2009_Book_FoodChemistry

302 4 Carbohydrates
The solubility in water, viscosity and stability of
solutions are all increased by binding neutral substituents
to linear polysaccharide chains. Thus the
properties shown by methyl, ethyl and hydroxypropyl
cellulose correspond to those of guaran
and locust bean gum. The effect is explained by
interference of the alkyl substituents in chain interactions,
which then facilitates hydration of the
molecule. An increased degree of substitution increases
the hydrophobicity of the molecules and,
thereby, increases their solubility in organic solvents.
4.4.3.7.2 Derivatization
with Acidic Substituents
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 solutions.
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.
Binding acid groups to a polysaccharide (carboxymethyl,
sulfate or phosphate groups) also results
in increased solubility and viscosity for reasons
already outlined. Some derivatized polysaccharides,
when moistened, have a pasty consistence.
4.4.4 Individual Polysaccharides
4.4.4.1 Agar
4.4.4.1.1 Occurrence, Isolation
Agar is a gelatinous product isolated from seaweed
(red algae class, Rhodophyceae), e. g., Gelidium
spp., Pterocladia spp. and Gracilaria 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 related
polysaccharides having the same backbone
chain structure. The main components of the
chain are β-D-galactopyranose and 3,6-anhydroα-L-galactopyranose,
which alternate through
1 → 4and1→ 3 linkages:
4.4.3.7.1 Derivatization
with Neutral Substituents
(4.135)