Studies on the in vitro starch digestibility and the glycemic index of six different indigenous rice cultivars from the Philippines

Starch degradability was studied in six indigenous Philippine rice cultivars differing in amylose contents. An in vitro enzymatic starch digestion method was applied in order to estimate the expected glycemic index in vivo based on the kinetics of starch hydrolysis in vitro. Two different treatments were investigated: first, samples were cooked and analysed immediately; second, samples were cooked and stored in a refrigerator for 24 h at a temperature of 4 °C in order to induce a retrogradation effect. The results indicate substantial differences in the estimated glycemic index between rice cultivars. Values ranged between 68 and 109 for cooked rice and between 64 and 87 for stored rice containing retrograded starch. Starch hydrolysis tended to be more rapid and more complete for waxy cultivars than for high amylose cultivars. Storing rice in the refrigerator led to a reduction of the estimated glycemic index for all cultivars. The highest decrease in starch hydrolysis after cool storing was seen for the waxy cultivars.