Effect of electric field distribution on energy use efficiency for berry puree under microwave drying

Berry fruit rich in bioactive compounds (anthocyanins and vitamins) has fairly high health and nutritional benefits in terms of anti-aging, antioxidant ability, and anti-carcinogenic activity. However, fresh berry fruit is difficult to be stored, due to its high moisture content and temperature in the harvesting season. Consequently, microwave drying has been widely applied in the dehydration process for berry fruit, with the advantages of high efficiency, great processing capacity, and easy control of dryer. In microwave heating, the energy absorption depends on the intensity of electric fields and dielectric properties inside the processed material. Maxwell's equation was followed by the transmission of an electric field in the microwave cavity, where a time-varying electric field was accompanied by a corresponding time-varying magnetic field. The behavior of electromagnetic fields can be completely defined inside a microwave applicator and coupled with the appropriate boundary conditions. The electric field is primarily determined by magnetron arrangement, the shape, and size of the cavity. However, the complex and variable distribution of electric field inside the material has posed severe effects on the utilization of microwave energy and drying uniformity of the final product. Taking the raspberry puree as a representative processed material, an adjustable regulation of active magnetrons was introduced to mount on the ceiling of the microwave cavity in a continuous dryer, in order to elucidate the effect of electric field distribution in the material layer on the energy utilization, particularly with the rising temperature and declining moisture. The input powers of the microwave were set as 12, 15, 18, and 21 kW. Four modes of active magnetrons in amount and positions were used to reveal the change of electric field. A coupling model with the electromagnetic, heat, and mass transfer was established to characterize the distribution of electric field and energy absorption inside the puree layer. The microwave propagation inside berry puree layers and utilization efficiency of energy were clarified in the microwave dryer, considering the coupling model and drying properties of berry puree at different moisture and temperature. Results show that the Root Mean Square Errors (RMSE) of simulated and measured temperatures were 5.8, 4.1, 6.7, and 6.9℃ at the microwave powers of 12, 15, 18, and 21 kW, respectively, indicating that the simulation model was reliable for the distribution of electric field in the pulp material layer. The amount and positions of active magnetrons determined the Transverse Electric (TE) or Transverse Magnetic (TM) plane wave in the microwave cavity, where the incident angle of the plane wave dominated the conversion capacity of microwave energy to thermal energy. There were higher impacts on the dimensional size of the inner wall in the microwave cavity, and the relative position between the raspberry puree and guidance ports, compared with the mounts of active magnetrons. There was a greater uniform distribution of electric field along the longitudinal direction than along the crossing direction. The active mode of magnetrons was parallel arrangement, and the distance between the central points of magnetrons was odd times of quarter microwave wavelength in the direction of the long side of the magnetron. As such, the uniformity of the electric field improved in the whole material layer, due to the cross distribution with high and low spots in the electric field. High uniformity of electric field strength can be used to enhance the absorption and conversion efficiency of microwave energy, thereby achieving a more uniform distribution of temperature in the final dried material. The research findings can gain significant insight to promote energy efficiency and drying uniformity in the large-scale continuous microwave dryer. © 2021, Editorial Department of the Transactions of the Chinese Society of Agricultural Engineering. All right reserved.