The importance of an oral digestion step in evaluating simulated in vitro digestibility of starch from cooked rice grain.

To examine the effect of oral digestion step in a simulated in vitro starch digestion model, the digestibility of intact, homogenized and actual chewed cooked rice grains was investigated and analyzed. The kinetics of starch digestibility were calculated from changes in the hydrolysis percent of starch that were achieved during simulated small intestinal digestion stage. Morphological and histological microscopic tissue structures were also examined. Compared with the trend of starch hydrolysis changes of the actual chewed grain, 1.3U/ml of salivary α-amylase concentration treated for 60min was regarded as a mimicked condition to the simulate in vitro oral digestion step in this study. The results showed that the equilibrium percent of starch hydrolysis for all of the samples ranged from 84.2% to 95.9% with no significant differences observed regardless of whether the oral digestion step was included (p>0.05). In contrast, the kinetic constant, which is one of the measure of starch digestion rate during small intestinal stage, significantly increased with the degree of grain homogenization increased: 120s>actual chewed ≥1s>intact, for both the gastrointestinal and oral plus gastrointestinal processes. These results indicated that the kinetic constant was influenced by the change of cooked rice grain structure in oral digestion step that would be related to increase in enzyme accessibility to rice starch. Thus, rice grain digestibility was affected by grain-scale structural changes, including grain tissue damages which were normally observed during the oral digestion step.