Study on Heating Uniformity of Ready-to-Eat Rice under Continuous Microwave Reheating

To improve the heating uniformity of ready-to-eat rice during continuous microwave reheating, the microwave power, heating time, and combination arrangement of ready-to-eat rice were selected as influencing factors, and moisture content, average temperature, and temperature uniformity served as evaluation indexes. The single-factor experimental design was employed to investigate the effects of continuous microwave reheating parameters on the temperature and moisture content distribution inside ready-to-eat rice and obtain the optimal processing parameters under continuous microwave reheating. The results demonstrated that microwave power and heating time had significant effects on the uniformity of temperature distribution inside the arrangement of packaged ready-to-eat rice. The temperature distribution inside the combination arrangement of ready-to-eat rice in the cavity of the continuous microwave dryer showed a clear “corner effect” and “rim overheating” due to the intensity decay of incident microwave contacting the arrangement of ready-to-eat rice. The ready-to-eat rice arranged in four columns had the highest average temperature and uniform temperature distribution. However, the average temperature of the ready-to-eat rice in two columns was relatively low with the non-uniform distribution. The ready-to-eat rice container laid along the width direction of the conveyor belt in the continuous microwave dryer had the obvious uniformity of temperature distribution which is conducive to microwave energy absorption and utilization efficiency. It was appropriate to determine the four-column arrangement of the ready-to-eat rice. The average temperature of ready-to-eat rice increased with microwave power, temperature uniformity initially deteriorated and ultimately dropped with microwave power, and the highest heating uniformity may be achieved at a microwave power of 17.1 kW (19 kW*0.90) with a pulse duty cycle of 0.90. The temperature distribution exhibited a “center focus effect” with an increase of the microwave power, where the conversion point from “corner effect” to “center focus effect” occurred at the microwave power of 16.15 kW (19 kW*0.85) with a pulse duty cycle of 0.85. During the continuous microwave reheating process of ready-to-eat rice, the top, middle, and bottom layers of ready-to-eat rice showed “hot spots” at the corners and “cold spots” at the center, demonstrating a clear “corner effect” under the total heating time in the range of 120～240 s. The “center focus effect” occurs at a total heating duration of 300 to 360 s. The average temperature gradually increased with the total heating time, and so does the uniformity of temperature distribution. According to the temperature distribution of the ready-to-eat rice’s overall arrangement during a continuous microwave reheating for 300 s, the average temperature of the top layer is the highest at the initial stage of reheating (60 s), the temperature of the middle layer gradually is higher than the top layer at the middle stage of reheating (120～180 s), and the average temperature of the bottom layer remained the lowest throughout the reheating process (0～300 s). According to the temperature distribution in the top, middle, and bottom layers of ready-to-eat rice during the continuous microwave reheating with a total heating period of 300 s, the temperature distribution trend was similar to the distribution of overall arrangement temperature. The color of the water-containing color-changing silica gel was introduced to determine the changing trend in the moisture content of ready-to-eat rice. It was found that the evaporation trends of the moisture in ready-to-eat rice were similar to its temperature distribution under the continuous microwave reheating process. There was a significant phenomenon of high edge temperature and corner thermal focusing in the rectangular rice packaging boxes placed inside the microwave cavity, and a similar distribution also existed in single packaging box. As the increase of pulse duty cycle (microwave power), the depth of microwave transmission into the interior of the rice, and the generation of microwave volumetric heating, the "cold spot" moved towards the center area at gradually falling trend, which improved the uniformity of microwave heating. The arrangement method had little effect on heating uniformity. For boxed rice arranged in 2, 3, and 4 columns in a continuous microwave heating device, the uniformity of rice heating inside the box changed from high to low due to the focusing of microwaves on the edge surface of the boxed rice, attenuation during internal transmission, and the low to high dielectric properties between the boxes and their interior. In order to fully utilize the heating capacity and cavity space size of continuous microwave equipment, the optimal process parameters for ready-to-eat rice under continuous microwave reheating were obtained as microwave power of 17.1 kW (19 kW*0.90) with a pulse duty ratio of 0.90, a heating time of 300 s, and four ready-to-eat rice packaging containers arranged across the width of the conveyor belt of the continuous microwave dryer. The research findings can provide the feasible approaches for reheating cooked-rice food with high quality.