Heating uniformity and differential heating of insects in almonds associated with radio frequency energy

Radio frequency (RF) treatments have potential as alternatives to chemical fumigation for phytosanitary disinfestation treatments in the dried nut industry. To develop effective RF treatment protocols for almonds, it is desirable to determine heating uniformity and the occurrence of RF differential heating of insects. This study compared heating uniformity in almonds (Nonpareil) heated by RF and by forced hot air. A mathematical model suggested a 4.7 and 6.0 °C RF preferential heating of the target pest navel orangeworm (Amyelois transitella [Walker]) over almonds at heating rates of 5 and 10 °C min−1, respectively, 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 m−2 °C−1. To validate the model, a gellan gel with dielectric properties similar to those of the target pest 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−1, respectively. These values corresponded to a heating rate for the model insect of 1.2 times greater than that for almond kernels. Slight preferential heating of insects in almonds using RF energy would improve the efficacy of large-scale RF treatments.