Properties of Rice Starch

Abstract

The texture of cooked rice appears to be greatly dependent upon the properties of starch. Here, the structures and properties of rice starch were discussed based on our recent knowledge. Non-waxy rice starches having low (Sasanishiki, Koshihikari), intermediate (unknown, IR48, IR64) and high (IR32, IR36, IR42) iodine affinity (AI) were said to have low, intermediate and high amylose contents, respectively. However, from the analyses of AI of amylose, amylopectin and starch, each starch was found to have similar amounts of amylose, because the AT of amylopectins of high AT starch were high but those of amyloses were similar. Amylopectins with higher AI contained increased amounts of long chains, which were not found in waxy rice. The mode of each chain connection was analyzed by β, i, β-degradation. The longer B chains carried increased numbers of A chains, and the structures of the chain linking within a cluster have been suggested to be the random Meyer structure. Molecular weights of amylopectins with high IA were lower than those of others, and [η] did not correlate with molecular weight. However, the amylopectins with high AT had high [η]. This implies that amylopectins with higher AI had more elongated shapes than others in an alkaline solution (1 N NaOH, 22.5°C). Amyloses from rice starches had the similar dn p. n and d. p. w. However, the ratio of linear and branched molecules and β-amylolysis limit differed by cultivars. The β-amylolysis limit of branched amylose was found to be 39%. The d.p.n of the branched amylase was 1.8-fold that of linear molecules. The side chains of branched molecules were mostly short chains with average chain-length of 20. The [η] of amylose had good correlation with the amount of branched molecules by weight. The branched molecules probably are more stable than linear molecules in aqueous solutions and have less retrogradation tendency.