Structural characterization of legume starches. I. Studies on amylose, amylopectin, and beta-limit dextrins

Abstract

Nine purified legume starches (from smooth and wrinkled peas; adzuki, garbanzo, mung, red kidney, navy, and faba beans; and green lentil) were fractionated into their amylose and amylopectin components. The fine structures of the isolated starch components and starch β-limit dextrins were investigated using hydrolytic enzymes (pullulanase, β-amylase, and glucoamylase) in conjunction with gel filtration chromatography (Biogel P-10). The unit-chain profiles of the debranched legume amylopectins were characterized by two overlapping distributions with average degrees of polymerization of 45-55 (II-chains) and 14-18 (III-chains). The average chain length for most of these amylopectins ranged from 20 to 26. Wrinkled pea starch contained branched polysaccaride molecules with a chain length greater than that of normal amylopectin. The limiting viscosity numbers of the legume amylose fractions ranged from 136 to 251 ml/g. The incomplete β-amylolysis of the legume amyloses (78.4-89.4%) was due to the presence of a relatively small number of α-(1→6) linkages in the molecule. A hypothesis was developed to explain starch gelatinization and granule rigidity based on the molecular characteristics of the branched polysaccharide fractions of these starches.