Molecular Components

Although commercial starch granules, especially those from cereal grains, contain small amounts of protein, lipids (especially phosphoglycerides and free fatty acids), and other components, the principal components are amylose and/or amylopectin (Tables 1 and 2) (1,4).

Amylose is an essentially linear polysaccharide composed of (l->4)-linked a-d-glucopyranosyl units (see the article Carbohydrates: classification, chemistry, labeling). Its degree of polymerization (DP) is 1,000 to 16,000 (MW 160,000-2,650,000), depending on the source and method of preparation. Amylose can have several conformations. In the solid state, it probably exists most often as a left-handed, sixfold helix. In solution, it seems to be a loosely wound and extended helix that behaves as a random coil. Because of its helical structure, amylose is able to complex with hydrophobic molecules. In foods, amylose often complexes with mono- and diglycerides and/or free fatty acids or their salts. Complexed amylose molecules retrograde less effectively (see the section "Retrogradation"). Hence, molecules complexed with hydrocarbon chains provide greater stability to foods, so emulsifiers and other surfactants are added to baked goods to retard staling, which at least in part results from r├ętrogradation.

Amylopectin has a branch-on-branch structure. Amy-lopectin molecules are composed of chains of (l->4)-linked a-d-glucopyranosyl units; branches are formed by joining these chains with a-d-(l->6) linkages. The average chain length is 20 to 30 units, although branch points are not equally spaced. The currently accepted architecture of an amylopectin molecule is that of a long main chain to which are attached clusters of branches, some of which are themselves branched (Fig. 1). This model has been termed the cluster model. The molecular weight of amylopectin has been measured as 5 x 107 - 4 x 108 (DP 3 x 10® - 2.5 x 106), depending on the source and method of preparation. Potato starch amylopectin occurs as a natural phosphate ester.

Corn and rice cultivars with altered polysaccharide composition have been developed. Normal corn starch is composed of approximately 28% of the linear polysaccharide amylose and approximately 72% of the branched polysaccharide amylopectin. Those starches that result from a mutation that makes them composed of essentially 100%

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