Differential Heating of Insects in Dried Nuts and Fruits Associated With Radio Frequency and Microwave Treatments

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.

Elimination of Cruptolestes Ferrungineus S. In Wheat by Radio Frequency Dielectric Heating at Different Moisture Contents

“…In general, RF is preferred to MW in controlling insects in stored products [16,17], and has been used at industrial scale for disinfestations of walnuts [18]. There is a need to investigate selective heating of one of the most troublesome insect pests, the Cryptolestes ferrungineus S., in the stored Canadian western red spring (CWRS) wheat, cv.…”

Microwave technology for disinfestation of cereals and pulses: An overview

“…Hence, model insects having dielectric properties similar to those of codling moth larvae have been developed using gel. Temperature measurements with model insects revealed 1.4-1.7 times greater heating of insects than walnuts at 27 MHz but no preferential heating of insects was detected at 915 MHz (Wang et al 2003a). The effect of soft electron treatment was studied by Imamura et al (2004) on the life stages of T. castaneum, P. interpunctella, and Callosobruchus chinensis (L.).…”

“…There is a possibility of advantageous selective heating in mixtures of different substances (Hamid et al 1968;Nelson 1972;Ikediala et al 1999;Antic and Hill 2003;Wang et al 2003a). When a mixture of dry food stuffs and insects are heated, the insects are heated upto lethal temperature because they contain more water as compared to the food stuff which is either left unaffected or gets slightly heated (Hurlock et al 1979;Wang and Tang 2001).…”

Applications of Microwave Heating in Agricultural and Forestry Related Industries

“…An important key to developing successful thermal treatments is to balance the need for complete insect mortality with minimal impact on the product quality. A common difficulty in using conventional hot-air disinfestation is the slow heating rate, non-uniform temperature distribution, and possible heat damage to heat-sensitive commodities [35]. A more promising approach is to heat the commodity rapidly using radio frequency (RF) or microwave dielectric heating to control insects [35,36].…”

“…The rate of temperature increase also depends on the density and thermal capacity of the heated material [35,36]:…”