Effects of ohmic heating on the cooking process energy consumption and quality of cooked rice

Abstract: Rice is one of the most important food source for a major portion of the world's population. In cooking rice, a traditional electric cooker has low efficiency of electro-thermal conversion, and low accuracy of temperature control. Alternatively, an ohmic heating has attract much attention, due to its high electro-thermal conversion efficiency, simple structure, easy control of equipment, as well as uniform and rapid heating. In this study, an ohmic heating experimental system was designed for cooking rice. The system was consisted of a control system, an ohmic heating container, and a personal computer. Its function was to collect the voltage, current, and temperature signals, then to send the collected data to the computer. The feedforward compensation was used to control the heating rate, whilst the PID controller was used to control the heat preservation temperature. PWM signals were both output with different duty cycles to adjust the output voltage, achieving the purposes of constant heating rate and heat preservation temperature. A systematic investigation was made to study the effects of soaking time (5, 15, 30, 50, 80 min) and water-rice ratio (1.25:1, 1.50:1, 1.75:1, and 2.00:1 mL/g) on the electrical conductivity of water-rice mixture, ohmic heating rate, energy consumption, and rice quality (expansion ratio, water absorption ratio, physical property value, and color) during ohmic heating. The effect of ohmic heating rate on the rice quality was explored, in order to compare energy consumption and quality of rice cooked by ohmic heating and electric cooker, where the same material and heating process were used during cooking rice. The results showed that during soaking rice, the electrical conductivity of rice-water mixture increased, due to the conductive material inside the rice, indicating an increase in the ohmic heating rate. The soaking rice reduced the energy consumption (P＜0.05), further to obtain the soft rice. Nevertheless, there was no any influence on the water absorption ratio, expansion ratio, stickiness, springiness, and color of rice, indicating that the cooking rice by ohmic heating was completely feasible under the situation that the rice had been soaked for 30 min. During the process of ohmic cooking rice, the electrical conductivity of rice-water mixture initially increased with the increase of cooking time, and then gradually decreased during the heat preservation stage. The electrical conductivity of water-rice mixture became greater, as the water-rice ratio increased. At the same time, the volume of water-rice mixture and the rice both showed the trend of being unchanged first, then increasing, and finally tending to balance. The total volume of rice was larger after cooking, as the water-rice ratio increased. The hardness of rice increased (P＜0.05), as the heating rate of ohmic cooking was higher, but the stickiness and springiness decreased slightly (P＜0.05). The heating rate can be within the range of 3-9 ℃/min. When the water-rice ratio was from 1.25:1 mL/g to 1.50:1 mL/g, the rice tasted better, indicating that the more suitable water-rice ratio was 1.50:1 mL/g to 1.75:1 mL/g in a electric cooker. In addition, the ohmic cooking rice had no heating hysteresis, where the temperature of heating and heat preservation can be control accurately, compared with an electric cooker. In the same amount of rice, the energy consumption of ohmic heating was only 20%-25% of energy required for electric cooker. Compared with an electric cooker, the hardness of rice cooked by ohmic heating can be reduced by 21%, and the expansion ratio and water absorption ratio were larger (P＜0.05), while the whiteness value (L*) of rice were lower (P＜0.05). This finding can provide a technical support to develop automatic rice cooking device with a high efficiency using ohmic heating.