Heating uniformity in radio frequency treated walnut kernels with different size and density

Radio frequency (RF) treatment is one of the most potential methods for disinfestations and pasteurizations of postharvest agriculture products. Non-uniform heating is the main issue for RF treatment to be used in a largescale industrial application. A 6 kW, 27 MHz polit-scale RF system was used to reveal the heating uniformity of walnut kernels associated with different size and density. The results showed that the thicker sample in the container indicated the higher heating rate, but the worse heating uniformity when the sample was heated by RF energy with the same weight but larger size. The heating rate of samples decreased with increasing electrode gap, which was beneficial for improving the RF heating uniformity. Meanwhile, the RF heating uniformity index of samples increased with increasing kernel size and decreased with increasing density when the samples were treated with the same volume and heating rate. The heating uniformity indexes were 0.124 for whole kernels, 0.115 for half kernels, 0.104 for quarter kernels, and 0.082 for cracking kernels after RF thermal treatment with the same volume and electrode gap of 98 mm. Furthermore, the heating uniformity of mixing kernels was improved by increasing percentages of cracking kernels to whole or half kernels. The results of this research may provide valuable information to improve the RF heating uniformity in walnut kernels for large-scale industrial applications.