Food Chemistry

302 4 Carbohydrates

Fig. 4.16.Schematic representation of the “effective
volumes” of linear, branched and linearly branched
types of polysaccharides

H-bridges. However, a divalent cation is needed
to achieve gel setting in a neutral solution.

4.4.3.6 Polysaccharides with Strongly Acidic Groups

Polysaccharides with strongly acidic residues,
present as esters along the polymer chains
(sulfuric, phosphoric acids, as in furcellaran,
carrageenan or modified starch), are also very
soluble in water and form highly viscous so-
lutions. Unlike polysaccharides with carboxyl
groups, in strongly acidic media these solutions
are distinctly stable.

4.4.3.7 Modified Polysaccharides

Modification of polysaccharides, even to a low
substitution degree, brings about substantial
changes in their properties.

4.4.3.7.1 Derivatization with Neutral Substituents

The solubility in water, viscosity and stability of
solutions are all increased by binding neutral sub-

stituents to linear polysaccharide chains. Thus the

properties shown by methyl, ethyl and hydroxy-

propyl cellulose correspond to those of guaran

and locust bean gum. The effect is explained by

interference of the alkyl substituents in chain in-

teractions, which then facilitates hydration of the

molecule. An increased degree of substitution in-

creases the hydrophobicity of the molecules and,

thereby, increases their solubility in organic sol-

vents.

4.4.3.7.2 Derivatization with Acidic Substituents

Binding acid groups to a polysaccharide (carb-

oxymethyl, sulfate or phosphate groups) also re-

sults in increased solubility and viscosity for rea-

sons already outlined. Some derivatized polysac-

charides, when moistened, have a pasty consis-

tence.

4.4.4 Individual Polysaccharides................................

4.4.4.1 Agar

4.4.4.1.1 Occurrence,Isolation

Agar is a gelatinous product isolated from sea-

weed (red algae class,Rhodophyceae), e. g.,Ge-

lidium spp., Pterocladia spp.andGracilaria spp.,

by a hot water extraction process. Purification is

possible by congealing the gel.

4.4.4.1.2 Structure,Properties.....................................

Agar is a heterogenous complex mixture of re-

lated polysaccharides having the same backbone

chain structure. The main components of the

chain areβ-D-galactopyranose and 3,6-anhydro-

α-L-galactopyranose, which alternate through

1 →4and1→3 linkages:

(4.135)