FOOD CHEMISTRY

Lesson 11. POLYSACCHARIDE - LINEAR, BRANCHED AND MODIFIED

Module 4. Food carbohydrates

Lesson 11
POLYSACCHARIDES - LINEAR, BRANCHED AND MODIFIED

11.1 Introduction

The carbohydrates constitute one of nature’s three most abundant classes of organic compounds, the other two being the fats and the proteins. Carbohydrates are essentially substances that are made up of carbon, hydrogen and oxygen only. The carbohydrates are divided into three broad categories, namely, monosaccharides, oligosaccharides and polysaccharides. Monosaccharides represent the group of carbohydrates that cannot be further hydrolyzed to smaller molecules. They form the building blocks of the more complex carbohydrates. Oligosaccharides comprise the low molecular weight polymers that include the disaccharides and trisaccharides and compounds with as many as ten monosaccharides linked into single molecules.

11.2 Classification of Carbohydrates

Carbohydrates may be broadly classified as follows:

I. Monosaccharides

1. Trioses, C₃H₆O₃, e.g. glyceraldehydes and dihydroxy acetone

2. Tetroses, C₄H₈O₄, e.g. erythrose, threose

3. Pentoses, C₅H₁₀O₅, e.g. arabinose, xylose, ribose, deoxyribose

4. Hexoses, C₆H₁₂O₆, e.g. glucose, galactose, fructose, mannose

II. Oligosaccharides

1. Disaccharides, C₁₂H₂₂O₁₁, e.g. sucrose, lactose, maltose

2. Trisaccharides, C₁₈H₃₂O₁₆, e.g. raffinose

3. Tetrasaccharides, C₂₄H₄₂O₂₁, e.g. Stachyose

III. Polysaccharides

1. Pentosans, e.g., araban, xylan

2. Hexosans, e.g., Starch, glycogen, cellulose, mannan, gallactan

3. Complex polysaccharides, e.g. hemicelluloses, gums, pectins

11.3 Polysaccharides

Polysaccharides are the carbohydrates which contain more than 10 monosaccharide units. They can be hydrolyzed into hundred or even thousands of monosaccharide units.

The suffix –ose in sugar is changed to –ans to describe the corresponding polysaccharide.

Examples:

(1) Pentosans

(a) Arabans

(b) Xylans

(2) Hexosans

(a) Glucans à starch, dextrin, glycogen, cellulose, inulin

(b) Mannans

(c) Galactans

(3) Complex polysaccharides

(a) Pectins or pectic substances

(b) Gums

(c) Mucilages

(d) Algal polysaccharides à Alginic acid and carrageenan.

(e) Bacterial polysaccharides à Xanthan gum.

11.4 Classification of Polysaccharides

Polysaccharides occur in an infinite variety of different structural types which can be broadly classified into homopolysaccharides and heteropolysaccharides.

(1) Homopolysaccharides

They contain the same structural units throughout. For example, the glucans (starch and glycogen), fructans, mannans etc. These polymers can possess either simple linear structure or branched structures of varying complexity with more than one type of inter unit linkage.

Perfectly linear polysaccharides: They compounds with single neutral monosaccharides structural unit with only one type of linkage are denoted as perfectly linear polysaccharides. They are usually insoluble in water and can be solublized only under drastic conditions. They also have tendency to precipitate from solution retrogradation.

Branched polysaccharides:They are more soluble in water than linear polysaccharides as the chain-chain interaction are less pronounced. Compare to the linear polysaccharides of equal molecular weight and concentration, solution of branched polysaccharides have the lowest viscosity and lower tendency to precipitate. They have the ability to form the sticky paste at higher concentration, probably due to side chain – side chain interaction. So they are suitable as binder or adhesives.

Linearly branched polysaccharides: They are polymers with long backbone chains and with mainly short chain viz. alkyl cellulose. They have properties, which are a combination of perfect linear and branched polymers. The long backbone chain is responsible for high viscosity of the solution. The presence of numerous short-side chains weakens the interaction between molecules and thereby gives it good solubility and rehydration rates and also provides stability to highly concentrated solutions.

Modified Polysaccharides: The properties of polysaccharides can be modified by physical and chemical methods that result in products suitable for specific purposes in the food industry. The solubility in water, viscosity and stability of solutions are all increased by binding neutral subsituents to linear polysaccharide chain e.g. hydroxypropyl cellulose. Binding acid groups (carboxymethyl, sulphate groups) also results in increased solubility and viscosity.

(2) Heteropolysaccharides

They contain two or more types of different monomer units. For example, the arabinoxylans, glucomannans etc. These biopolymers can be linear or branched to varying degrees with different types of branch points. Typical sugars units found in polysaccharides which occur in food are

a. D- Fructose (Fru)
b. D-mannose (Man)
c. D-Glucose (Glu)
d. D-Mannuronic acid (Man A)
e. D-Glucuronic acid (Glu A)
f. D-Xylose (Xyl)
g. D-Galactose (Gal A)
h. L-Arabinose (Ara)
i. D-Galacturonic acid (Gal A)
j. L-Rhamnose (Rha)

Table 11.1 Homopolysaccharides occurring or used in foodstuffs

Type	Linkage	Structure	Polysaccharide	Occurrence
Glucans	α, 1 → 4	Linear	amylose	Starchy material
	α, 1 → 4 α, 1 → 6	Branched	amylopectin	Starchy material
	α, 1 → 4 α, 1 → 6	Branched	glycogen	Animal liver
	β, 1→ 4	Linear	Cellulose	Cell walls of all plants
	β, 1→ 3 β, 1→ 4	Linear	β - glucan	Cereal grains (oats, barley)
Fructans	β, 2→ 6 β, 2→ 1	Branched	Fructans	Various plants (wheat endosperm)
	β, 2→ 1	Linear	Inulin	Jerusalem artichokes
Arabinans	α, 1 → 3 α, 1 → 5	Branched	Pectic substances	Sugar beet, citrus pectins
Xylans	β, 1→ 4	Linear	Xylans	Cell walls of plants

Table 11.2 Heteropolysaccharides occurring or used in foodstuff

Units	Structure	Polysaccharide	Occurrence
Ara; Xyl	Branched	Arabinoxylans	Plant cell walls (wheat flour)
Glu A; Xyl	Branched	Glucuronoxylans	Plant cell walls
Glu; Man	Linear	Glucomannans	Seeds
Glu A ; Man A	Linear	Alginic acid	Brown seaweeds
Gal; Man	Branched	Guar/carbo gum	Leguminous seeds
Anhydro Gal; Gal sulphate	Linear	carrageenan	Brown seaweeds
Gal A; Rha	Linear	Pectic materials	All plant material
Ara; Rha; Gal; Glu A; Glu	Branched	Gum arabic	Trees (Acacia spp.)
Gal A; Xyl; Gal; Fuc	Branched	Gum tragacanth	Trees (Astragalu spp.)