2009_Book_FoodChemistry

300 4 Carbohydrates
4.4.3 Properties
4.4.3.1 General Remarks
Polysaccharides are widely and abundantly distributed
in nature, fulfilling roles as:
• Structure-forming skeletal substances (cellulose,
hemicellulose and pectin in plants; chitin,
mucopolysaccharides in animals).
• Assimilative reserve substances (starch, dextrins,
inulin in plants; glycogen in animals).
• Water-binding substances (agar, pectin and alginate
in plants; mucopolysaccharides in animals).
As a consequence, polysaccharides occur in many
food products and even then they often retain
their natural role as skeletal substances (fruits and
vegetables) or assimilative nutritive substances
(cereals, potatoes, legumes). Isolated polysaccharides
are utilized to a great extent in food processing,
either in native or modified form, as: thickening
or gel-setting agents (starch, alginate, pectin,
guaran gum); stabilizers for emulsions and dispersions;
film-forming, coating substances to protect
sensitive food from undesired change; and
inert fillers to increase the proportion of indigestible
ballast substances in a diet (cf. 15.2.4.2).
Table 4.19 gives an overview of uses in food technology.
The outlined functions of polysaccharides are
based on their highly variable properties. They
vary from insoluble forms (cellulose) to those
with good swelling power and solubility in
hot and cold water (starch, guaran gum). The
solutions may exhibit low viscosities even at
very high concentrations (gum arabic), or may
have exceptionally high viscosities even at low
concentrations (guaran gum). Some polysaccharides,
even at a low concentration, set into
a thermoreversible gel (alginates, pectin). While
most of the gels melt at elevated temperatures,
some cellulose derivatives set into a gel.
These properties and their utilization in food
products are described in more detail in section
4.4.4, where individual polysaccharides are
covered. Here, only a brief account will be given
to relate their properties to their structures in
a general way.
4.4.3.2 Perfectly Linear Polysaccharides
Compounds with a single neutral monosaccharide
structural unit and with one type of linkage
(as occurs in cellulose or amylose) are denoted
as perfectly linear polysaccharides. They
are usually insoluble in water and can be solubilized
only under drastic conditions, e. g. at high
temperature, or by cleaving H-bonds with alkalies
or other suitable reagents. They readily precipitate
from solution (example: starch retrogradation).
The reason for these properties is the existence
of an optimum structural prerequisite for
the formation of an orderly conformation within
the chain and also for chain–chain interaction.
Often, the conformation is so orderly that a partial
crystallinity state develops. Large differences
in properties are found within these groups of
polysaccharides when there is a change in structural
unit, linkage type or molecular weight. This
is shown by properties of cellulose, amylose or
β-1,3-glucan macromolecules.
4.4.3.3 Branched Polysaccharides
Branched polysaccharides (amylopectin, glycogen)
are more soluble in water than their perfectly
linear counterparts since the chain–chain interaction
is less pronounced and there is a greater
extent of solvation of the molecules. Solutions of
branched polysaccharides, once dried, are readily
rehydrated. Compared to their linear counterparts
of equal molecular weights and equal concentrations,
solutions of branched polysaccharides
have a lower viscosity. It is assumed that the
viscosity reflects the “effective volume” of the
macromolecule. The “effective volume” is the
volume of a sphere with diameter determined
by the longest linear extension of the molecule.
These volumes are generally larger for linear than
for branched molecules (Fig. 4.16). Exceptions
are found with highly pleated linear chains.
The tendency of branched polysaccharides to
precipitate is low. They form a sticky paste
at higher concentrations, probably due to side
chain–side chain interactions (interpenetration,
entanglement). Thus, branched polysaccharides
are suitable as binders or adhesives.