Dielectric properties of in-shell peanuts with radio frequency and microwave heating treatment and RF heating performance

Radio frequency (RF) heating technology has potential advantages for drying or sterilization of in-shell peanuts, with a critical need for understanding the dielectric properties governing energy conversion. In this study, the dielectric properties of peanut shells and kernels were measured over the frequency range of 10 MHz ∼ 3 GHz, spanning temperatures from 20 to 80 °C. Results showed that the dielectric constant of all the samples decreased rapidly with increasing frequency, the loss factor of peanut shells and kernels with lower moisture content exhibited an initial decrease within the RF band followed by a slight increase within the microwave (MW) band, while loss factor of peanut kernels with higher moisture content decreased continuously with increase in frequency. A high temperature will enhance the sample polarization, while an elevated moisture content will provide the more polar molecules and release the more ions, so that both of them have the positive effect on the dielectric properties. It should be noted that the disruption of the coarse fiber structure and clumping phenomenon in the samples might mitigate the temperature effect. The numerical simulation results of RF heating based on the cubic regression model of dielectric properties showed that: since the loss factor was more sensitive to temperature and moisture content than dielectric constant, the loss angle tangent of the samples increased significantly with increasing moisture content, which subsequently hindered the heating rate of the samples. Meanwhile, this simulation results derived the optimal conditions, with the fastest heating rate at 20% moisture content and the most uniform RF heating (UI=0.16331) at 30% moisture content. Consequently, the loading thickness of in-shell peanuts should be controlled within 11 cm when designing a 27.12 MHz RF dryer. This research has a profound meaning to promote the efficiency of RF heat treatment of in-shell peanuts.