Combination of parallel and sequential digestion kinetics reveals the nature of digestive characteristics of short-term retrograded rice starches

Abstract

Cooked rice is commonly consumed after a short period of cooling, which results in the short-term starch retrogradation. Starch fine molecular structures that are important for its in vitro digestibility after short-term retrogradation were thus investigated in this study. A modified first-order kinetics model was developed in order to consider both the sequential and simultaneous digestion phases in the system. Correlation analysis between the resulted digestion parameters with starch fine molecular structures, i.e. chain-length distribution and molecular size distribution, showed that the number of digestion phases was determined by the amount of amylose short to medium chains and starch molecular size; relatively lower amount of these chains and larger starch molecules could result in two digestible fractions (fast versus slow) that possess a combination of sequential and parallel digestion pattern. Among the starches with two digestible fractions, a relatively larger amylose molecules, higher amount of amylose short to medium chains, shorter amylose medium chains, as well as longer amylopectin medium chains would result in a slower digestion rate for both the fast- and slow-digestible starch fraction. The initiation time of the digestion of slow-digestible starch fraction was positively correlated with the amount of amylose short to medium chains, while negatively correlated with the amylose molecular size. These results could enable rice breeders to develop rice varieties with slower starch digestibility by introducing desirable starch fine molecular structures.