Evaluation of Digestibility of Cooked Rice Grain using in vitro Digestion Technique

Impact of postharvest and/or food processing (such as flour milling to agricultural and food-resource materials) on the digestibility of foodstuffs was investigated through a simulated gastro-intestinal in vitro digestion technique. The differences among the starch digestibilities of cooked rice grain and slurry were examined. In vitro digestion techniques are commonly applied to starch-based milled materials such as flours used for bread making. However, rice is not usually milled to flour but cooked as a whole for consumption. Therefore, maintaining an intact structure of cooked grain during in vitro digestion is very important to evaluate its starch digestibility. Two hundred grams of polished rice grain were cooked using electric rice cooker with 300 ml RO water after soaking for 30 min at 30 °C. A part of cooked grain was grinded using a cutter mill to produce cooked rice slurry. The starch hydrolysis of the rice sample was measured and calculated during in vitro digestion process and regarded as sample digestibility (%). Changes in grain tissue structure during in vitro digestion process were also measured and evaluated. The overall digestion percentage at the end of in vitro small intestine digestion for the cooked rice slurry and grain was approximately 94%. However, nearly 65% of cooked rice slurry was digested within 5 min of small intestinal digestion, whereas the cooked rice grain digestibility (%) ranged at 24% at the same time period. These results indicated that the digestion rate of grain was different from the slurry. The kinetic constant of the slurry digestion, which was calculated using curve fitting method and regarded as an estimated digestion rate, was 8 times larger than the grain. To examine the differences in in vitro digestion rate, microstructural changes in the grain during in vitro digestion process were observed using fluorescent microscopic techniques. As a result, the aleurone and the sub-aleurone layers of endosperm appeared as thin-film like materials during in vitro digestion, therefore may be regarded as less digestive materials during the cooked rice grain digestion. These results may be useful in predicting the glucose release and other glycaemic properties of whole grain-based, starch-based slurries and milled starchy foods during human digestion