal enzyme mixtures, being significantly higher than that produced by T. reesei [26]. Cel7A (CBHI) has an important role in hydrolysis since it efficiently breaks down the bulk of crystalline cellulose of the substrate. Relatively wide variation, 10 %, was allowed in the proportions of CBHI and CBHII practically without effect in the hydrolysis yield (Figs. 3 and 4). Generally less Cel7A (CBHI) was needed in longer 72 h hydrolysis than in 24 h. Probably more crystalline substrate is available for CBH in the beginning than in the later part of hydrolysis. The only exception was steam-exploded bagasse, for which the lowest optimal share of CBHI (57 %) was in hydrolysis lasting 24 h. With SE bagasse, the high hydrolysis degree after 48 and 72 h resulted in broad optimal areas instead of a single optimal point, and therefore, enzyme mixtures containing 50–80 % Cel7A (CBHI) gave almost the same hydrolysis yield (Fig. 4). If enzyme dosage is high enough and the hydrolysis time is long, even a suboptimal enzyme mixture can produce high hydrolysis yields. The Role of Endoglucanases The role of endoglucanases was clearly less important than that of CBHs. The EG content varied from 5 to 27 % in optimized mixtures. However, the addition of Cel7B (EGI) to T. reesei mixture caused clear increase in the degree of hydrolysis. In preliminary optimization, only 35 % degree of hydrolysis was obtained with a dosage of 10.5 mg g−1 without Cel7B (EGI), whereas in optimization experiments, 56 % hydrolysis was reached with a dosage of 6 mg g−1 . The high increase indicates the significant role of Cel7B (EGI) in hydrolysis with T. reesei enzymes. Similar conclusions were also drawn in the studies of Billard et al. on steam-pretreated wheat straw [9]. In an optimal mixture of T. reesei enzymes, 20–26 % of Cel7B (EGI) and only 1–2 % of Cel5A (EGII) were required. Banerjee et al. [2] suggested that the overlapping functions of Cel7B (EGI) and Cel5A (EGII) can result in situations in which major changes in the relative proportions of these two enzymes cause only small shifts in the hydrolysis yield. According to the model, increasing the EGII content from 1 to 11 % and decreasing EGI from 26 to 16 % Appl Biochem Biotechnol
- 171. IV/15 caused only 2 % decrease in hydrolysis yield. In the mixture of thermostable enzymes, the role of EG was less significant and the mixtures contained 5–16 % of TaCel5A (EGII). However, TaCel5A has been shown to be efficient in the liquefaction of pretreated wheat straw [19]. The Role of Xylanase The amount of xylanase in the mixtures was limited to 1–12 % (or 1–15 % with AlkOx bagasse) of total protein. A relatively high proportion of xylanase, 12 %, was needed in the hydrolysis of hydrothermally pretreated wheat straw and AlkOx bagasse with the optimal mixtures of thermostable enzymes. In AlkOx bagasse, the required high xylanase amount could be explained by high xylan content of the substrate. Xylanase activity has been shown to be important in the hydrolysis of hydrothermally pretreated wheat straw although its xylan content is low [7]. Interestingly, only a minimal amount of xylanase was needed in the hydrolysis of wheat straw with a T. reesei enzyme mixture, which may be due to the fact that T. reesei endoglucanase Cel7B (EGI) has been shown to be active on arabinoxylan and xylan [35, 36], and thus, a high content of Cel7B could at least partly replace xylanase or vice versa [11]. The improving effect of T. reesei EGI on hydrolysis yield may thus also be due to its xylanolytic activity. The T. reesei and thermostable enzyme mixtures also contained xylanases from different GH families, which can affect their role in the enzyme mixture. It has been reported that the hydrolysis products of family 10 xylanases are shorter than family 11 xylanases and family 10 xylanases are less specific to xylan than family 11 xylanases [37]. Shorter hydrolysis products giving potentially higher response in reducing sugars assay and broader specificity might have made the role of TaXyn10A more important than that of TrXyn11 in this experimental setup. In the preliminary optimization study, the xylanase (NfXyn11) dosage was 2,500 nkat g−1 , corresponding to 5.7 and 2.5 % of the total protein for thermostable enzymes and Trichoderma enzymes, respectively. It appears that more than 2,500 nkat g−1 was required in the efficient hydrolysis of AlkOx bagasse and pretreated wheat straw. TaXyn10 has shown higher efficien- cy in solubilizing xylan in pretreated wheat straw than NfXyn11 [7]. The too low xylanase dosage might also have been one reason for the low degree of hydrolysis by the T. reesei mixture in the preliminary optimization. The high amount of even more efficient xylanase in the optimized mixtures of thermostable enzymes underlines the importance of an adequate level of xylanolytic activities in total hydrolysis of the wheat straw substrate. The Role of β-Glucosidase The design of the experiments was made by limiting the βG content to 1–5 % of protein. Most of the optimal mixtures of thermostable enzymes contained 2–3 % of βG. Interestingly, only 1–2 % of βG or even less was needed with T. reesei enzymes in the hydrolysis of wheat straw. Zhou et al. [14] reported 6 % of βG in optimized mixtures for hydrolysis of SE corn stover, whereas studies by Banerjee et al. [2] indicated that the optimal mixture for hydrolysis of various pretreated substrates contained 4–12 % of βG when the minimum tested value was 4 %. It is possible that even a lower proportion of βG could provide better hydrolysis yields with some materials. For pretreated dried distillers grains with solubles (DDGS), the need was even higher (20–43 %) and it is clear that the required βG proportion for hydrolysis of different biomasses can vary considerably. The present study as well as the optimizations in the literature (Table 5) was carried out at 1 % d.m. content, and it is possible that the required amount of βG would be higher in conditions with higher consistency, for example because of the increased inhibition of βG by high glucose concentration. Appl Biochem Biotechnol
- 172. IV/16 Evaluation of the Performance of Optimal Mixtures The performance of the preliminary mixtures of the five major enzymes (Cel7A (CBHI), Cel6A (CBHII), Cel5A (EGII), xylanase and β-glucosidase) either from T. reesei or from thermostable sources was compared to that of the commercial enzymes. Hydrolysis of wheat straw for 72 h with a commercial mixture (Celluclast+Novozym188) resulted in 75 % hydrolysis yield, which was higher than that obtained by the mixtures of purified enzymes (Fig. 2). This comparison was rather unfair due to the high protein dosage and high load of accessory enzymes of Novozym188, but results indicated that there was still potential to improve the performance of Trichoderma and thermostable enzyme mixtures by further optimization. The hydrolysis of pretreated wheat straw by Trichoderma enzymes was significantly increased by full optimization compared to the results in preliminary optimization. In optimi- Fig. 5 Hydrolysis of a alkaline oxidised bagasse and b steam- exploded bagasse with optimal thermostable enzyme mixture and a mixture of Cellic Ctec2 and Cellic Htec at 8 % d.m. content. The enzyme dosage was 10 mg g−1 with alkaline oxidised bagasse and 15 mg g−1 with steam-exploded bagasse and the ratio of Ctec2/ Htec was 85:15 Appl Biochem Biotechnol
- 173. IV/17 zation experiments, a maximum of 56 % hydrolysis yield was obtained with a dosage of 6 mg g−1 , whereas in pre-tests with a dosage of 10.5 mg g−1 , the maximal hydrolysis yield was only 35 %. The corresponding increase was less marked with thermostable enzymes (from 70 to 74 %), but when considering the lower enzyme dosage in optimization (6 mg g−1 ), the improvement was clear. The optimized mixture of thermostable enzymes also showed higher efficiency when compared to the hydrolysis yield obtained using the commercial Celluclast- Novozym 188 mixture with a clearly higher dosage. The performance of the optimal thermostable enzyme mixture was also evaluated in the hydrolysis of wheat straw with never-dried substrate at 1 and 12 % d.m. contents; 1 % substrate was hydrolysed with the dosage of 6 mg g−1 very easily, giving 70, 86 and 96 % yield in 24, 48 and 72 h, respectively. It is obvious that the never-dried substrate was hydrolysed more easily than the dried handsheet disks. Hydrolysis at 12 % d.m. content was clearly slower and only 52, 61 and 66 % yield was obtained in 24, 48 and 72 h, respectively. It is also possible that hydrolysis at high dry matter concentration would require a different optimal enzyme mixture than hydrolysis at 1 % d.m. content. The performance of the optimal enzyme mixtures of thermostable enzymes (mixtures for 72 h hydrolysis) in hydrolysis of bagasse was compared to that of a commercial enzyme Cellic at 8 % d.m. content. With AlkOx bagasse, the hydrolysis was very efficient with both the optimal mixture and the commercial enzyme (Fig. 5a). However, the commercial enzyme mixture reached the maximum degree of hydrolysis clearly earlier than the optimal enzyme mixture. With SE bagasse, the optimal enzyme mixture was more efficient in the beginning of hydrolysis but the hydrolysis then slowed down more steeply (Fig. 5b). With AFEX-treated corn stover, a six-component enzyme mixture could not reach the hydrolysis yield obtained by commercial enzyme mixtures even with high enzyme dosages, whereas an 11-component mixture was almost as efficient in the longer hydrolysis [12]. It has also been suggested that the importance of accessory enzymes increases in the later stages of hydrolysis. The rate and degree of hydrolysis, especially in the final stage of hydrolysis, was probably improved by these accessory enzymes present in the full commercial product but not in the thermostable enzyme mixture, composed essentially of five monocomponent preparations. As a conclusion, by careful selection and optimization of the enzyme components, it was possible to reach a hydrolysis performance close to that of the commercial products, although it became evident that the optimal enzyme mixtures could further be improved by including other additional enzymes needed for complete hydrolysis of cell wall carbohydrates. Conclusions Enzyme mixtures were optimized by statistically designed experiments. Preliminary studies showed that a CBH/EG ratio of 4:1 or higher gave maximal synergy in the hydrolysis of hydrothermally pretreated wheat straw by thermostable enzymes. The composition of optimal enzyme mixtures depended clearly on the substrate and enzyme system studied. The optimal thermostable enzyme mixture contained Cel7A as a major component, whereas in the Trichoderma reesei mixture, relatively high proportions of Cel6A and Cel7B were also needed. Hydrolysis of wheat straw with thermostable enzymes also required more xylanase than hydrolysis with T. reesei enzymes, in which the high proportion of Cel7B (EGI) appeared to provide the required xylanase activity. The raw material and pretreatment also had an effect on the optimal enzyme mixture. Even materials with relatively low xylan content required a high proportion of xylanase, possibly due to otherwise decreased accessibility of the cellulose Appl Biochem Biotechnol
- 174. IV/18 substrate. The results also indicated that wide variation was possible for enzyme components without a major effect on the hydrolysis yield. Comparison with commercial enzymes showed that optimal enzyme mixtures of thermostable enzymes had high hydrolysis performance in the hydrolysis of pretreated wheat straw and bagasse. Acknowledgments Financial support of the EU 7th framework program project HYPE grant number 213139 and the Finnish Funding Agency for Technology and Innovation (Tekes) Biorefine program project SugarTech is gratefully acknowledged. The authors thank Jari Leino and Juha Kaunisto for carrying out the steam explosion and alkaline oxidation pretreatments, Markku Saloheimo for providing T. reesei supernatant with CtCel6A enzyme and Roosa Luode, Ulla Vornamo and Jenni Lehtonen for carrying out the hydrolysis experiments. References 1. Nidetzky, B., Hayn, M., Macarron, R., Steiner, W. (1993). Biotechnology Letters, 15, 71–76. 2. Banerjee, G., Car, S., Scott-Craig, J., Borrusch, M., Walton, J. (2010). Biotechnology and Biofuels, 3, 22. 3. Kumar, R., Wyman, C. E. (2009). Bioresource Technology, 100, 4203–4213. 4. Hu, J., Arantes, V., Saddler, J. (2011). Biotechnology and Biofuels, 4, 36. 5. Selig, M. J., Knoshaug, E. P., Adney, W. S., Himmel, M. E., Decker, S. R. (2008). Bioresource Technology, 99, 4997–5005. 6. Varnai, A., Huikko, L., Pere, J., Siika-aho, M., Viikari, L. (2011). Bioresource Technology, 102, 9096– 9104. 7. Zhang, J., Tuomainen, P., Siika-aho, M., Viikari, L. (2011). Bioresource Technology, 102, 9090–9095. 8. Rosgaard, L., Pedersen, S., Langston, J., Akerhielm, D., Cherry, J. R., Meyer, A. S. (2007). Biotechnology Progress, 23, 1270–1276. 9. Billard, H., Faraj, A., Lopes, F. N., Menir, S., Heiss-Blanquet, S. (2012). Biotechnology and Biofuels, 5, 9. 10. Gao, D., Chundawat, S. S., Liu, T., Hermanson, S., Gowda, K., Brumm, P., Dale, B., Balan, V. (2010). Bioenergy Research, 3, 67–81. 11. Banerjee, G., Car, S., Scott-Craig, J. S., Borrusch, M. S., Aslam, N., Walton, J. D. (2010). Biotechnology and Bioengineering, 106, 707–720. 12. Banerjee, G., Car, S., Scott-Craig, J. S., Borrusch, M. S., Bongers, M., Walton, J. D. (2010). Bioresource Technology, 101, 9097–9105. 13. Gao, D., Chundawat, S. P. S., Krishnan, C., Balan, V., Dale, B. E. (2010). Bioresource Technology, 101, 2770–2781. 14. Zhou, J., Wang, Y., Chu, J., Luo, L., Zhuang, Y., Zhang, S. (2009). Bioresource Technology, 100, 819–825. 15. Gusakov, A. V., Salanovich, T. N., Antonov, A. I., Ustinov, B. B., Okunev, O. N., Burlingame, R., Emalfarb, M., Baez, M., Sinitsyn, A. P. (2007). Biotechnology and Bioengineering, 97, 1028–1038. 16. Berlin, A., Maximenko, V., Gilkes, N., Saddler, J. (2007). Biotechnology and Bioengineering, 97, 287– 296. 17. Garlock, R. J., Balan, V., Dale, B. E. (2012). Bioresource Technology, 104, 757–768. 18. Viikari, L., Alapuranen, M., Puranen, T., Vehmaanperä, J., Siika-Aho, M. (2007). Advances in Biochemical Engineering/Biotechnology, 108, 121–145. 19. Szijarto, N., Horan, E., Zhang, J., Puranen, T., Siika-aho, M., Viikari, L. (2011). Biotechnology and Biofuels, 4, 2. 20. Petersen, M. Ø., Larsen, J., Thomsen, M. H. (2009). Biomass and Bioenergy, 33, 834–840. 21. Kallioinen, A., Hakola, M., Riekkola, T., Repo, T., Leskelä, M., von Weymarn, N., Siika-aho, M. (2013). Bioresource Technology, 140, 414–420. 22. Puls, J., Poutanen, K., Körner, H., Viikari, L. (1985). Applied Microbiology and Biotechnology, 22, 416– 423. 23. Tenkanen, M., Siika-aho, M. (2000). Journal of Biotechnology, 78, 149–161. 24. Suurnäkki, A., Tenkanen, M., Siika-aho, M., Niku-Paavola, M., Viikari, L., Buchert, J. (2000). Cellulose, 7, 189–209. 25. Tenkanen, M., Puls, J., Poutanen, K. (1992). Enzyme and Microbial Technology, 14, 566–574. 26. Sipos, B., Benko, Z., Dienes, D., Reczey, K., Viikari, L., Siika-aho, M. (2010). Applied Biochemistry and Biotechnology, 161, 347–364. 27. Paloheimo, M., Mäntylä, A., Kallio, J., Suominen, P. (2003). Applied and Environmental Microbiology, 69, 7073–7082. Appl Biochem Biotechnol
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- 177. Series title and number VTT Science 56 Title Development of pretreatment technology and enzymatic hydrolysis for biorefineries Author(s) Anne Kallioinen Abstract The growing demand for energy, materials and food, depletion of fossil raw material reservoirs and increasing environmental concerns have all increased interest in renewable resources. Lignocellulosic biomass is an alternative for replacing fossil raw materials in the production of fuels, materials and various chemicals. Lignocellulose present in plant cell walls consists mainly of polysaccharides, cellulose and hemicellulose, and aromatic lignin. These major components form a complex structure that is resistant to microbial and enzymatic activity. Due to the recalcitrant structure of plant cell walls, lignocellulosic raw materials must be pretreated before their enzymatic hydrolysis to monosaccharides. Various pretreatment methods; chemical, physical, biological or their combinations, have been developed. After pretreatment, polysaccharides can be hydrolysed enzymatically to monosaccharides, which in turn can be fermented to different products such as ethanol. Currently the first commercial scale lignocellulosic ethanol plants have started production. A secure supply of biomass is one of the key factors for a feasible biorefinery, and new alternative feedstocks are still required especially in northern climates in order to fulfil the raw material demands of biorefineries in a sustainable way. In addition, development of new pretreatment technologies and more efficient enzymatic hydrolysis are needed. New lignocellulosic feedstocks and improved pretreatment methods were studied in the work de- scribed in this thesis. Reed canary grass and barley straw were found to be interesting carbohydrate-rich raw materials that could be pretreated by steam explosion and hydrolysed enzymatically with yields comparable to those obtained from wheat straw. Selection of the most favourable harvest time for reed canary grass, autumn or spring, was studied in relation to pretreatment and hydrolysis yields. Spring harvested reed canary grass was found to be the more suitable raw material as it had a higher cellulose content and the pretreated fibre was hydrolysed more efficiently compared to autumn harvested material. A new pretreatment method using sodium carbonate and oxygen pressure was developed. The alkaline oxidation method fractionated biomass into a carbohydrate-rich fibre and a dissolved fraction containing most of the lignin. The produced carbohydrate-rich fibre could be efficiently hydrolysed by enzymes and the hydrolysis was also efficient at 12% dry matter content. Compared to the 52% total glucose yield obtained in enzyme hydrolysis of spruce after pretreatment by steam explosion, a significantly higher glucose yield of 84% was obtained in hydrolysis after alkaline oxidation. Different kinds of raw materials, such as spruce, birch and sugar cane bagasse, could be efficiently pretreated by alkaline oxidation. The main effects of alkaline oxidation pretreatment were dissolution and partial degradation of lignin and hemicellulose. Some galactoglucomannan and xylan was solubilised and further oxidised to other products, and therefore relatively low yields of hemicellulose were obtained. Organic acids were formed as degradation products of lignin and carbohydrates. Process conditions were partially optimized using spruce as raw material in order to improve the efficiency of alkaline oxidation. The pretreatment could be accelerated by increasing the treatment temperature, by the use of copper-phenanthroline catalyst, and by decreasing the particle size of the raw material. Further optimization of e.g. alkali dosage and the solid to liquid ratio is still required to improve hemicellulose yield and economical feasibility. Fibre fractions of alkaline oxidation could be hydrolysed by low enzyme dosages, 2–4 FPU/g dry matter. Significantly higher enzyme dosages were required in the hydrolysis of steam exploded materials, probably due to the inhibitory effect of the high residual lignin content after the pretreatment. The efficient hydrolysis of alkaline oxidised materials by low enzyme dosages can decrease enzyme costs or enable shorter hydrolysis time. In order to further improve the hydrolysis efficiency and decrease the required enzyme dosage, en- zyme mixtures were optimized regarding the major enzymes needed in biomass hydrolysis. Optimized mixtures of thermostable enzymes were found to have significantly different proportions of cellobiohydro- lases, endoglucanases and xylanase than the optimized mixtures of Trichoderma reesei enzymes. Although different, the significant role of cellobiohydrolases was demonstrated in both types of mixtures. The results also indicated that high xylanase activity was required in the hydrolysis of pretreated materials having decreased enzyme accessibility to cellulose due to high xylan content or possibly due to drying of the substrate. The hydrolysis performance of optimized enzyme mixtures of five thermostable enzyme components was shown to be close to that of state-of-the-art commercial mixtures. ISBN, ISSN ISBN 978-951-38-8143-6 (Soft back ed.) ISBN 978-951-38-8144-3 (URL: http://www.vtt.fi/publications/index.jsp) ISSN-L 2242-119X ISSN 2242-119X (Print) ISSN 2242-1203 (Online) Date April 2014 Language English, Finnish abstract Pages 107 p. + app. 64 p. Keywords Lignocellulose, pretreatment, enzymatic hydrolysis, optimal enzyme mixture Publisher VTT Technical Research Centre of Finland P.O. Box 1000, FI-02044 VTT, Finland, Tel. +358 20 722 111
- 179. Julkaisun sarja ja numero VTT Science 56 Nimeke Esikäsittely- ja entsyymihydrolyysiteknologioiden kehittäminen biojalostamosovelluksiin Tekijä(t) Anne Kallioinen Tiivistelmä Kasvavat energian, materiaalien ja ruuan tarpeet, fossiilisten raaka-ainevarojen vähentyminen ja huoli ympäristöstä ovat lisänneet kiinnostusta uusiutuviin luonnonvaroihin. Lignoselluloosapohjainen bio- massa on vaihtoehto fossiilisille raaka-aineille polttoaineiden, materiaalien ja monien kemikaalien tuotannossa. Kasvien soluseinän lignoselluloosa koostuu pääosin polysakkarideista, kuten selluloo- sasta ja hemiselluloosasta, sekä aromaattisesta ligniinistä. Nämä pääkomponentit muodostavat monimutkaisen rakenteen, joka on hyvin kestävä mikrobien ja entsyymien hajotukselle. Koska kasvien soluseinat ovat lujia, lignoselluloosapohjaiset raaka-aineet täytyy esikäsitellä ennen entsymaattista hydrolyysia monosakkarideiksi. Erilaisia kemiallisia, fysikaalisia ja biologisia esikäsittelymenetelmiä tai niiden yhdistelmiä on kehitetty. Esikäsittelyn jälkeen polysakkaridit voidaan hydrolysoida entsymaatti- sesti monosakkarideiksi, jotka voidaan edelleen fermentoida erilaisiksi tuotteiksi, kuten etanoliksi. Tällä hetkellä ensimmäiset kaupallisen mittakaavan lignoselluloosapohjaista bioetanolia valmistavat tehtaat ovat aloittaneet tuotannon. Koska kannattava biojalostamo vaatii turvatut raaka-ainelähteet, uusia ja vaihtoehtoisia biomassoja tarvitaan edelleen erityisesti pohjoisessa ilmastossa täyttämään biojalosta- moiden raaka-ainetarve. Myös uusien esikäsittelytekniikoiden kehitystä ja nykyistä tehokkaampaa entsyymihydrolyysiä tarvitaan. Tässä työssä tutkittiin uusia lignoselluloosaraaka-aineita ja kehitettiin esikäsittelymenetelmiä. Tutki- muksessa havaittiin, että ruokohelpi ja ohran olki olivat kiinnostavia korkean hiilihydraattipitoisuuden omaavia raaka-aineita, jotka voitiin esikäsitellä höyryräjäytyksellä ja hydrolysoida entsymaattisesti vehnän olkeen verrattavilla saannoilla. Ruokohelven korjuuajankohdan vaikutusta esikäsittelyyn ja hydrolyysisaantoon tutkittiin syksyllä ja keväällä korjatulla ruokohelvellä. Keväällä korjatun ruokohelven havaittiin olevan sopivampi raaka-aine, sillä sen selluloosapitoisuus oli suurempi ja siitä saatu esikäsi- telty kuitu hydrolysoitui paremmin verrattuna syksyllä korjattuun materiaaliin. Tutkimuksessa kehitettiin uusi esikäsittelymenetelmä, joka perustui alkaliseen käsittelyyn hapetta- vissa olosuhteissa käyttämällä kemikaaleina natriumkarbonaattia ja happea. Alkalihapetus fraktioi bio- massan hiilihydraattipitoiseen kuituun ja ligniinipitoiseen liuenneeseen fraktioon. Tuotettu kuitu voitiin hydrolysoida tehokkaasti entsyymeillä, ja hydrolyysi oli tehokas myös 12 %:n kuiva-ainepitoisuudessa. Verrattuna höyryräjäytetyllä kuusella saatuun 52 %:n glukoosisaantoon esikäsittelyssä ja hydro- lyysisssä, merkittävästi korkeampi 84 %:n glukoosisaanto saatiin käyttämällä alkalihapetusta esikäsit- telynä. Menetelmä soveltui erilaisten raaka-aineiden, kuten kuusen, koivun ja sokeriruokobagassin, esikäsittelyyn. Alkalihapetuksen päävaikutukset olivat ligniinin ja hemiselluloosan liukeneminen ja osittainen ha- joaminen. Osa galaktoglukomannaanista ja ksylaanista liukeni ja hapettui edelleen muiksi tuotteiksi, ja siksi hemiselluloosasaanto oli suhteellisen alhainen. Alkalihapetuksessa muodostui ligniinin ja hiilihyd- raattien hajoamistuotteina orgaanisia happoja. Prosessiolosuhteita optimoitiin alkalihapetuskäsittelyn tehokkuuden parantamiseksi käyttämällä kuusta raaka-aineena. Esikäsittelyä voitiin tehostaa nosta- malla käsittelylämpötilaa, käyttämällä kupari-fenantroliini-katalyyttiä ja pienentämällä raaka-aineen partikkelikokoa. Hemiselluloosasaannon ja taloudellisen tehokkuuden parantamiseksi tarvitaan esi- merkiksi alkaliannostuksen ja kiintoaine-neste-suhteen lisäoptimointia. Alkalihapetus-esikäsittelystä saadut kuitufraktiot hydrolysoituivat tehokkaasti jo alhaisilla entsyy- miannoksilla, 2–4 FPU/g kuiva-ainetta. Merkittävästi korkeampia entsyymiannoksia tarvittiin höyryrä- jäytettyjen materiaalien hydrolyysissä, mikä johtui todennäköisesti korkean ligniinipitoisuuden aiheut- tamasta inhiboivasta vaikutuksesta. Alkalihapetettujen materiaalien tehokas hydrolyysi alhaisilla entsyymiannostuksilla voi alentaa entsyymikustannuksia tai mahdollistaa lyhyen hydrolyysiajan. Biomassan hydrolyysissa tarvittavien pääentsyymien seoksien koostumusta optimoitiin, jotta hyd- rolyysiä voitaisiin edelleen tehostaa ja alentaa entsyymiannoksia. Lämpöstabiilien entsyymien opti- moiduissa seoksissa oli eri suhteissa sellobiohydrolaaseja, endoglukanaaseja ja ksylanaasia kuin Trichoderma reesei -homeen entsyymeistä koostetuissa optimoiduissa seoksissa. Sellobiohydrolaasit olivat kuitenkin merkittävin entsyymi molemmissa seoksissa. Saadut tulokset viittaavat siihen, että korkeaa ksylanaasiaktiivisuutta tarvitaan sellaisten esikäsiteltyjen materiaalien hydrolyysissä, joissa entsyymien pääsy selluloosaan on heikentynyt korkeasta ksylaanipitoisuudesta tai mahdollisesti raaka-aineen kuivaamisesta johtuen. Viiden termostabiilin entsyymin optimaaliset seokset vastasivat hydrolyysitehokkuudeltaan kaupallisia entsyymiseoksia. ISBN, ISSN ISBN 978-951-38-8143-6 (nid.) ISBN 978-951-38-8144-3 (URL: http://www.vtt.fi/publications/index.jsp) ISSN-L 2242-119X ISSN 2242-119X (painettu) ISSN 2242-1203 (verkkojulkaisu) Julkaisuaika Huhtikuu 2014 Kieli Englanti, suomenkielinen tiivistelmä Sivumäärä 107 s. + liitt. 64 s. Avainsanat Lignocellulose, pretreatment, enzymatic hydrolysis, optimal enzyme mixture Julkaisija VTT PL 1000, 02044 VTT, puh. 020 722 111
- 180. VTTSCIENCE56 Developmentofpretreatmenttechnologyandenzymatichydrolysisfor... Development of pretreatment technology and enzymatic hydrolysis for biorefineries The growing demand for energy, materials and food, finite fossil raw material reservoirs and increasing environmental concerns have all increased interest in renewable resources. Lignocellulosic biomass is a widely available alternative for replacing fossil raw materials in the production of fuels, materials and various chemicals. Lignocellulose present in plant cell walls has a recalcitrant structure and therefore needs to be pretreated before enzymatic hydrolysis. Various pretreatment methods; chemical, physical, biological or their combinations, have been developed. After pretreatment, polysaccharides can be hydrolysed enzymatically to monosaccharides, which in turn can be fermented to different products such as ethanol. A secure supply of biomass is one of the key factors for a feasible biorefinery, and thus new feedstocks are required. In addition, development of new pretreatment technologies and more efficient enzymatic hydrolysis are needed. In this work new alternative lignocellulosic raw materials – reed canary grass and barley straw – were investigated as feedstocks for a biorefinery in northern climates. In addition, an alkaline oxidative pretreatment method was developed which can fractionate the main components of lignocellulose and provide a well-hydrolysable cellulose fraction. Furthermore, optimal enzyme mixtures were developed for the hydrolysis of pretreated materials. ISBN 978-951-38-8143-6 (Soft back ed.) ISBN 978-951-38-8144-3 (URL: http://www.vtt.fi/publications/index.jsp) ISSN-L 2242-119X ISSN 2242-119X (Print) ISSN 2242-1203 (Online)