Submitted to: Journal of Stored Products Research
Publication Type: Peer reviewed journal
Publication Acceptance Date: 6/21/2013
Publication Date: 10/15/2013
Publication URL: http://handle.nal.usda.gov/10113/58240
Citation: Wang, S., Tang, J., Johnson, J.A., Cavalieri, R. 2013. Heating uniformity and differential heating of insects in almonds associated with radio frequency energy. Journal of Stored Products Research. 55:15-20. Interpretive Summary: A major problem in the marketing of California almonds is infestation by various insects, particularly larvae of the navel orangeworm. Increased restrictions on the chemical fumigants normally used to disinfest almonds of these pests and the growth of the organic industry has generated interest in non-chemical alternatives. Radio frequency heating, which heats product rapidly and thoroughly, is one potential alternative. The heating uniformity of almonds under radio frequency treatment was shown to be better than conventional hot forced air, which suggests radio frequency treatments would have less impact on product quality. A mathematical model was designed that predicted that pest insects would heat faster than almonds, and this was demonstrated using model insects. When almond kernels were heated under radio frequencies of 27.12 MHz to 55°C, final temperatures for model insects under the same conditions were 60 and 61°C at heating rates of 5 and 10°C, respectively. This observed differential heating of pest insects improves the efficacy of the proposed treatment. Such a treatment would allow the nearly $3 billion California almond industry to maintain their critical export markets with a non-chemical alternative.
Technical Abstract: 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 differential heating of insects. We compared the heating uniformity in almonds (Nonpareil) heated by RF and by forced hot air. A mathematical model suggested a 4.7 and 6.0°C preferential heating of the target pest navel orangeworm (Amyelois transitella [Walker]) over almonds at heating rates of 5 and 10°C/min, 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/m2 °C. To test the validity of 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, 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.