Modulating fracture properties of mixed protein systems

Modulating fracture properties of mixed protein systems
Carsten Ersch a, b, Irene ter Laak b, Erik van der Linden a, b, Paul Venema b,
Anneke Martin a, c, *
a Top Institute Food & Nutrition (TIFN), PO Box 557, 6700 AN Wageningen, The Netherlands
b Wageningen University and Research Centre, Laboratory of Physics and Physical Chemistry of Foods, PO Box 8129, 6700 EV Wageningen, The Netherlands
c TNO, PO Box 360, 3700 AJ Zeist, The Netherlands
a r t i c l e i n f o
Article history:
Received 25 April 2014
Accepted 4 September 2014
Available online 16 September 2014
Keywords:
Fracture
Soy protein
Gelatin
Texture
Protein mixture
Formulation flexibility
a b s t r a c t
To design foods with desired textures it is important to understand structure build-up and breakdown.
One can obtain a wide range of structures using mixtures of different structuring ingredients such as for
example protein mixtures. Mixed soy protein isolate (SPI)/gelatine gels were analyzed for their linear
rheological properties, fracture properties and microstructure. The two ingredients were found to form
independent networks despite changes in the SPI microstructure, which were attributed to micro phase
separation. It is shown that mixing of SPI and gelatine allows to arrive at a large variety of fracture
properties. This provides opportunities for tailoring textures in foods using mixed independent gel
networks. The fracture stress of mixed gels corresponded to the fracture stress of the strongest of the two
gels. At constant fracture stress, increasing Young's modulus of the mixed independent gels resulted in
reduced fracture strain.
© 2014 Elsevier Ltd. All rights reserved.
1. Introduction
The demand for a sustainable production of protein containing
products leads to an increased interest in the use of alternative
(mostly plant) proteins. This leads to challenges like employing
these proteins in existing foods without changing texture, taste
and appearance. Because simple replacement of one protein with
another usually leads to different properties one needs to resort to
mixtures. Studying the fracture properties of mixed protein sys-tems
in which one is of plant origin is therefore essential to gain
insight on how textural properties can be modulated when
introducing new proteins.
To employ biopolymer mixtures in foods, a basic understanding
of their interactions and phase behaviour is important. Thermo-dynamic
(in)compatibility of macro constituents is one of the most
important and mostly researched topics in mixed systems. Poly-saccharide
(PS)/protein mixtures as the most commonly researched
biopolymer mixture (Çakir & Foegeding, 2011; Doublier, Garnier,
Renard, & Sanchez, 2000; Martínez & Pilosof, 2012; Ould Eleya &
Turgeon, 2000; Pires Vilela, Cavallieri, & Lopes da Cunha, 2011;
Tolstoguzov, 1991; Turgeon & Beaulieu, 2001; van den Berg,
2008) usually show phase separation at relatively low (total)
polymer concentration (Doublier et al., 2000). Protein mixtures, on
the other hand, are thermodynamically compatible over a wide
concentration range dependent on their Osbourne classification,
hydrophobicity and surface charges (Polyakov, Kireyeva, Grinberg,
& Tolstoguzov, 1985). However, changes such as denaturation, ag-gregation
and/or coilehelix transitions reduce their compatibility
with other polymers and interaction with water (i.e. solubility)
drastically. This can induce phase separation and therefore struc-tural
changes in single and mixed systems (Ako, Nicolai, Durand, &
Brotons, 2009; Fitzsimons, Mulvihill, & Morris, 2008; Polyakov,
Grinberg, & Tolstoguzov, 1997).
The rheological properties and microstructure has been
described in detail for several polysaccharide/protein (Çakir &
Foegeding, 2011; Doublier et al., 2000; Martínez & Pilosof,
2012; Ould Eleya & Turgeon, 2000; Pires Vilela et al., 2011;
Tolstoguzov, 1991; Turgeon & Beaulieu, 2001; van den Berg,
2008) and protein/fat (Kim, Renkema, & Van Vliet, 2001; Sala,
2007) mixtures. van den Berg (2008) and Sala (2007) have
linked large deformation properties for two examples of the
above mentioned mixtures to sensory attributes by mixing whey
protein with different polysaccharides (van den Berg, 2008) or
using different types of proteins in emulsion filled gels (Sala,
2007). For mixed protein systems, however, these relationships
have not yet been established. First steps towards understanding
the structure formation and breakdown properties have been
* Corresponding author. Top Institute Food & Nutrition (TIFN), PO Box 557, 6700
AN Wageningen, The Netherlands. Tel.: þ31 (0)88 8661780.
E-mail address: anneke.martin@tno.nl (A. Martin).
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Food Hydrocolloids
journal homepage: www.elsevier.com/locate/foodhyd
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Food Hydrocolloids 44 (2015) 59e65

60 C. Ersch et al. / Food Hydrocolloids 44 (2015) 59e65
undertaken for mixed protein systems containing gelatine and a
globular protein. Here, gelatine was shown to gel inside existing
networks (gels) of globular proteins whereas the reverse situa-tion
is more challenging (Walkenstr€om & Hermansson, 1997).
Rheological properties of mixed gels ranged from gelatine-like to
whey protein (Brink, Langton, Stading, & Hermansson, 2007;
Walkenstr€om & Hermansson, 1994, 1996; Walkenstr€om et al.,
1997) or egg white-like (Ziegler, 1991). Rheological changes
were attributed to segregative phase separation driven by poly-merization
of one of the two proteins (e.g. aggregation or coil to
helix transition (Fitzsimons et al., 2008)). However, these results
were only obtained for a limited number of proteins (not
including plant proteins), protein concentrations and mixing
ratios. Therefore, using plant proteins and at the same time
mapping rheological properties at all possible concentrations and
mixing ratios provides new insight and opportunities for
tailoring mixed gelatine/globular protein systems.
Soy protein is one of the most researched and readily avail-able
plant storage proteins and therefore, of high interest for
research and industrial applications. Only few related studies on
mixed gels including soy protein isolate (SPI) have been pub-lished
(Chronakis & Kasapis, 1993; Comfort & Howell, 2002).
Moreover, the interpretation thereof is often difficult since SPI
itself can be considered a mixed system (Kasapis & Tay, 2009;
Renkema, Knabben, & van Vliet, 2001; Sok, Kasapis, & Han,
2008). Gelatine, on the other hand, is frequently studied in
mixed systems (Alevisopoulos, Kasapis, & Abeysekera, 1996;
Almrhag et al., 2012, 2013; Altay & Gunasekaran, 2013; Ares
et al., 2007; Fiszman, Lluch, & Salvador, 1999; Fiszman &
Salvador, 1999; Fitzsimons et al., 2008; Salvador & Fiszman,
1998; Shrinivas, Kasapis, & Tongdang, 2009; Walkenstr€om
et al., 1994, 1996, 1997; Ziegler, 1991; Ziegler & Rizvi, 1989).
Gelatine is thermo-reversible and rheologically well distin-guishable
from heat induced, globular protein gels such as the
brittle and weak (tofu-like) SPI gel. Due to its lack of tertiary
structure it compares well to many polysaccharides, when in the
coil conformation (Doublier et al., 2000). Nevertheless, it is
thermodynamically compatible with most proteins over a wider
range of concentrations other than most polysaccharides
(Tolstoguzov, 1988).
The objective of this study is to map and understand the
origin of the fracture properties of mixed gelatine/SPI gels.
Differential scanning calorimetry, small deformation measure-ments
and microscopy are combined to gain better under-standing
of the structure build-up of mixed gelatine/soy protein
gels and its relation with fracture properties. These results will
help one to employ protein mixtures of plant origin in
designing foods.
2. Material and methods
2.1. Material
Defatted soybean flour was obtained from Cargill having ~50%
w/w protein. Pork skin gelatine type A was generously provided by
Rousselot B.V. (Ghent, Belgium) having a nominal bloom strength
of 150 (determined by manufacturer following a standardized
procedure), a protein content of 89.6% (Kjeldahl, N-factor 5.5), an
isoelectric point around 8 (Isoelectric focussing and QC-RLT) and
negligible amounts of salts (ICP-AES). Chemicals were obtained
from Sigma Aldrich (St Louis, MO, USA) and were of analytical
grade. They were used without any further purification using
demineralized water (conductivity 1.5 mS/cm).
2.2. Sample preparation
2.2.1. Soy protein isolate and gelatine stock solution preparation
Soy protein isolate (SPI) was prepared by isoelectric precipita-tion
and subsequent washing steps as described in literature
(Urbonaite, de Jongh, Linden, & Pouvreau, 2014). The extracted SPI
was kept as solution (pH 7) and had a protein content of 11%
(Kjeldahl, N-factor 6.38). For microbiological stability 0.02%
sodium-azide was added. Solutions were stored at 4 "C and used
within 4 weeks after protein extraction. Within this time no
physical changes were observed. Gelatine solutions were prepared
by dispersing gelatine granules in demineralized water. For sam-ples
with 10% SPI, gelatinewas dissolved directly in the SPI solution.
Gelatine solutions were heated at 60 "C for 1 h in a water bath and
stored at 4 "C overnight.
2.2.2. Sample preparation
All solutions (including demineralized water) were degassed
before usage and handling was done at a sample temperature of
40 "C to avoid gelling of the gelatine. Protein stock solutions (12%
SPI, 20% gelatine) were mixed with demineralized water, NaCl and
Na-azide stock solutions to obtain desired protein concentrations
(0e10% SPI, 0e14% gelatine) and constant concentrations of 300
millimolal NaCl and 20 millimolal Na-Azide. MOPS buffer stock
solution (1 M, pH 6.8) was added to reach a final buffer concen-tration
of 20 millimolal.
2.3. Rheological techniques
2.3.1. Large deformation
Samples were placed in pre-lubricated (paraffin oil) sealed
plastic tubes, tempered for 1 h at 40 "C, heated at 95 "C for 30 min
and afterwards stored in an acclimatised room overnight at 25 "C.
Gels were cut into cylindrical pieces (2 cm # 2 cm) using a steel
wire. For each sample 4 cylindrical pieces were measured and
average values are shown. A 90% strain compression test was per-formed
using a texture analyzer (TA-XT plus, Stable Micro Systems
Ltd., Godalming, U.K.) equipped with a 500 N load cell. The probe
had a much larger diameter than the cylinders and paraffin oil was
applied to all surfaces allowing sufficient lubrication. Samples were
compressed in a single compression test to 90% of their initial
height at a compression speed of 1 mm/s. True stress, true strain
and Young's modulus were calculated as described elsewhere (de
Jong & van de Velde, 2007).
2.3.2. Small deformation
After preparation, samples were stored in the freezer ($20 "C)
and thawed in a water bath prior to analysis at 40 "C for 30 min
before rheological measurements (Anton Paar MC502 rheometer
Graz, Austria). Gelling behaviour was compared with non-frozen
samples and their behaviour did not change significantly upon
freezing and thawing. A sand-blasted cup-bob geometry (CC17), a
strain of 0.5% and a frequency of 1 Hz were used throughout the
experiment. Samples were covered with a thin layer of paraffin oil
to prevent evaporation and heated from 40 "C to 95 "C. They were
kept at 95 "C for 1 h before cooling back to 15 "C. The temperature
was then kept at 15 "C for 1 h before re-heating again to 95 "C.
Heating and cooling rates were set at 5 "C/min.
2.4. Differential scanning calorimetry (DSC)
Samples were placed in aluminium pans, equilibrated at 40 "C
inside the equipment (TA Instruments DSC Q1000 NewCastle, USA)
and then cooled to 15 "C before starting the measurements. The
samples were heated at 1 "C/min to 110 "C and subsequently cooled

C. Ersch et al. / Food Hydrocolloids 44 (2015) 59e65 61
again to 15 "C. Samples were kept at both 15 and 110 "C for 5 min.
Analysis was performed using the manufacturer software (TA
Universal Analysis). Data smoothing was done at 2 "C/min and
baseline rotation was done between 40 "C and 60 "C.
2.5. Confocal laser scanning microscopy (CLSM)
For CLSM analysis, 0.05% (w/w) rhodamine B was added to the
samples and gels were made by heating at 95 "C for 30 min inside a
sealed cuvettes cuvettes (Product Name: Gene Frame® 125 ml, ob-tained
from Thermo Scientific) in awater bath. Afterwards, cuvettes
were cooled down to room temperature and samples analyzed
using a Leica DMI6000 (Wetzlar, Germany) equipment. Several
pictures at different location in the samples were scanned and
shown structures were found throughout the sample.
3. Results and discussion
3.1. Transition and denaturation temperatures of single and mixed
systems
The thermogram for single SPI, single gelatine and their mix-tures
is given in Fig. 1. The two endothermic peaks in single SPI
represent the denaturation temperatures of the two main soy
protein fractions beta conglycinin (77 "C) and glycinin (97 "C)
(Bainy, Tosh, Corredig, Woodrow, & Poysa, 2008; Margatan, Ruud,
Wang, Markowski, & Ismail, 2013; Tseng, Xiong, & Boatright,
2008). When reheated these peaks are absent in all curves, point-ing
to the irreversible nature of SPI denaturation.
For single gelatine, the endothermic peak around 28 "C corre-sponds
to the helix to coil transition (melting). During cooling helix
to coil formation of gelatine resulted in one exothermic peak. Re-sults
for the cooling ramps are not shown since they show the same
trends as the heating ramps. Gelling and melting peaks are present
upon re-measuring the samples indicating the reversibility of the
coil helix transition. Transition temperature values agree with
literature for gelatine (Bainy et al., 2008; Dranca & Vyazovkin,
2009; Soeda, Kaneko, Hokazono, Tujimoto, & Murakami, 2005).
When gelatine and SPI are mixed, all the peaks belonging to the
single proteins were observed. The SPI denaturation temperatures
were not influenced by the presence of gelatine. On the other hand
the melting (and also gelling) temperature of gelatine increased
with increasing gelatine ratio. This is most likely due to increasing
total gelatine concentration when changing polymer ratios. This
presumably kinetic effect is in line with earlier reported DSC
measurements on gelatine at different gelatine concentrations
(Koh, Matia Merino, & Dickinson, 2002).
Since no significant changes in the enthalpic peaks were found
upon mixing, it is generally accepted (Kasapis, 2008; Kasapis,
Morris, Norton, & Brown, 1993; Ould Eleya & Turgeon, 2000) that
no specific interactions between SPI and gelatine are to be
expected.
3.2. Structure build-up in single and mixed systems
Gel formation was analyzed by measuring the storage modulus
(G0) while applying a thermal treatment as presented in Fig. 2. For
single SPI, a sharp increase in G0 occurred at a gelling time of 30 min
which for ease of wording, will be referred to as gelling time. For all
tested samples, tan(d) was <0.5 independent of the gelatine con-centration
at the end of the heating time. Upon cooling (Fig. 2,
70e90 min), non-covalent bonds such as hydrogen bonds
strengthen the network further leading to an additional increase in
G0 (Beliciu & Moraru; Renkema, 2001). On the other hand, single
gelatine forms a gel once cooled below the coil to helix transition
after ~90 min in Fig. 2. Re-heating this gel leads to melting of the
gelatine network (at ~150 min) indicated by a sharp drop in G0.
In mixed systems of SPI and gelatine, addition of gelatine to SPI
leads to a significant reduction in gelling time for the SPI network
accompanied by an increase in the plateau value of G0 at 95 "C
(70 min). This reduction in gelling time can be caused by a decrease
in repulsive forces between proteins (Verheul & Roefs, 1998) but it
can also be caused by an increase in overall protein concentration
(Fernandes, 1994). Since 300 millimolal NaCl was added, electro-static
interactions are strongly screened. It is therefore unlikely that
addition of gelatine has an effect on electrostatic interactions be-tween
soy proteins. This suggests that the cause for the reduced
gelling time is based on a concentration effect where SPI is effec-tively
concentrated by the presence of gelatine. Higher effective SPI
concentrations are also in line with the increased G0 plateau values
Fig. 1. Thermogram for single SPI (10%w/w, solid line), single gelatine (10%w/w, solid line with dots) and mixed SPI/gelatine having 10% gelatine and 2% SPI (dashed line with dots)
and 10% SPI and 2% gelatine (dashed line).

10000
1000
100
10
1
0.1
0 50 100 150
100
80
60
40
20
0
Storage Modulus [Pa]
Temperature [°C]
Time [min]
Fig. 2. Storage modulus during heating and cooling for 4% single SPI (long dashes) and
mixed systems containing 4% SPI plus 5% gelatine (short dashes) and 4% SPI with 10%
gelatine (dashes with dots). As a reference 5% single gelatine (solid line) is added. Also
shown is the temperature profile (dash dot dot) with heating/cooling performed at
5 "C/min.
compared to single SPI gels observed at 95 "C (72 min). A similar
effect is observed for gelatine gels (~140 min) where increasing G0
values upon SPI addition are observed.
When comparing G0 values of the mixed systems before gelatine
gelation (corresponding to 75 min in Fig. 2) and after gelatine
melting (corresponding to 150 min in Fig. 2) they are comparable.
This suggests that the gelation of gelatine in the presence of a
continuous SPI gel does not affect the continuity of the SPI network.
Together with the DSC results this suggests that both proteins form
independent gels.
The microstructure of single and mixed systems was probed by
CLSM (Fig. 3). Single SPI gels as well as gelatine gels (results not
shown) do not show any structures above the resolution of the
CLSM independent of protein concentrations. However, mixed gels
do show structures with an increase in coarseness (typical distance
between areas with higher and lower intensity) upon addition of
gelatine at a given initial SPI concentration. Such an increase in
coarseness is commonly seen in heat-set globular protein gels (such
as SPI) upon addition of salts, generally attributed to phase sepa-ration
on a microscopic level (Ako, Nicolai, et al., 2009). The
development of coarseness occurred during heat treatment above
70 "C. However, no macroscopic nor microscopic phase separation
was observed in the temperature range between 40 and 60 "C. In
mixed systems, an increase in coarseness, i.e. microscopic phase
separation, can be caused by an increased tendency for segregative
phase separation due to denaturation and polymerization as seen
on a more macroscopic scale in other protein/protein as well as
protein/polysaccharide systems (Fitzsimons et al., 2008; Ould Eleya
& Turgeon, 2000; Tobin, Fitzsimons, Chaurin, Kelly, & Fenelon,
2012). The occurrence of phase separation on a microscopic
length scale is further supported by an increase in G0 and decrease
in gelling time as presented in Fig. 2 (Ako, Durand, Nicolai, & Becu,
2009; Ako, Nicolai, et al., 2009; Çakir & Foegeding, 2011; Verheul &
Roefs, 1998; Verheul, Roefs, Mellema, & de Kruif, 1998).
3.3. Fracture properties of mixed protein gels
Large deformation tests were performed on single and mixed
SPI/gelatine samples. Fig. 4 shows fracture stress, fracture strain
and Young's modulus as a function of gelatine concentration at
different SPI concentrations. For single SPI, only a concentration of
10% gave a self-supporting gel and therefore no values for lower
concentrations could be measured. For single gelatine, gels above
8% were strong enough to be measured. Their fracture stress and
Young's modulus increased with protein concentration while the
fracture strain is concentration independent.
The fracture stress (Fig. 4A) of mixed gels closely follows the
behaviour of the single gelatine gels deviating only at low gelatine
(<5%) and high SPI concentration. At these deviating conditions,
gelatine alone forms a weak gel with lower fracture stress than the
(in this case stronger) SPI gel. Nevertheless, in most samples gela-tine
forms stronger gels and dominates the fracture stress of the
mixed gels. Knowing that in mixed gels SPI and gelatine networks
co-exist leads to the hypothesis that during compression both
Fig. 3. CLSM pictures (at different magnification top/bottom) of 10% SPI (left) and mixed systems containing 7.5% SPI plus 2% (middle) and 4.5% (right) gelatine. Samples were
heated to 95 "C and cooled to room temperature before analysis (both proteins are in the gel state). Scale bars are 250 mm (top) and 50 mm (bottom).

100000
10000
2.0
1.5
1.0
0.5
100000
10000
networks are able to break independently. The measured fracture
stress corresponds to the fracture stress of the stronger network
implying that the other network has already been broken at a lower
stress. Unfortunately, due to limited sensitivity in large deforma-tion
experiments the fracturing of the weaker network stays un-detected
as was also reported earlier by Brink et al. (2007).
The Young's modulus (GE) is defined by the initial slope of the
stress versus strain relationship during compression. In mixed gels,
GE increases with increasing gelatine and increasing SPI concen-tration
(Fig. 4C). The general shape of the GE versus gelatine con-centration
curve, however, is not significantly altered by the
presence of SPI. Higher GE values lead to a “faster” increase in stress
over strain. Samples reach their fracture stress at a lower defor-mation
(strain). This effect is represented in the reduced fracture
strain values upon SPI addition at constant gelatine concentration
as presented in Fig. 4B.
Fracture properties of mixed SPI/gelatine gels (and possibly
other globular protein/gelatine gels) were found to be dependent
on only two factors, the fracture stress of the stronger of the two
networks (as known for single systems) and the Young's modulus
of the system. To enable one to modulate the fracture properties of
a mixed system, the main question that remains is the mechanism
underlying the increase in GE upon addition of either one of the
proteins. In general, one can think of two possible mechanisms for
increased GE values in mixed SPI/gelatine gels.
First, if both proteins would be present at sufficiently high
protein concentration they both form a gel independent from each
other. The gelation of SPI occurs upon heating and in mixed
systems, resulting gels are slightly firmer than in single SPI system
at the corresponding SPI concentration (Figs. 2 and 4). Upon cool-ing,
gelatine forms a gel within the existing SPI network as shown
in Fig. 2 and also reported earlier for WPI/gelatine mixtures
(Walkenstr€om et al., 1994, 1996). In these mixed gels where both
proteins form a gel, measurements of GE will probe both networks
simultaneously and therefore an increase in GE was found with
increasing gelatine and SPI concentration. Nevertheless, the
modulus of the mixed system is not a simple addition of the moduli
of the corresponding single systems.
Secondly, if SPI would be present below its gelling concentra-tion
it forms soluble aggregates when heated (Campbell, Gu,
Dewar, & Euston, 2009). In the mixed gels, presence of aggre-gates
increases the GE and G0 by either reducing the amount of
available water for gelatine (concentrating the gelatine) or by
acting as filler particles. Unfortunately, with rheological mea-surements
in this research the possible mechanisms could not be
differentiated from each other.
3.4. Opportunities for mixed protein systems
A wide range in breakdown properties of mixed gels can be
obtained using a combination of SPI and gelatine. In Fig. 5 a texture
map as commonly used for food systems is presented.
SPI gels are commonly brittle and soft whereas gelatine gels are
deformable and hard (for definition of terms please refer to Fig. 5).
By combining SPI and gelatine, gels with intermediate values in
hardness and deformability can be prepared. Increasing the SPI
concentration at fixed gelatine concentration leads to a reduced
100000
10000
1000
True Fracture Stress [Pa]
A
Hard
Soft
Brittle Deformable
100000
10000
1000
Increasing SPI
Increasing Gelatine
0 0.5 1 1.5 2
True Fracture Strain [1]
B
Hard
Soft
Brittle Deformable
Fig. 5. Texture map of mixed gels as analyzed in this research (B) and an overlay of the
texture maps of different literature values with the current research (A); details graph
B: samples contain 0e14% gelatine and 0% (þ), 2% (#), 4% (*), 6% (
), 8% (-) and 10%
▫
(B) SPI, dotted line shows outline as used in graph A; details graph A: outline from
this research (solid line) together with WPI/kappa-carrageenan cold set gels (van den
Berg, 2008) (short dashes), different tofu model systems (long dashes) (Urbonaite
et al., 2014), gelatin/xanthan mixed gels (interrupted dashes) (Altay & Gunasekaran,
2013) and WPI/kappa-carrageenan (lines with points) (Çakir & Foegeding, 2011)
with the continuous phase being kappa-carrageenan (open circles) or WPI (closed
triangles).
1000
A
0.0
True Fracture Strain [1]
B
Single SPI
Single Gelatine
1000
0% 4% 8% 12% 16%
Young's Modulus [Pa]
Gelatine Concentration [% g/gSolution]
C
Single Gelatine
Single SPI
Fig. 4. True fracture stress (A), true fracture strain (B) and Young's modulus (C) of
mixed gelatine/SPI gels as a function of gelatine concentration with SPI concentrations
of 0% (#) 2% (
), 4% (B), 6% (
▵
▫
), 8% (>) and 10% (;). Gels were measured at room
temperature after heat treatment to allow both proteins to form a gel.

fracture strain making the gels more brittle. Gelatine on the other
hand increases fracture strain and fracture stress making the gels
harder and more deformable. This allows one to modulate the
fracture properties and obtain combinations of fracture properties
which are not possible with just SPI or gelatine. Especially being
able to increase or decrease fracture stress and strain partially
independently in these type of gels is of high importance in
developing novel foods or food ingredients.
Mixed SPI/gelatine samples cover a wide range of fracture
stress (10e100 kPa) and fracture strain (0.7e1.6). To compare this
range in fracture properties to other type of mixed systems, the
outlined area from Fig. 5B is displayed in Fig. 5A together with
data from other studies. Harder, elastic SPI/gelatine samples are
in the same region as different tofu model system (Urbonaite
et al., 2014) and the whey protein continuous part of a whey
protein/k-carrageenan study (Çakir & Foegeding, 2011). Weaker
more brittle samples show overlap with the phase inverted (k-carrageenan
continuous) part of the same study (Çakir &
Foegeding, 2011). Also within the range of SPI/gelatine are
studies on the textural properties of a system with polysaccharide
addition to whey protein aggregate gels (van den Berg, 2008) as
well as mixed gelatine/xanthan gum gels (Çakır et al., 2012).
The wide range in fracture properties of mixed SPI/gelatine
gels and the overlap with other (mixed) biopolymer systems
indicate the application possibilities that this system offers.
Selectively mixing of proteins may lead to design of new textures
with controllable sensory properties. However, the physical as-pects
of sensory perception are not limited to location in the
texture map only. In fact, the texture map mainly relates to sen-sory
perception “first bite” and neglects all other oral processing
steps (Stokes, Boehm, & Baier, 2013). For a more detailed
description of sensory, factors such as water release, recoverable
energy, microstructure and rheological properties after fracture
(Çakır et al., 2012) but also the balance between energy going into
fracture and dissipation (van den Berg, 2008) are of importance.
4. Conclusions
SPI and gelatine formed mixed gels upon applying an appro-priate
heating and cooling profile, in which both proteins are able
to form independent gel network structure. Interactions between
proteins are non-specific and lead to micro-phase separation
during heat induced gelation of SPI. Mixed gels had increased
moduli (G0 and GE) while the fracture stress was determined by
whichever single gel formed a stronger gel. Gelatine tends to
dominate rheological properties of mixed protein/gelatine sys-tems
as reported in literature (Walkenstr€om et al., 1994, 1996,
1997; Ziegler & Rizvi, 1989). However, the fact that gelatine is
able to dominate mixed gel fracture properties is mainly based on
the fact that gelatine is able to form strong gels already at low
total polymer concentration. Using the SPI/gelatine gels as an
example, these results suggest that for any mixed gel with two
independent networks the fracture behaviour will always be
dominated by the stronger of the two gels. However, the rela-tionship
between the modulus of single and mixed gels still re-quires
more attention to be able to entirely predict the fracture
properties of mixed gels based on the properties of the single
gels.
Fracture properties of SPI/gelatine mixed gels were found to
overlap with existing studies having significant differences in
sensorial properties. This proves the system to be interesting for
future textural studies and developments of new foods. Mixing SPI/
gelatine allows one to modulate the breakdown properties (and
possibly textures) and increases the formulation flexibility during
the development of foods as ingredients can be chosen partially
independent of the products fracture properties.
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