Effect of starch retrogradation on molecular dynamics of cooked rice by quasi-elastic neutron scattering

Abstract

Starch retrogradation can be evaluated using X-ray diffraction (XRD) techniques, which can estimate the crystallinity of starch. It is well known that the crystallinity is reflected in the mechanical properties of foods and materials. Crystallinity should be also related to the structural dynamics of starch, but there are no reports on the relationship between these properties. In this study, we used quasi-elastic neutron scattering (QENS) to investigate changes in the molecular dynamics of cooked rice starch during retrogradation. The width of the measured QENS narrowed with retrogradation. The elastic incoherent structure factor (EISF) increased, which indicated that the molecular dynamics are spatially suppressed upon retrogradation. However, the width of the quasi-elastic scattering component was independent of retrogradation in the measured Q-range. Analysis of EISF with a bimodal continuous diffusion model, where low and high mobilities are assumed to correspond to crystalline and amorphous phases, respectively, showed that the fraction of the low-mobility component increases with retrogradation. The suppression of the molecular dynamics during retrogradation was correlated with the crystallinity evaluated by XRD. The molecular dynamics of the crystalline region that appeared after gelatinization and retrogradation were different from that of raw rice starch. These results suggest that molecular dynamics reflect the state of retrogradation and can be used to monitor retrogradation of starch in foods.