Comparison of morphological changes and in vitro starch digestibility of rice cooked by microwave and conductive heating

Abstract

This study was aimed at evaluating the effect of two cooking methods on in vitro digestibility of starch and the change in its microstructure. Rice was cooked by microwave and conductive heating (CH), then stored at −18, 4, 25°C, the in vitro digestibility of starch was measured by in vitro simulative digestion method and the micromorphology was obtained by SEM. Rice starch heated by microwave had greater swell and smaller gaps between starch granules when compared with that heated in a conductive way. The concentrations of reducing sugar and glucose of in vitro digestibility of the rice starch cooked by microwave heating (MH) are higher than that by CH (p≤0.01). After storage, the in vitro digestibility of starch lowered, namely, the content of rapidly digestible starch (RDS) decreased, however, the content of slowly digestible starch (SDS) and resistant starch (RS) increased. After 72 h of storage the in vitro digestibility of rice starch cooked by MH was inferior to that by CH. The higher the storage temperature was, the worse the in vitro digestibility of starch was. This applied to both heating methods. The best digestibility effect was achieved when the rice was cooked by MH and stored at −18°C. Meanwhile, the kinetic model of in vitro digestibility of starch demonstrated that the equilibrium percentage and kinetic constant of the reducing sugar and glucose declined as the storage time increased.