Structure of rice starch and its relation to cooked-rice texture

Abstract

Starch from seven varieties of rice, known to cook from very soft to very hard texture, was fractionated by gel-permeation chromatography on Sepharose CL-2B column. The high-molecular-weight fraction (Sepharose FRI) and the low-molecular-weight fraction (Sepharose FRII, further sub-divided into FRIIa, IIb and IIc) were debranched using isoamylase and fractionated on Biogel P-10. All the four fractions in all the different varieties of rice gave a similar trimodal chain profile, indicating the presence of branched molecules in all of them. Clearly, the branched component of starch (‘amylopectin’) is not necessarily big in size, but includes very small to very big molecules. The presence or absence of the largely linear, and relatively small molecule, ‘amylose’, could not be settled either way with the technique employed. However, based on certain assumptions, amylose content was calculated to be in the range of 7%–11% in the samples, much less than generally thought. The content of long-B chains of the branched molecule in the four Sepharose fractions individually and in aggregate, as well as the calculated amylose content, correlated well with the sensory tenderness of cooked rice. It was observed that the content of all long linear chains, including amylose if any, govern the texture of cooked rice.