Transactions of the ASAE Vol. 46(4): 1175–1182 E 2003 American Society of Agricultural Engineers ISSN 0001–2351 1175 DIFFERENTIAL HEATING OF INSECTS IN DRIED NUTS AND FRUITS ASSOCIATED WITH RADIO FREQUENCY AND MICROWAVE TREATMENTS S. Wang, J. Tang, R. P. Cavalieri, D. C. Davis ABSTRACT. 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. Keywords. Codling moth, Dielectric properties, Differential heating, Microwaves, Radio frequencies, Walnuts. ried nuts (including walnuts) and fruits are treated by chemical fumigation to control field and storage pests before being shipped to domestic and international markets. Because of increasing public concern about adverse impacts of chemical fumigation on humans and the environment, there is a heightened interest in developing non–chemical pest control methods, especially thermal methods. An important key to developing successful thermal treatments is to balance needs for a complete kill of insects with a minimal thermal impact on product quality. A common difficulty in using conventional hot–air disinfestation methods is the slow heating rate, non–uniform temperature distribution, and possible heat damage to heat–sensitive commodities (Hansen, 1992; Tang et al., 2000; Wang et al., 2001b). A more promising approach is to heat the commodity rapidly by radio Article was submitted for review in December 2002; approved for publication by Food & Process Engineering Institute Division of ASAE in May 2003. Presented at the 2002 ASAE Annual Meeting as Paper No. 026002. The authors are Shaojin Wang, ASAE Member Engineer, Research Associate, Juming Tang, ASAE Member Engineer, Professor, Ralph P. Cavalieri, ASAE Member Engineer, Associate Dean, and Denny C. Davis, ASAE Member Engineer, Professor; Department of Biological Systems Engineering, Washington State University, Pullman, Washington. Corresponding author: Juming Tang, Department of Biological Systems Engineering, Washington State University, 213 L. J. Smith Hall, Pullman, WA 99164–6120; phone: 509–335–2140; fax: 509–335–2722; e–mail: jtang@mail.wsu.edu. frequency (RF) or microwave dielectric heating to control the insects in commodities (Nelson and Payne, 1982; Ikediala et al., 1999; Tang et al., 2000; Wang et al., 2001a). Tang et al. (2000) and Wang et al. (2002b) proposed thermal treatments based on RF energy to replace chemical fumigation to control the codling moth, Cydia pomonella (L.) (Lepidoptera: Tortricidae), and other insect pests in walnuts (Juglans regia L.). Electromagnetic energy in RF (1 to 300 MHz) or microwave (300 to 30,000 MHz) heating has been studied to control insects in other commodities for many years. Headlee and Burdette (1929) reported results for determining lethal exposures for honeybees in a 12 MHz RF field. They observed that frequency, time, and voltage gradient were three important factors in killing insects. An attractive feature of insect control using an electromagnetic field is the possibility that the insects may be heated at a faster rate than the dry nuts and fruits they infest. If this were true, then the insects would reach a lethal temperature while the product would be heated to lower temperatures that do not cause quality loss. By exploring differences in the rate of heating between insect pests and host materials during RF or microwave heating, we can potentially reduce the time and the product temperature needed for effective treatments, thereby reducing adverse effects on product quality and enabling a greater throughput of product in a processing plant. In general, temperature rise in insects and agricultural commodities when exposed to the same electromagnetic field depends not only on their dielectric properties but also D