Change in the Amounts of Reducing Sugars and Free Amino Acids in Rice during the Cooking Processes

Abstract

Rice (Nipponbare from Shiga Prefecture) with a 1.5 (w/w) ratio of water to rice was cooked. The amounts of reducing sugars and free amino acids which were contained in the rice grains and eluted into the cooking water were measured during soaking (1 h for 20°C), heating (at the rate of 6.6°C/min) and murashi (15 min) duration after the initial washing (20°C). The reducing sugars in the raw rice were reduced to two thirds by washing. They showed the largest increase in the temperature range of 40-60°C during cooking in both the rice grains and cooking water. This was due to the enzyme-catalyzed decomposition of starch to reducing sugars that was accelerated in this temperature range. The most abundantly contained amino acids, aspartic and glutamic acids, were reduced to one fifth as a result of washing. These amino acids increased in the cooking water during soaking and increased in the rice grains in the temperature range of 80-100°C during cooking.