Effect of the ratio of water to rice on the molecular dynamics of cooked rice starch during retrogradation: Implications for amorphous structure in gelatinized state

The ratio of water to rice during gelatinization affects starch retrogradation in cooked rice; a high ratio is more inhibitory to retrogradation. The structural state of the crystalline phase and also of the amorphous phase can be clarified by an indicator of molecular dynamics. In this study, we used quasi-elastic neutron scattering (QENS) to investigate the effect of the ratio of water to rice on the molecular dynamics of cooked rice starch during retrogradation. X-ray diffraction and differential scanning calorimetric measurements revealed that the degree of recrystallization and change in enthalpy (Delta H) were smaller by increasing the amount of water added for cooking rice, whereas little difference in the crystallinity of the gelatinized rice starch was detected. The QENS measurements determined that the elastic incoherent structure factor (EISF) values of gelatinized samples were smaller by increasing the amount of water added for cooking rice, indicating that the molecular dynamics of gelatinized rice starch with higher added water content were spatially more extended. In addition, reduction in the molecular dynamics was correlated with the increase in starch crystallinity during retrogradation. These results suggest that differences in the molecular dynamics of gelatinized rice starch might be associated with the suppression of retrogradation.