Effects of storage temperatures on the starch digestibility of whole rice with distinct starch fine molecular structure.

Although the retrogradation of rice starch has been extensively investigated, there remain questions as to how storage temperature affects starch inter- and intramolecular interactions in cooked white rice, and the relationship of these interactions with the digestion rate. To this end, micromorphology, crystallinity polymorphisms, molecular interaction patterns and in vitro starch digestibility of 3 rice varieties kept under 5 different storage temperature programs (room temperature (RT), 4 °C, -18 °C, 4 °C/RT (4 °C for 24 h and then RT for 24 h), -18 °C/RT (-18 °C for 24 h and then RT for 24 h)) were investigated. As expected, a significant variance in starch digestibility was observed for samples after storage at different temperatures. Overall, storage at 4 °C could most effectively decrease the starch digestibility of retrograded rice. The digestion rate constant was for the first time found to be determined by short-range amylopectin intermolecular interactions rather than long-range starch molecular interactions, for all different storage conditions. Furthermore, the digestion extent was determined by both inter- and intramolecular interactions among starch molecules as well as by the long-range order of the retrograded double helices. These results could prove useful to devise storage regimes which use retrogradation to produce cooked rice with lower glycemic index.