Physicochemical properties and digestibility of eleven Vietnamese rice starches with varying amylose contents†
The physicochemical properties of eleven Vietnamese rice starches with apparent amylose contents ranging from 0.2% to 28.4% were investigated to identify the interplaying factors contributing to the in vitro digestibility of starch: in uncooked, cooked in excess water and under limited water conditions. The amylose content of rice starches governed the physicochemical properties of starch such as swelling power, pasting and gelatinization of starch which strongly correlated with raw starch digestibility. Amylopectin predominantly contributed to the crystallinity of starches both in short range and long range orders (observed by FT-IR and X-ray diffraction respectively). This study found that cooking destructively changed the molecular structure of starch that lead to an increase in the rate of starch digestion. Cooking in excess water resulted in a higher rate of starch digestion at least by 2 fold in all examined starches, suggesting that under these cooking conditions, factors that would normally limit the rate of digestion were eliminated. Cooking in limited water that was similar to domestic cooking conditions, seemed to allow rice starch granules to rearrange and interact together and limited the rate of digestion compared to cooking in excess water. It can be concluded that the physicochemical properties of starch can predict the digestibility of raw starches only but not cooked starches. The digestion of rice starch cooked in limited water cannot be predicted by just studying the uncooked starches or cooking in excess water.