Chemical and Functional Properties of Food Saccharides

© 2004 by CRC Press LLC
15.4 RESERVE POLYSACCHARIDES
Seed-bearing plants deposit energy-containing reserves to support growth of the
embryo within the seed. Seed reserves can be of protein, lipid, or polysaccharide
character. Although the major reserve polysaccharide is indisputably starch, a number
of other polysaccharides are used, particularly within the plant family Leguminoseae.
Unlike starch, which is stored in amyloplasts in the cytoplasm, nonstarch
reserve polysaccharides in seeds are deposited in the cell wall. Reserve gums are
typically found in seeds of plants growing in arid regions, where the ability of the
polysaccharide to bind water increases survival of germinating seeds in conditions
of fluctuating water availability. Plants can also deposit energy reserves in underground
storage organs such as roots or tubers, and some root-derived polysaccharides
are used in the food industry.
15.4.1 LEGUME SEED GALACTOMANNANS
The most important plant gums by volume used in the food industry are the galactomannans
derived from legume seeds. Four main gums used at present, though
many other potential species might provide useful hydrocolloids as yet untested in
the laboratory or the market. There is guar gum from the seeds of Cyamopsis
tetragonoloba, locust bean gum (LBG) from seeds of Ceratonia siliqua (carob tree),
tara gum from seeds of Cesalpinia spinosa, and cassia gum from seeds of Cassia
obtusifolia. All seed galactomannans possess the same basic structure of a β(1-4)linked
mannan backbone with varying degrees of substitution with α(1-6)-linked
galactose residues. The ratio of galactose to mannose is roughly constant for a given
species and provides a useful diagnostic to identify gum samples. The distribution
of galactose along the mannan chain in both LBG and guar gum is not uniform but
tends to be grouped in blocks of high substitution (hairy regions), which are separated
by intervening stretches with few galactose residues (smooth regions).
Preparation of the gums involves dehulling of seeds, crushing to remove the
embryo, followed by milling of the endosperm to produce crude flour. The flour
can be purified by dissolving in hot water, followed by filtration and precipitation
with isopropanol to remove impurities.
15.4.1.1 Locust Bean Gum
In the Mediterranean and the Middle East, LBG has been used in traditional food
preparation for hundreds of years as well as in other nonfood uses as described by
classical authors. The main LBG-producing regions remain in the Mediterranean
and Middle East.
LBG has galactose-to-mannose ratio of 1:4 and molecular weights around
300,000. The rheology shows pseudoplastic behavior and decreases with temperature.
31 Cold-water solubility of LBG is low and dispersions need to be heated to
85°C to achieve good dissolution, concentrations of up to 5% w/v being possible.
Low solubility is due to the tendency of the linear mannan chains to strongly
hydrogen bond to each other in unsubstituted regions of the chain, limiting
opportunities for interaction with water molecules. Different grades of LBG are