Effects of cooking methods on starch hydrolysis kinetics and digestion-resistant fractions of rice and soybean

In this study, we investigated the effects of cooking methods and digestion-resistant fractions on the starch hydrolysis kinetics of rice and soybeans. The RS1 contents in unheated rice and soybeans were found to be high at 5.3 and 4.9%, respectively. After heating, the RS1 levels of the rice and soybeans were significantly reduced to 0.4 and 2.4%, respectively. The total nonstarch polysaccharide (NSP) content was not significantly different according to heat treatment, but the heat treatments affected the amounts of water insoluble nonstarch polysaccharides (WISNSP) and water soluble nonstarch polysaccharides (WSNSP). The level of WISNSP in the unheated samples was higher than the WSNSP of both samples; however, the WISNSP level showed a reducing trend after heating. Also, total pentosan and water soluble pentosan contents showed similar trends with total NSP and WSNSP. The hydrolysis indices (HI) of the unheated rice and soybeans were low (36.2 and 40.3%), while the heated group had high levels (89.5 and 45.0%). Among the cooking methods, the highest kinetic constants (0.284 and 0.199) were found with autoclaving for both rice and soybeans, followed by electric cooker (0.282 and 0.170), microwave oven (0.256 and 0.155), and stone pot (0.238 and 0.167). Particularly, all soybean samples appeared to have lower starch hydrolysis kinetics than the rice samples. The RS contents of the rice and soybeans were highest in the samples that were cooked in the stone pot, at 1.2 and 3.8%, respectively.