Effects of Cooking Methods and Starch Structures on Starch Hydrolysis Rates of Rice

Authors


Direct inquiries to author Jane (E-mail: jjane@iastate.edu).

Abstract

This study aimed to understand effects of different cooking methods, including steamed, pilaf, and traditional stir-fried, on starch hydrolysis rates of rice. Rice grains of 3 varieties, japonica, indica, and waxy, were used for the study. Rice starch was isolated from the grain and characterized. Amylose contents of starches from japonica, indica, and waxy rice were 13.5%, 18.0%, and 0.9%, respectively. The onset gelatinization temperature of indica starch (71.6 °C) was higher than that of the japonica and waxy starch (56.0 and 56.8 °C, respectively). The difference was attributed to longer amylopectin branch chains of the indica starch. Starch hydrolysis rates and resistant starch (RS) contents of the rice varieties differed after they were cooked using different methods. Stir-fried rice displayed the least starch hydrolysis rate followed by pilaf rice and steamed rice for each rice variety. RS contents of freshly steamed japonica, indica, and waxy rice were 0.7%, 6.6%, and 1.3%, respectively; those of rice pilaf were 12.1%, 13.2%, and 3.4%, respectively; and the stir-fried rice displayed the largest RS contents of 15.8%, 16.6%, and 12.1%, respectively. Mechanisms of the large RS contents of the stir-fried rice were studied. With the least starch hydrolysis rate and the largest RS content, stir-fried rice would be a desirable way of preparing rice for food to reduce postprandial blood glucose and insulin responses and to improve colon health of humans.

Practical Application

After rice was cooked using different methods, including steamed, pilaf, and stir-fried, the stir-fried indica rice displayed the least starch hydrolysis rate and the largest resistant starch (RS) content. These results showed that cold storage of steamed normal rice at 4 °C for 24 h followed by stir-frying with corn oil (10%) reduced the rate of starch hydrolysis and increased the RS content. Ingesting stir-fried rice therefore can reduce the postprandial blood–glucose concentration and insulin response, which benefits the health of diabetics and prediabetics. The large RS content of the stir-fried normal rice could also provide health benefits to the colon.

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