Chemical and Functional Properties of Food Saccharides

© 2004 by CRC Press LLC
15.1.1 STRUCTURE OF PLANT AND ALGAL GUMS
As natural products, plant gums display a wide range of structures, which can vary
both between and within species. Composition and size of polysaccharide molecules
can further fluctuate, depending on genotype of the source crop, environmental
conditions during cultivation, and age at harvest. Precise structures are therefore not
so useful, and typically a gum is described in terms of the percentage or ratio of its
main monosaccharide components, which can carry a range of minor substituents.
The impact such variation in structure might have must be determined experimentally
for each batch of gum unless guaranteed by the supplier. Algal gums show a similar
variation in structure but differ as a class from plant gums by the inclusion of some
different residues that are not represented in plants and by the high levels of sulfation
that can occur. Table 15.2 summarizes the principal monosaccharide components of
major gums.
The functional properties of gums largely depend on the conformation the
polysaccharide can adopt in solution, which, in turn, depends on the composition
and linkages of the main polysaccharide chain and the presence of side groups. 9 The
properties of polysaccharides in solution are dominated by hydrogen bonding, and
the two major conformations adopted by polysaccharide chains are either a random
coil or a helix stabilized by regular intrachain hydrogen bonds. In helix-forming
polysaccharides, double helices can often form where hydrogen bonds occur between
two chains.
15.1.2 GUM RHEOLOGY
The key property of plant and algal gums that has led to their widespread use in
food is their ability to modify the rheology of aqueous systems at low levels of
addition. Apart from pectins, plant gums do not readily form gels, but due to their
large molecular size, when hydrated, they provide large increases in viscosity. Rheology
is generally pseudoplastic, with a decrease in viscosity at high shear rates,
though some gums show Newtonian behavior where viscosity remains constant.
Neutral polysaccharides can show rheology that is pH-stable, but where ionizable
groups are present an optimum pH for maximum viscosity is observed. Table 15.3
summarizes the viscosity for solutions of different hydrocolloids. However, all gums
can show a wide range of viscosities, depending on the average chain length of the
sample used, and the values given are only broadly indicative of the general range
for the class of hydrocolloid. Algal gums are typically gel forming. Gel formation
requires the ability for cross-links to form between chains without binding along
the entire length of the chain. A number of books on the rheology of food polysaccharides
provide good coverage of plant and algal gums. 10
15.2 WALL POLYSACCHARIDES
A major site of polysaccharide synthesis in any plant cell is in the production of
polysaccharides for the formation of the cell wall. The principal component of all
plant cell walls is cellulose. Natural cellulose cannot function as a hydrocolloid due