The Study Of Temperature-cycled Retrogradation For Preparing Rice Slowly Digestible Starch

Slowly digestible starch is the portion that digested slowly by amylase from 20 to 120 min in small intestine, and has significant implications on human health, such as, cardiovascular diseases, non-insulin diabetes, obesity and providing sustained and stable energy for athletes. Recently, there are a number of techniques to prepare slowly digestible starch, such as pullulanase debranching, hydrothermal treatment, microwave heat treatment and a citric acid modification.The paper studied the influence of autoclaving and heat treatment on the digestibility of rice starch and found that waxy rice starch treated by heated gelatinization and temperature-cycled retrogradation was a new technique to prepare slowly digestible starch. The paper also studied the influence of cycled-temperature, cycled-time, cycled-time interval, moisture on the forming of slowly digestible starch, and determinated the optimal SDS content by orthogonal test. It was found that the maximum SDS content was 51.62%, when the starch: water was 1:2, stored under cycled-temperature 4/25°C for 7 days and cycled-time interval for 24 h.The results indicated that there was a higher onset temperature (To), a narrower melting temperature range (Tc-To), a lower enthalpy change and a higher ratio (1047 cm-1/1022 cm-1 was 0.899) of the slowly digestible starch products prepared under the temperature cycled condition than that under the isothermal retrogradation. Furthermore, it was evident that the in vitro glycemic index and swelling factors of slowly digestible starch products with temperature cycled retrogradation was reduced more than that with isothermal storage. Therefore, the waxy rice starch stored under cycled-temperature was more difficult to hydrolyzed by amylase than that under isothermal storage.At last, amylopectin of waxy rice starch recrystallized more quickly at 4/25°C (0.785) than at isothermal temperature (0.725, 0.0570) and its crystal nucleus formed once for all as same as that at 4°C while crystal nucleus at 25°C formed step by step . Meanwhile, our results showed the similar kinetics of crystallization characters in normal rice starch stored at 4°C, 25°C and 4/25°C. The results from the enzymatic hydrolysis procedure found that waxy rice starch stored under cycled temperature showed a more ordered and smooth surface, and a solid section appeared after hydrolysed for 120 min. Besides, the SDS content and melting temperature range was negative correlation. The mechanism of temperature-cycled retrogradation formed slowly digestible starch was that crystal nucleus formed once for all, and amylopectin recrystallised and a large amount of imperfect and homogeneous crystals formed.