Carbohydrates: Part 1

Contents
Introduction
Functions
Classification
Blood Group Substances
Qualitative tests

Introduction
 Carbohydrates may be defined as poly-hydroxy
aldehydes or ketones or compounds which produce
them on hydrolysis.
 The term ‘sugar’ is applied to carbohydrates soluble
in water and sweet to taste.
 They are primarily composed of the elements carbon,
hydrogen and oxygen.

Functions
 Carbohydrates are the most abundant dietary source of energy (4 Cal/g) for all
organisms.
 They are precursors for many organic compounds (fats, amino acids, etc.).
 Carbohydrates (as glycoproteins and glycolipids) participate in the structure of cell
membrane and cellular functions such as cell growth, adhesion and fertilization.
 Carbohydrates also serve as the storage form of energy (glycogen) to meet the
immediate energy demands of the body.
 They are structural components of many organisms. These include the fibre (cellulose)
of plants, exoskeleton of some insects and the cell wall of microorganisms.
 It helps in breakdown of fatty acids and prevention of ketosis.
 Carbohydrates form a part of DNA and RNA in the form of deoxyribose and ribose
sugars.

Monosaccharides
 Monosaccharides are the simplest group of carbohydrates.
 They are often referred to as simple sugars.
 They have the general formula Cn(H2O)n.
 They cannot be further hydrolysed.
 They are colourless crystalline compounds readily soluble in water and sweet in taste.
 Their solution are optically active and exhibit the phenomenon of Mutarotation.
 In our nature, most of D-form of sugars occurs.
 Sugars may have cyclic structure most of alpha and beta form.

Monosaccharides
 The monosaccharides are divided into different categories based on:
 Stereoisomerism is an important character of monosaccharides. The number of
asymmetric carbon atoms (n) determines the possible isomers of a given compound
which is equal to 2n.
FUNCTIONAL GROUP
Classification
[[[
• When the functional group is an
aldehyde, they are known as
aldoses. For eg. Glyceraldehyde,
Glucose, etc.
• When the functional group is a
ketone, they are known as ketoses.
For eg., Dihydroxyacetone,
Fructose, etc.
NUMBER OF CARBON ATOMS
• Based on the number of carbon
atoms, the monosaccharides are
regarded as
• These terms along with functional
groups are used while naming
monosaccharides. For eg. Glucose
is an aldohexose while Fructose is
a ketohexose.
Aldoses:
Ketoses:
Trioses Tetroses Pentoses
Hexoses Heptoses
(3C) (4C) (5C)
(6C) (7C)

Structural Relationship between D- Aldoses
(Fischer Projection)
D- Glyceraldehyde
D- Erythrose D- Threose
D- Ribose D- Arabinose D- Xylose D- Lyxose
D- Allose D- Altrose D- Glucose D- Mannose D- Gulose D- Idose D- Galactose D- Talose
Aldo-Trioses
(3C)
Aldo-Tetroses
(4C)
Aldo-Pentoses
(5C)
Aldo-Hexoses
(6C)

Structural Relationship between D- Ketoses
(Fischer Projection)
Dihydroxyacetone
D- Erythrulose
D- Ribulose D- Xylulose
D- Psicose D- Fructose D- Sorbose D- Tagatose
Keto-Triose
(3C)
Keto-Tetrose
(4C)
Keto-Pentoses
(5C)
Keto-Hexoses
(6C)

Oligosaccharides
 Oligosaccharides contain 2-10 monosaccharide molecules which are liberated on
hydrolysis. They are held together by a glycosidic bond.
 They are of two types:
1. Reducing oligosaccharides with free aldehyde or ketone group. For eg., maltose,
lactose, cellobiose, etc.
2. Non-reducing oligosaccharides with no free aldehyde or ketone group. For eg.,
sucrose, trehalose, raffinose, etc.
 Based on the number of monosaccharide units present, the oligosaccharides are further
subdivided to disaccharides, trisaccharides, tetrasaccharides, pentasaccharides, etc.

Oligosaccharides
Disaccharides
 A disaccharide consists of two monosaccharide units (similar or dissimilar) held together
by a glycosidic bond either with 1-2 or 1-4 linkage.
 They are crystalline, water-soluble and sweet to taste.
Maltose:
Lactose:
Sucrose:
Composed of two α-D-glucose units with
1-4 α linkage. Reducing sugar. Sunflower
shaped osazone crystals.
Composed of β-D-galactose and β-D-
glucose units with 1-4 β linkage.
Reducing sugar. Cotton-ball shaped
osazone crystals. It is commonly known
as milk sugar.
Composed of α-D-glucose and α-D-
fructose units with 1-2 β linkage. Non-
Reducing sugar. Needle shaped osazone
crystals.

Oligosaccharides
Disaccharides
Trehalose: Composed of two α-D-glucose
units with 1-1 α linkage. Non-
reducing sugar. It is resistant to
acid hydrolysis, and stable in
solution at high temperatures, even
under acidic conditions. Some
bacteria, fungi, plants and
invertebrate animals synthesize it
as a source of energy, and to
survive freezing and lack of water.
Cellobiose: Composed of two β-D-glucose
units with 1-4 β linkage. Reducing
sugar. It can be hydrolyzed to
glucose enzymatically or with acid.
It can be obtained by enzymatic or
acidic hydrolysis of cellulose and
cellulose-rich materials such as
cotton, jute, or paper.

Oligosaccharides
Trisaccharides
 Trisaccharides are oligosaccharides composed of three monosaccharides with two
glycosidic bonds connecting them. For eg., Raffinose, Melezitose, Rhaminose, Rabinose,
etc.
Raffinose: Composed of α-D-Galactose and Sucrose
units with 1-6 α linkage. Non-reducing
sugar. It is commonly found in plants. It
can be hydrolysed to D-galactose and
sucrose by the enzyme α-galactosidase,
an enzyme not found in the human
digestive tract.
Melezitose: Composed of Sucrose and α-D-Glucose
units with 1-3 α linkage. Non-reducing
sugar. It is produced by many plant sap
eating insects by an enzyme reaction.
This is useful to the insects, as it reduces
the stress of osmosis by reducing their
own water potential. It can be partially
hydrolyzed to glucose and turanose (1-3
α linkage; isomer of sucrose).

Oligosaccharides
Tetrasaccharides
 A tetrasaccharide is a carbohydrate which gives upon hydrolysis four molecules of the
same or different monosaccharides. For eg. Stachyose, Maltotetraose, etc.
 The general formula of a tetrasaccharide is typically C24H42O21.
Stachyose: Composed of two α-D-galactose units, one α-D-glucose unit, and one β-D-
fructose unit sequentially linked as gal(α1→6)gal(α1→6)glc(α1↔2β)fru.
Non-reducing sugar. It is commonly found in plants. Stachyose upon
hydrolysis gives one molecule each of glucose and fructose and two
molecules of galactose. It is less sweet than sucrose, at about 28% on a
weight basis. It is mainly used as a bulk sweetener. Stachyose is not
completely digestible by humans and delivers 1.5 to 2.4 kcal/g (6 to 10 kJ/g).

Oligosaccharides
Pentasaccharides
 A tetrasaccharide is a carbohydrate which gives upon hydrolysis four molecules of the
same or different monosaccharides. For eg. Stachyose, Maltotetraose, etc.
 The general formula of a tetrasaccharide is typically C24H42O21.
Verbascose: Composed of three α-D-galactose units, one α-D-glucose unit, and
one β-D-fructose unit sequentially linked as
2gal(α1→6)gal(α1→6)glc(α1↔2β)fru. Non-reducing sugar. Upon
hydrolysis, verbascose gives one molecule each
of glucose and fructose and three molecules of galactose.

Relationship of all Oligosaccharides

Polysaccharides
 Polysaccharides are the most abundant Carbohydrate found in food.
 They are long chain polymeric carbohydrates composed of monosaccharide units (with
high molecular weight up to a million) bound together by glycosidic linkages.
 They are usually tasteless (non-sugars), amorphous and insoluble in water.
 Polysaccharides have a general formula of Cx(H2O)y where x is usually a large number
between 200 and 2500. When the repeating units in the polymer backbone are six-carbon
monosaccharides, as is often the case, the general formula simplifies to (C6H10O5)n, where
typically 40 ≤ n ≤ 3000.
 Polysaccharides are an important class of biological polymers. Their function in living
organisms is usually either structure- or storage-related.
 Polysaccharides are linear as well as branched polymers.
 The polysaccharides are of two types –
• Homopolysaccharides
• Heteropolysaccharides

Polysaccharides
Homopolysaccharides
 When all the monosaccharides in a polysaccharide are the same type, the polysaccharide
is called a Homopolysaccharides or homoglycan. For eg. Starch, cellulose, etc.
Starch:
Starch is a polysaccharide consisting of numerous glucose units joined by glycosidic bonds.
This is produced by most green plants as energy storage. It is the most common carbohydrate
in human diets and is contained in large amounts in staple foods like potatoes, maize (corn),
rice, and cassava.
Pure starch is a white, granular, tasteless and odorless
powder that is insoluble in cold water or alcohol. The
basic chemical formula of the starch molecule is
(C6H10O5)n. It consists of two types of molecules: the
linear and helical amylose and the branched amylopectin
joined in α 1,4 linkages. Depending on the plant, starch
generally contains 20 to 25% amylose and 75 to 80%
amylopectin by weight.

Polysaccharides
Glycogen:
Glycogen is a multi-branched polysaccharide of glucose that serves as a form of energy
storage in animals, fungi, and bacteria. The polysaccharide structure represents the main
storage form of glucose in the body. It functions as one of two forms of energy reserves,
glycogen being for short-term and the other form being triglyceride stores in adipose tissue
(i.e., body fat) for long-term storage. Glycogen is the called as animal starch. In humans,
glycogen is made and stored primarily in the cells of the liver and skeletal muscles. Small
amounts of glycogen are also found in other tissues and cells, including the kidneys, red blood
cells, white blood cells, and glial cells in the brain. The uterus also stores glycogen during
pregnancy to nourish the embryo.
Glycogen is a branched biopolymer consisting of
linear chains of glucose residues with an average
chain length of approximately 8–12 glucose units.
Glucose units are linked together linearly by
α(1→4) glycosidic bonds from one glucose to the
next. Branches are linked to the chains from which
they are branching off by α(1→6) glycosidic
bonds between the first glucose of the new branch
and a glucose on the stem chain.

Polysaccharides
Dextrin:
Dextrins are white, yellow, or brown powder that are partially or fully water-soluble, yielding
optically active solutions of low viscosity. Most of them can be detected with iodine solution,
giving a red coloration. White and yellow dextrins from starch roasted with little or no acid
are called British gum.
Dextrins are a group of low-molecular-weight carbohydrates produced by the hydrolysis of
starch or glycogen. These are mixtures of polymers of D-glucose units linked by α-(1→4) or α-
(1→6) glycosidic bonds. Dextrins can be produced from starch using enzymes like amylases,
as during digestion in the human body and during malting and mashing, or by applying dry
heat under acidic conditions (pyrolysis or roasting). Dextrins produced by heat are also known
as pyrodextrins.
Yellow dextrins are water-soluble gums basically used adhesive or glues in various industries.
White dextrins are used as a crispness enhancer for food
processing; a textile finishing and coating agent to increase
weight and stiffness of textile fabrics; a thickening and
binding agent in pharmaceuticals and paper coatings; a
pyrotechnic binder and fuel; and a stabilizing agent for
certain explosive metal azides, particularly Lead(II) azide

Polysaccharides
Cellulose:
Cellulose is a polysaccharide consisting of a linear chain of several hundred to many thousands
of β(1→4) linked D-glucose units. It is an important structural component of the primary cell
wall of green plants, many forms of algae and the oomycetes. Some species of bacteria secrete
it to form biofilms. Cellulose is the most abundant organic polymer on Earth. The cellulose
content of cotton fiber is 90%, that of wood is 40–50%, and that of dried hemp is
approximately 57%.
Cellulose is tasteless, odourless, hydrophilic, insoluble in water and most organic solvents,
chiral and biodegradable. It melts at 467 °C. It can be broken down chemically into its glucose
units by treating it with concentrated mineral acids at high temperature.
Cellulose is used to produce paperboard, paper,
cellophane, rayon, wood pulp and cotton.
In human nutrition, cellulose is a non-digestible
constituent of insoluble dietary fiber, acting as a
hydrophilic bulking agent for feces and potentially
aiding in defecation.

Polysaccharides
Chitin:
Chitin is a modified polysaccharide that contains nitrogen; it is synthesized from units of N-
acetyl-D-glucosamine [2-(acetylamino)-2-deoxy-D-glucose]. These units form covalent β-
(1→4)-linkages (like the linkages between glucose units forming cellulose). Therefore, chitin
may be described as cellulose with one hydroxyl group on each monomer replaced with an
acetyl amine group. This allows for increased hydrogen bonding between adjacent polymers,
giving the chitin-polymer matrix increased strength.
Chitin is a primary component of cell walls in
fungi, the exoskeletons of arthropods, such as
crustaceans and insects, the radulae of molluscs,
cephalopod beaks, and the scales of fish and
lissamphibians.
In its pure, unmodified form, chitin is translucent,
pliable, resilient, and quite tough.

Polysaccharides
Heteropolysaccharides
 When more than one type of monosaccharide is present they are
called Heteropolysaccharides or heteroglycans. For eg. Heparin.
Hyaluronic acid:
Hyaluronic acid is a polymer of disaccharides, themselves composed of D-glucuronic acid
and N-acetyl-D-glucosamine, linked via alternating β-(1→4) and β-(1→3) glycosidic bonds.
Hyaluronic acid is an anionic, nonsulfated glycosaminoglycan distributed widely throughout
connective, epithelial, and neural tissues. As one of the chief components of the extracellular
matrix, hyaluronan contributes significantly to cell proliferation and migration, and may also
be involved in the progression of some malignant tumors.
The average 70 kg (154 lb) person has
roughly 15 grams of hyaluronan in the
body, one-third of which is turned over
(degraded and synthesized) every day.
Hyaluronic acid is also a component of the
group A streptococcal extracellular
capsule, and is believed to play a role in
virulence.

Polysaccharides
Heparin:
Heparin is a medication and naturally occurring glycosaminoglycan.
As a medication it is used as an anticoagulant (blood thinner)
preventing the formation of clots and extension of existing clots
within the blood. While heparin does not break down clots that have
already formed (unlike tissue plasminogen activator), it allows the
body's natural clot lysis mechanisms to work normally to break
down clots that have formed.. Specifically it is also used in the
treatment of heart attacks and unstable angina.
It is given by injection into a vein or under the
skin. Other uses include inside test tubes and
kidney dialysis machines.
Heparin appears to be relatively safe for use
during pregnancy and breastfeeding. Heparin is
produced by basophils and mast cells in all
mammals.

Polysaccharides
Chondroitin sulphate:
Chondroitin sulfate is a sulfated glycosaminoglycan (GAG) composed of a chain of
alternating sugars (N-acetylgalactosamine and glucuronic acid). It is usually found attached to
proteins as part of a proteoglycan. A chondroitin chain can have over 100 individual sugars,
each of which can be sulfated in variable positions and quantities. Chondroitin sulfate is an
important structural component of cartilage and provides much of its resistance to
compression. Along with glucosamine, chondroitin sulfate has become a widely used dietary
supplement for treatment of osteoarthritis.

Blood Group Substances
The blood group antigens (A, B and O) of erythrocyte membrane contain carbohydrates as
glycoproteins or glycolipids. These antigens, upon reaction with specific antibodies, cause
agglutination of the cells to which they are attached. N-acetyl galactosamine, galactose,
fucose, sialic acid etc. are found in the blood group substances.
Fucose is a hexose deoxy sugar which is equivalent to 6-deoxy-l-
galactose. It is found on N-linked glycans on the mammalian, insect
and plant cell surface. In human N-linked glycans, fucose is most
commonly linked α-1,6 to the reducing terminal β-N-acetyl
glucosamine. However, fucose at the non-reducing termini linked α-
1,2 to galactose forms the H antigen, the substructure of the A and B
blood group antigens.

Qualitative Tests of Carbohydrates
Scheme For Detection Of Unknown Carbohydrate Sample:

MOLISCH’S TEST
Principle: This is a general test for carbohydrates. On treatment with strong Sulphuric acid,
carbohydrates undergo dehydration to give furfural derivative which on condensation with α-
naphthol yields a violet or purple coloured complex at the junction of two liquids.
Reagents: Molisch reagent (α-naphthol in ethanol)
Procedure:
i. At first, 2ml of carbohydrate solution was taken in a clean and
dry test tube.
ii. Then 2 drops of ethanolic α-naphthol is added and the solution
was mixed properly.
iii. Then the test tube was inclined and 2ml of conc. H2SO4 was
added by the side of the test tube so that the acid formed a layer
under the carbohydrate solution.
iv. Then the test tube was gently rotated between the palms of the
hands to bring about slight mixing at the surface.
Inference: An appearance of reddish violet or purple colored ring at the
junction of two liquids is observed in a positive Molisch test.

IODINE TEST
Principle: This test shows whether the carbohydrate sample is a monosaccharide or
disaccharide or a polysaccharide. If colour change occurs on addition of carbohydrate sample
to the iodine solution, then it is a polysaccharide or else a mono or disaccharide. Iodine forms
a co-ordinate complex between the helically coiled polysaccharide chain and iodine, centrally
located within the helix due to adsorption. The colour obtained depends upon the length of the
linear chain available for complex formation.
Reagents: Iodine solution
Procedure:
A few drops of iodine solution added to about 1ml of the sample solution.
Inference:
Amylose: A linear chain component of starch, gives a deep blue color
Amylopectin: A branched chain component of starch, gives a purple color
Glycogen: Gives a reddish brown color
Dextrins: Amylodextrin, Eryhthrodextrin and Achrodextrins, formed as
intermediates during hydrolysis of starch give violet, red and no color
with iodine respectively.

BENEDICT’S QUALITATIVE TEST
Principle: This test is for the reducing sugars only. Sugars possessing a free, or potentially
free, aldehyde or ketone group act as reducing agents. Such sugars reduce cupric ion in
alkaline solution at high temperature to insoluble yellow or red cuprous oxide.
Reagents: Benedict’s qualitative reagent contains:
a. Copper sulphate that furnishes cupric ion in the solution.
b. Sodium carbonate provides alkaline medium.
c. Sodium citrate prevents the precipitation of cupric ion as cupric hydroxide by
forming a loosely bound cupric sodium citrate complex which on dissociation
gives a continuous supply of cupric ions.

BENEDICT’S QUALITATIVE TEST
Procedure:
i. At first, take 5ml of Benedict’s reagent in a clean and dry test tube.
ii. Then add 8 drops of carbohydrate solution.
iii. Then boil the solution in the test tube over a flame or in boiling water bath for 2 mins.
iv. Then the solution was cooled down.
Inference:
Green colour: upto 0.5% present (+)
Green precipitate: 0.5 – 1.0% present (++)
Yellow precipitate: 1.0 – 1.5% present (+++)
Orange precipitate: 1.5 – 2.0% present (++++)
Brick red precipitate: >20% present (+++++)

FEHLING’S TEST
Principle: This test is also for the reducing sugars only. Sugars possessing a free, or
potentially free, aldehyde or ketone group act as reducing agents. Such sugars reduce cupric
ion in alkaline solution at high temperature to insoluble yellow or red cuprous oxide.
RCHO + 2Cu2+ + 4OH− RCOOH + Cu2O↓ + H2O
Reagents: Fehling’s reagent contains:
Fehling Solution A: It contains copper sulphate solution. It is prepared by dissolving 34.65 gm
of CuSO4.5H2O in 500 ml of distilled water.
Fehling Solution B: It contains potassium hydroxide and Rochelle salt (Sodium potassium
tartarate). It is prepared by dissolving 125 gm of KOH and 173 gm of Rochelle salt in 500 ml
of distilled water.
Equal volume of Fehling A and Fehling B should be mixed before use.

FEHLING’S TEST
Procedure:
i. At first, 2ml of Fehling’s reagent was taken in a clean and dry test tube.
ii. Then 2 ml of carbohydrate solution was added.
iii. Then the solution in the test tube was mixed and boiled.
Inference: Appearance of yellow or red precipitate is observed in a positive reaction

BARFOED’S TEST
Principle: Aldoses and ketoses can reduce cupric ions even in acidic conditions. This test is
used to distinguish reducing monosaccharides from disaccharides by controlling pH and time
of heating. Monosaccharides react very fast whereas disaccharides react very slowly. It is
based on the reduction of copper (II) acetate to copper (I) oxide (Cu2O), which forms a brick-
red precipitate.
Reagents:
Barfoed’s reagent is prepared by dissolving 24 gm of copper acetate in 400 ml of boiling water.
To this add 25 ml of 8.5% lactic acid solution. Stir, cool the solution and dilute to 500 ml.
Procedure:
i. At first, 2ml of Barfoed’s reagent was taken in a clean and dry test tube.
ii. Then 2 ml of carbohydrate solution was added.
iii. Then the test tube was kept in boiling water bath for 3 minutes only and cooled.
Overheating should be avoided.
Inference: Appearance of scanty brick red precipitate is observed in a positive reaction

SELIWANOFF’S TEST
Reagents: Resorcinol in concentrated hydrochloric acid (diluted 1:1 with
water).
Procedure:
At first, take 3ml of Seliwanoff’s reagent in a clean and dry test tube.
Then add 3 drops of carbohydrate solution.
Then heat the test tube over a flame for 30 seconds and cool.
Inference: An appearance of cherry red colour is observed in a positive
reaction.
Principle: This test is positive for ketohexose only and hence is used in the detection of
fructose. On treatment with hydrochloric acid, fructose forms 5-hydroxymethyl furfural; which
on condensation with resorcinol gives a cherry red coloured complex. This test distinguishes
between fructose and glucose. Overheating of the solution is avoided because on continuous
boiling, Aldoses will also give this positive because of their conversion to ketoses by
hydrochloric acid.
Sucrose will also give Seliwanoff’s test positive because the acidity of reagent is
sufficient enough to hydrolyze sucrose to glucose and fructose but Benedict’s test will be
negative.

OSAZONE TEST
Principle: A solution of reducing sugar when heated with phenyl hydrazine, characteristic
yellow crystalline compounds called ‘Osazone’ is formed. These crystals have definite
structure, precipitation time and melting point for different reducing sugars.
Reagents: Equal part of Phenyl hydrazine hydrochloride and anhydrous sodium acetate. It
has to be freshly prepared at the time of the reaction.
Procedure:
i. Roughly 0.5g of phenyl hydrazine mixture was taken in a clean and dry test tube.
ii. Then 5 ml of disaccharide solution and 2 drops of glacial acetic acid were added and
mixed properly.
iii. Then the test tube was placed in boiling water for 30 minutes.
iv. After 30 minutes, the test tube was taken out from the boiling water bath and allowed
to stand to cool by itself in a test tube rack. (NOTE: the test tube should not be
disturbed in between when cooling as the osazones of disaccharides separates out on
slow cooling). Yellow coloured crystals formed was observed under the microscope.

OSAZONE TEST
Inference:
Needle shaped crystals: Osazones of glucose, fructose and mannose
Rhombic plate like crystals: Osazone of galactose
Sunflower shaped crystals: Osazone of maltose
Cotton ball shaped crystals: Osazone of lactose

SUCROSE CONFIRMATORY TEST (HYDROLYSIS TEST)
Principle: Sucrose on hydrolysis with HCl is converted to glucose and fructose. The presence
of these two monosaccharides can be confirmed by Benedict’s and Seliwanoff’s tests.
Procedure:
i. At first, 5 ml of sucrose solution was taken in a clean and dry test tube.
ii. Then 2 drops of HCl and 1 drop of thymol blue was added to the sucrose solution.
The development of pink color indicates that the solution is acidic.
iii. Then the solution in the test tube was boiled for about 1 minute and cooled under tap
water.
iv. The contents in the test tube was neutralized by adding 2% sodium carbonate drop by
drop. Formation of blue color indicates neutralization.
v. Then the solution in the test tube was divided into two equal parts.
vi. One part is used for Benedict’s test and the other part is used for Seliwanoff’s test.
Inference: Sucrose is a non-reducing sugar, since it does not have free aldehyde or ketone
group to cause reduction; hence it gives a negative reaction with Benedict’s reagent. But upon
boiling with HCl, sucrose is hydrolyzed to yield glucose and fructose, which give positive
reactions with Benedict and Seliwanoff’s reagents.

ACID HYDROLYSIS TEST FOR STARCH
Principle: Acidified starch on complete hydrolysis with HCl is converted to glucose. It takes
about 20 minutes time. This can be checked by Benedict’s test and iodine test.
Procedure:
i. At first, take 20 ml of starch solution (1%) in a 100 ml conical flask.
ii. Then add 5 ml of conc. HCl.
iii. Divide the solution into 5 equal parts (5 ml each) in five different test tubes.
iv. Mark the test tubes as 1, 2, 3, 4 and 5 and keep in boiling water bath for 1, 5, 8, 12
and 20 minutes respectively.
v. Divide the solutions in each test tube into two parts.
vi. One part is used for Benedict’s test after neutralizing with sodium carbonate and the
other part is used for Iodine test.

ACID HYDROLYSIS TEST FOR STARCH
Inference:
Test
Tube
Time Colour
with
Iodine
Benedict’s
test
Reduction of
hydrolysis
Product
1 1 minute Blue Blue No reduction Starch
2 5 minutes Violet Green Reduction starts (+) Amylodextrins
3 8 minutes Reddish
violet
Red Initiation of reduction
(++)
Amylo and
erythrodextrins
4 12 minutes Red Red Partial reduction
(+++)
Achrodextrin
5 20 minutes No colour Red Completely reduced
(++++)
Glucose

Carbohydrates: Part 1

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