Molecular Structure and Pasting Properties of Legume Starches.

Abstract

The molecular structures and pasting properties of four legume starches (white Dutch runner; WDR, adzuki bean, pea and common bean) were examined. The apparent amylose content (27.0-30.0%) was 3.9-9.4% higher than the actual amylose content (17.6-25.5%) due to the high iodine affinity (IA) of the legume amylopectins. The crystalline type of pea starch was of Cb and those of the others were of the Cc. Adzuki bean starch had the highest maximum viscosity and breakdown. No breakdown was observed for pea starch. The legume amylopectins had IA of 1.32-2.28 g/100 g and number-average chain-length of 21-22. The phosphorus content of amylopectin varied between 13 and 820 ppm with WDR amylopectin having the highest content. All the legume amylopectins had 1-4% of long-chain fraction, being consistent with their high IA. Compared with starches of cereal amylopectins, the legume amylopectins seemed to contain a relatively small amount of A-chain fraction. Distribution pattern for chains of degree of polymerization (DP) 6-8 was similar to those of potato and sweet potato amylopectins in which the amount of these chains decreased with an increase in DP. The legume amyloses had number-average DP of 820-1350 and comprised 2.4-4.7 chains per molecule, being similar to cereal amyloses. Among the legume amyloses, adzuki bean amylose was the largest and most branched molecule. (author abst.)