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

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 degrees 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 degrees 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 approximate to 115 degrees 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 degrees 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 degrees C storage, a higher transition enthalpy was observed in waxy TCW 70 from day 7, although at 25 degrees C storage, no similar changes were observed throughout 21 days.