Effects of molecular size and structure of amylopectin on the retrogradation thermal properties of waxy rice and waxy cornstarches

Starches from waxy rice and waxy corn were treated in anhydrous methanol with 0.36% HCl at 25 °C for 1–15 days to obtain starches with different molecular weights. The retrogradation thermal properties of the HCl–methanol-treated starches, after gelatinized and stored at 4 °C for 7 days, were determined by using differential scanning calorimetry, and effects of molecular size and structure of amylopectin on the retrogradation thermal properties were elucidated. Results indicated that the degree of retrogradation (DR%) of waxy starches increased with decreasing molecular weight. The DR% of waxy starches was found negatively correlated with the logarithmic value of its weight-average degree of polymerization (DPw) (r2=0.847 for waxy rice and 0.951 for waxy corn). The increment of DR% with the decreasing DPw of waxy cornstarch (slope=−13.98) was profoundly lower than that of waxy rice starch (slope=−25.42). In spite of the botanical source of starch, the DR% and the melting temperature of retrograded waxy starch depended on not only the molecular size, but also the molecular structure such as average chain length and chain length distributions of amylopectin branch chains and its subfractions.