Effects of input power mode on the uniformity of berry pulp in microwave heating

Abstract: Microwave heating has been one of the most popular heating methods for foods and agricultural products. Hence, the core of the material can be heated up rapidly during microwave heating, and then the heat can flow outwards from the core to the surface. Among them, the uniform distribution of heat energy can greatly contribute to the thermal processing of commercial foods in microwave heating. Therefore, this study aims to determine the generation and change mechanism of temperature and moisture distribution in the pulpy food materials under microwave heating. A berry pulp was also introduced as a representative material with high moisture, high viscosity, and rich in heat-sensitive components. Four evaluation indexes were then selected to characterize the even heat distribution of the pulp, including the temperature dispersion (VT), moisture dispersion (VM), hot zone distribution (HTD), and temperature contrast value (CON). An attempt was also made to clarify the effects of the input power (constant and intermittent variable) modes on the heat distribution inside the berry pulp under microwave heating. The results show that the reflection, scattering, and non-resonance of microwave waves caused the non-uniformity of microwave heating, due to the interference of the low-density medium (air) propagating into the high-density medium (fruit pulp). The microwave volumetric heating inside the berry pulp also resulted in the generation and disappearance of vapor bubbles, indicating a dominated process of the heat and mass transfer. There were three successive stages inside the fruit pulp under microwave heating, including slow heating, stable temperature, and rapid heating. Such variations were then attributed to the uniformity index of the temperature and moisture distribution of the fruit pulp layer. Specifically, the non-uniformity of the moisture distribution inside the berry puree increased significantly, owing to the concentration and overheating of the electric field at the corners of the material layer during microwave heating. By contrast, the rapid change of temperature in hot sites was utilized to weaken the non-uniformity with increasing the temperature and hot zones, due to the formation and disappearance of bubbles inside the berry pulp under microwave heating. These uniformity indexes were used to assess the microwave heating in the berry pulp, where the VT and HTD increased in the heating zone, and then decreased in the temperature stability zone, whereas, the VM continued to rise, while the CON increased in the temperature stability zone, and then decreased in the rapid heating zone. These indicate that the overheating at the corners of berry pulp caused the non-uniformity of microwave heating, whereas, the reduction of temperature difference among cold and hot spots was used to enhance the heating uniformity in the later stage of drying. More importantly, the input power with an intermittent variable mode was used to significantly improve the uniformity of microwave heating inside the berry pulp. Furthermore, the uniformity improvement rate of the pulp, the VT, VM and HTD indicators increased, whereas, the CON increased to the stable temperature region (II), and then decreased in the rapid heating region (III), with the increase of the power transition point. Consequently, the heating uniformity was achieved, when reducing the microwave power ratio and the intermittent time higher than 8 min. But, the high-low microwave power ratio below 0.5 was led to the low heating efficiency. Specifically, an optimally variable power input of microwave heating was obtained for the higher heating uniformity and efficiency, where the power conversion point of 0, the power ratio of 0.5, and the intermittent time of 8 min. The findings can provide a potential mathematical model to evaluate the microwave heating uniformity of berry materials.