Changes in Starch Digestibility and Tissue Structure of Cooked Rice Grain Under Different In vitro Simulated Gastric Digestive Conditions

The objectives of this study were to investigate the changes in starch hydrolysis and tissue structure of cooked rice grain during gastro-intestinal digestion in vitro under different gastric digestive conditions. Packaged cooked rice prepared by microwave oven was used as the cooked grain samples. Starch hydrolysis of the samples was evaluated using simulated gastro-intestinal digestion in vitro. Four gastric digestive conditions were employed using combinations of low/high pH level and with/without pepsin content. Mechanical properties of the samples after gastric digestion were also evaluated. Changes in tissue structure of the samples during digestion were observed using a stereo microscope and scanning electron microscopy (SEM). Starch hydrolysis was almost 0 % during gastric digestion and increased from 88.1 to 100.3 % at the end of small intestinal digestion, regardless of the gastric digestive condition. The sample surface before gastric digestion was smooth, whereas disintegration of the tissue structure and fibers was observed after gastric digestion. Firmness and adhesiveness of the samples were decreased after gastric digestion. Numerous cell walls were observed on the sample surface after small intestinal digestion. This study indicated that gastric digestive conditions influenced the physical properties of samples; however, they did not influence starch hydrolysis and tissue structure during small intestinal digestion.