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

Starch from seven varieties of rice, known to cook from very soft to very h ard texture, was fractionated by gel-permeation chromatograghy on Sepharose CL-2B column. The high-molecular-weight fraction (Sepharose FRI) and the l ow-molecular-weight fraction (Sepharose FRII, further sub-divided into FRII a, 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 molecu les in all of them. Clearly, the branched component of starch ('amylopectin ') is not necessarily big in size, but includes very small to very big mole cules. The presence or absence of the largely linear, and relatively small molecule, 'amylose', could not be settled either way with the technique emp loyed. However, based on certain assumptions, amylose content was calculate d to be in the range of 7%-11% in the samples, much less than generally tho ught. The content of long-B chains of the branched molecule in the four Sep harose fractions individually and in aggregate, as well as the calculated a mylose content, correlated well with the sensory tenderness of cooked rice. It was observed that the content of all long linear chains, including amyl ose if any, govern the texture of cooked rice. (C) 1999 Elsevier Science Lt d. All rights reserved.