Effect of molecular size on gelatinization thermal properties before and after annealing of rice starch with different amylose contents

Rice starch isolated from varieties of TCW70 (waxy rice), TNG67 (japonica rice) and TCS17 (indica rice) with different amylose contents (0.1%, 18.3% and 29.2%, respectively) were treated in methanol containing 0.36% HCl at 25 °C for 1–15 days to prepare starches with different molecular sizes. The acid–methanol-treated starches were annealed at 50 °C for 72 h, and the gelatinization thermal properties and 13C CP/MAS NMR spectra of the starches before and after annealing were determined for elucidating the effect of molecular size on the annealing properties of rice starch. Results showed that the gelatinization onset temperature (To), peak temperature (Tp) and enthalpy (ΔH) of native rice starches and acid–methanol-treated starches after annealing were higher than those of the counterpart starches before annealing. The double helix content and relative crystallinity calculated from NMR spectra of starch showed no obvious change or a slight increase after annealing. Annealing was found to enhance the thermal stability of acid–methanol-treated rice starches. The dTo (change in To before and after annealing) of TCW70 rice starch increased with decreasing DPw of starch, and the same is true for the dTp (change in Tp before and after annealing). While both dTo and dTp of TNG67 and TCS17 rice starches decreased with decreasing DPw of starch when DPw was lower than 10,000. This suggests that the changes in gelatinization thermal properties of rice starches after annealing depend on not only the molecular size but also the presence of amylose.