Polysaccharides

Polysaccharides are polymers of monosaccharide units, which may be substituted with other groups such as ester, ether, and/or cyclic acetal moieties (1). Polysaccharides may be attached to proteins, in which case the substance is referred to as a protein-polysaccharide (when they originate from plants) or a proteoglycan (when they originate from animals), or to a lipid, in which case the substance is referred to as a lipopolysaccharide. Certain polysaccharides of plants, namely, hemicelluloses, may be covalently bound to lignin.

Polysaccharides of foods, either naturally occurring or added as ingredients, are classified in several different ways (3,4), by source (Table 2) and by structure (Table 3) being the most common ways. They may also be classified by composition (3) and by imparted functionalities (4). Water-soluble polysaccharides of foods, especially those added as ingredients, are known as food gums and hydro-colloids (see the article Gums).

One feature to note from Table 3 is that polysaccharides, unlike proteins, can be branched. Because each monosaccharide unit of a polysaccharide has several hy-droxyl groups that can be involved in glycosidic linkages, each unit can potentially be involved in multiple linkages. Therefore, branching can, and does, occur. Branched polysaccharide structures can have several shapes, including essentially linear structures with a very few, very long branches; linear structures decorated with short branches, either regularly spaced, irregularly spaced, or in clusters; branch-on-branch structures with branches in clusters (as in starch amylopectin) (1) (see the article Starch); and bushlike, branch-on-branch structures, with and without decoration with short branches (Fig. 8). Each polysaccharide has one and only one reducing end unit but may have many nonreducing end units.

The chemistry of polysaccharides, like that of oligosaccharides, is primarily the chemistry of hydroxyl groups and the chemistry of glycosidic linkages; in the case of polysaccharides containing uronic acid units, the chemistry of car-boxyl groups may also be involved (3). Hydroxyl groups may be reacted to form ethers (practiced with cellulose [1] and starches [1], primarily corn, waxy maize, and potato starches). Hydroxyl groups may also be reacted to form esters (practiced with starch) (1). Some naturally occurring ester groups are removed by saponification during processing of the food gums konjac mannan (acetate ester groups) and gellan (acetate and glycolate ester groups). Oxidants will convert hydroxyl groups into carbonyl functions; carboxyl groups may also be introduced during oxidation.

Oxidation is usually effected at alkaline pH values. Under these conditions, /ยก-eliminations may result in depoly-merization (1). Glycosidic bonds are also cleaved by acid-catalyzed hydrolysis (practiced with starches and gums in general). Starches are also depolymerized by enzyme-catalyzed hydrolyses in the formation of corn syrups and other hydrolytic products. Under low-moisture conditions, heating in the presence of an acid results in both glycosidic bond hydrolysis and transfer, the latter reaction resulting in a more highly branched product (production of dextrins from a starch).

Some of the uronic acid carboxyl groups of algins are esterified in the production of propylene glycol alginate (1). Some naturally occurring methyl ester groups of a high-methoxyl pectin are saponified during its conversion into a low-methoxyl pectin. In other low-methoxyl pectins, the methyl carboxylate groups are converted into carboxamide groups by treatment with ammonia (1).

Allowed label designations of food gums are given in Table 4.

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