The fine structure of rice-starch amylopectin and its relation to the texture of cooked rice

Abstract

Starches derived from 20 rice varieties containing from very low to very high total and hot-water-insoluble amylose-equivalent (AE) were fractionated by gelpermeation chromatography (GPC). Fraction I (amylopectin) and fraction II (amylose) correlated well with the insoluble-AE and soluble-AE, respectively, of the parent rice. Thus soluble-AE broadly represented the true rice amylose and insoluble-AE the iodine affinity of amylopectin. Amylopectins of eight representative varieties were therefore debranched and fractionated by GPC to study their chain profiles. Amylopectins from the highest-AE variety had the largest proportion of long B chains and the lowest proportion of short chains, while the reverse was true for waxy rice. Other varieties broadly followed this correlation between B-chain length and AE. In addition, when the eight amylopectins were first hydrolysed with β-amylase to remove the external chains and the β-limit dextrins were then debranched and fractionated, the greatest drop in the amount of long B chains occurred in the highest-insoluble-AE variety and the smallest drop (nil), in waxy rice. In other words, highest-insoluble-AE (i.e. high-iodine-affinity) amylopectin had not only the highest amount of long B chains, but the largest proportion of these chains was in the exterior region (carrying non-reducing ends), and vice versa. Difference in cooked rice texture seemed to be related to this difference in the fine structure of its amylopectin.