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Dielectric properties directly influence microwave drying characteristics of food products. A knowledge of dielectric properties of foods as a function of moisture content and temperature is essential in the design and control of microwave drying systems. Dielectric constant e4 and loss factor e, of Red Delicious apples (Malus domestica Borkh.) were measured over a moisture content range of 4% to 87.5% at 225 C and 605 C. At high moisture content (>70%), free water dispersion and ionic conduction accounted for the dielectric behavior. At medium moisture (23%), ionic conduction played a major role. At low moisture contents (4%), bound water accounted for the major dispersion mechanism. A decrease in moisture content resulted in a decrease in e4 and e,. Based on this study, we expect a strong moisture leveling effect when drying apples from 50% to 4% at elevated temperatures in 915 MHz or 2.45 GHz microwave drying systems.

This research was conducted to provide a theoretical basis and experimental evidence to support the hypothesis that insect larvae can be preferentially heated in dry nuts and fruits by radio frequency (RF) heating for pest control. We selected codling moth larvae as the target insect and in-shell walnuts as the host material for this study, and focused our attention on one RF frequency (27 MHz) and one microwave frequency (915 MHz). Dielectric properties measurements showed that the loss factor ratio between codling moth larvae and walnut kernels at 20°C was 397 at 27 MHz and 4 at 915 MHz. The theoretical prediction for a 3 min treatment at 0.27 kW/kg suggested 12.0°C preferential heating of insect larvae for the loss factor ratio of 397 (corresponding to 27 MHz) and 0.1°C for the ratio of 4 (corresponding to 915 MHz), when the heat transfer coefficient between insects and walnuts was set at 500 W/m 2 °C. To prove differential heating predicted by the theoretical model, a gellan gel with dielectric properties similar to those of insects was used as a model insect. When walnut kernels were heated at 27 MHz from 20°C to 53°C, the model insects were differentially heated from 12.6°C to 21.2°C higher than the kernel temperature, depending on the power used and the treatment time. These values corresponded to a heating rate for the model insect of 1.4 to 1.7 times greater than that for walnut kernels. As predicted by the theoretical model, microwave heating at 915 MHz caused no differential heating of insects. Preferential heating of insects in dry nuts and fruits at radio frequencies can be used in developing thermal treatments to control insects without adversely affecting product quality.

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