Heating Uniformity and Differential Heating of Insects in Almonds in Radio Frequency Systems

Abstract

Radio frequency (RF) treatments are one of the potential phytosanitary alternatives to chemical fumigation in nut industry. It is desirable to determine heating uniformity and differential heating of insects in almonds in developing effective RF treatment protocols. We selected hot air heating first to determine the heating uniformity in almonds (Nonpareil), and navel orangeworm (Amyelois transitella [Walker]) larvae as the targeted insect and almond as the host material for differential heating using RF energy. To prove the hypothesis of differential heating, a gellan gel with dielectric properties similar to those of insects was used as a model insect. When almond kernels were heated at 27.12 MHz from 21°C to 55°C, the model insects were differentially heated about 4.6°C and 5.6°C higher than the kernel temperatures at heating rates of 5 and 10°C/min, respectively. These values corresponded to a heating rate for the model insect of 1.2 times greater than that for almond kernels. Based on the differential heating model developed for codling moth larvae in walnuts, the theoretical prediction suggested 4.7 and 6.0°C preferential heating of model insects over almonds at the heating rates of 5 and 10°C/min for the loss factor ratio of 183 at 27.12 MHz, when the heat transfer coefficient between insects and almonds was set at 500 W/m2 °C. Slightly preferential heating of insects in almonds using RF energy would help development of effective large-scale treatments after taking the heating non-uniformity into account.