Effect of heat-moisture treatment of unpolished red rice on its starch properties and in vitro and in vivo digestibility

Abstract

Changes in physicochemical and digestible characteristics of starches isolated from untreated and heat-moisture treated unpolished rice grains were investigated in this study. Heat-moisture treatment (HMT) at moisture content of 20%, 25% or 30% and heating temperature of 100 °C or 120 °C were applied for treating unpolished rice grains. The results indicated that granular morphology and crystalline structure of starches remained almost unchanged. However, a significant difference in the format and degree of agglomeration of starches of the rice grains treated at moisture content of 30% was observed. The HMT also suppressed the swelling and breakdown of the starch granules when treating at high moisture content and heating temperature. After HMT, amounts of resistant starch (RS) in the treated rice grains significantly increased as compared to that in the untreated rice grains. Moreover, the rice grains were heat-moisture treated with higher moisture contents or heating temperatures had higher RS contents. As a result, the rice grains treated at moisture content of 30% and heating temperature of 120 °C contained the highest amount of RS (49.1%), released the lowest blood glucose level and had GI value of 55, which is considered as low GI food.