Effect of heat-moisture treatment to raw paddy rice (Oryza sativa L.) on cooked rice properties

Abstract

Heat moisture treatment is safe and non-chemical use method for modifying physicochemical properties and digestibility of starch. Slow starch digestion in rice product would provide numerous health benefits for the consumers. This study investigated the heat-moisture treatment to the raw paddy rice at 60 and 80 °C for 4 and 24 h, and their effects on the attributes of cooked rice. The firmness, adhesiveness, starch hydrolysis and morphological changes during in vitro digestion were examined. Results showed that heat-moisture treatment to raw paddy rice for 24 h significantly reduced the moisture content and could increase the total starch content of the rice grain. The apparent amylose content ranged from 23.83% to 25.65% at the variation in the values between treated and untreated rice. Enhancement in the firmness and reduction in the starch hydrolysis was found in rice treated by longer heating time of 24 h. Meanwhile, the adhesiveness of cooked rice was decreased when increasing the heating temperature. The morphological observation showed that the treated rice exhibited a honey-comb-like structure during simulated gastric digestion, while the untreated rice displayed a non-uniform structure. Noteworthy, the porous structure observed in all digested rice grains during simulated small intestinal digestion could favor the starch hydrolysis. This study showed that the heat-moisture treatment to the raw paddy rice at certain conditions obviously influenced the attributes of cooked rice, especially on the starch hydrolysis, which would provide a fundamental knowledge to enable development and improvement of slow digestion rice products.