Influence of ohmic heating on the electrical conductivity, volume, and rice quality of each component of the water–rice mixture

Abstract

Ohmic heating (OH) is a new heating method with high electrothermal conversion, simple equipment, high accuracy of temperature control, and rapid heating. It is used in cooking rice. This study aimed to investigate the effects of OH on the expansion behavior of rice grains, changes in the electrical conductivity (EC) of each component, temperature, and quality at different heating rates (3 °C/min, 6 °C/min, and 9 °C/min). The EC of the rice grains was calculated using the equivalent circuit method. The results showed that the total volume of the water–rice mixture did not change significantly during the heating stage of OH cooking of rice. The volume and the proportion of rice in the water–rice mixed layer increased slowly before 62.5 °C and then rapidly after that. The EC of the water layer rice soaking water was lower than that of the water–rice mixed layer rice soaking water. The regression models of the volume of the water–rice mixed layer, the proportion of rice in the mixed layer, and the EC of each component were established. Besides, the internal temperature distribution was uneven due to the difference in the EC of each part of the mixture, and the temperature gradually decreased from top to bottom. Hence, the upper rice grains softened faster. This finding provided technical support to develop an automatic rice cooking device with high efficiency using OH.