Structural differences between hot-water-soluble and hot-water-insoluble fractions of starch in waxy rice (Oryza sativa L.)

Amylose readily dissolves in hot water, unlike amylopectin which is largely insoluble. However, size distributions of amylose isolated in such a manner often show the presence of hyper-branched material consistent with amylopectin. The difference in solubility suggests that the hot-water-soluble (HWS) hyper-branched material might be structurally distinct from typical amylopectin. In the present paper, the structural properties of the two solubility fractions of hyper-branched material are explored in a set of traditional waxy rice varieties. The objective was to elucidate the nature of the HWS component, e.g. to see if it could be phytoglycogen, another water-soluble polysaccharide. We show that solubility is controlled by thermodynamic effects, rather than slow dissolution (kinetic effects). The average size, degree of branching and the debranched chain-length distributions indicate that the HWS fraction is structurally different from phytoglycogen. The debranched chain-length distributions of short chains and the degrees of branching in the HWS material are similar to those of the hot-water-insoluble (HWI) fractions but the chain-length distributions indicate that the HWI fractions carry longer chains than those in the HWS fractions. Light-scattering measurements show that the average size of whole molecules in the HWI component is significantly greater than in the HWS component. It is postulated that the structural differences limit solubility of the molecules in the HWI fraction, possibly due to co-crystallisation with adjacent molecules at more points than is possible for the shorter chains in HWS molecules.