Gel-chromatography fractionation and thermal characterization of rice starch affected by hydrothermal treatment.

Abstract

Two rice cultivars, high amylose TNS 19 and waxy TCW 70, were selected for a study of thermal behavior and molecular redistribution of rice starch after hydrothermal treatments. The treatment conditions included soaking for 2 hr at 40°C and steaming for 10 or 20 min at 0 and 18 psi steam pressure, respectively. Lower temperature endotherms, measured by differential scanning calorimetry, were observed at 65 and 72°C for the untreated TNS 19 and TCW 70, respectively. The hydrothermal treatments caused a shift to higher gelatinization temperatures and lower transition enthalpy changes. Simultaneously, a higher temperature endotherm was observed at 115°C for TNS 19 only. The thermal degradation of amylopectin apparently resulted in a decreased amount of large molecules and an increased amount of smaller molecules. The heat-moisture-treated rice flour was mixed with water and heated to gelatinization and then stored at 4 or 25°C. The transition enthalpies increased with concomitant decreases of transition peak temperatures during storage. These results indicate that the extent of retrogradation increases with storage time for both rice cultivars. At 4°C storage, a higher transition enthalpy was observed in waxy TCW 70 from day 7, although at 25°C storage, no similar changes were observed throughout 21 days.