Shaojin Wang

Dielectric properties of fruits and insect pests as related to radio frequency and microwave treatments.

Information on dielectric properties of commodities and insect pests is needed in developing thermal treatments for postharvest insect control based on radio frequency (RF) and microwave energy. Dielectric properties of six commodities along with four associated insect pests were measured between 1 and 1800 MHz using an open-ended coaxial-line probe technique and at temperatures between 20 and 608C. The dielectric loss factor of fresh fruits and insects decreased with increasing frequency at constant temperatures. The loss factor of fresh fruits and insects increased almost linearly with increasing temperature at 27 MHz radio frequency, but remained nearly constant at 915 MHz microwave frequency. Both dielectric constant and loss factor of nuts were very low compared to those of fresh fruits and insects. The temperature effect on dielectric properties of nuts was not significant at 27 MHz. The large difference in the loss factor between insects and nuts at 27 MHz suggests possible differential heating of insects in nuts when treated at the same time in a RF system.

High-temperature-short-time thermal quarantine methods

Abstract In this paper, kinetic models are discussed with respect to their uses in describing the intrinsic thermal mortality of insect pests. A unique heating block system was used to obtain kinetic information for the thermal mortality of codling moth larvae. The kinetic data demonstrated the possibility to develop high-temperature-short-time thermal treatments to control codling moth and reduce thermal impact on product quality. Equations are presented to evaluate cumulative effects of any time–temperature history on the thermal mortality of target insect pests and on the quality of host materials. Computer simulation results demonstrated that the cumulative thermal effects on product quality during the heating period in conventional hot air or hot water treatments are much more important than the cooling period. Radio frequency (RF) heating or microwave heating is suggested as an alternative to reduce adverse thermal impact to treated commodities during the heating period. A case study is presented to demonstrate the effect of RF heating in a high-temperature-short-time thermal treatment to control codling moth larvae in in-shell walnuts.

Radio frequency treatments to control codling moth in in-shell walnuts

‘Diamond’ Walnuts (Juglans regia L.) in the shell were treated with radio frequency (RF) energy in a 27 MHz pilot-scale system to determine the treatment effect on third- and fourth-instar codling moth, Cydia pomonella (L.) (Lepidoptera: Tortricidae), mortality and walnut quality. After 2 and 3 min of RF treatments, infested in-shell walnuts were heated to 43 and 53°C. The corresponding insect mortality reached 78.6 and 100%. The fatty acid (FA) concentration of treated walnuts was not affected by RF treatments. The FA values were B 0.1% after accelerated storage times up to 30 days at 35°C, simulating storage at 4°C for up to 3 years. The effect of RF treatments on walnut oil peroxide values (PV) was not significant. The PV value of walnuts was less than 1.0 meq:kg (the upper limit for good quality walnuts), after 20 days storage at 35°C that simulated 2 year storage at 4°C. The PV was about 1.2 meg:kg after 30 days storage at 35°C. RF treatments can, therefore, potentially provide an effective and rapid quarantine security protocol against codling moth larvae in walnuts as an alternative to methyl bromide fumigation.

Process protocols based on radio frequency energy to control field and storage pests in in-shell walnuts

A practical process protocol was developed to control insect pests in in-shell walnuts using a 27 MHz pilot scale radio frequency (RF) system. Fifth-instars, that had been determined to be the most heat resistant life stage for navel orangeworm (Amyelois transitella [Walker]) using a heating block system, were selected as the targeted insect in the protocol development. RF heating to 55 °C and holding in hot air for at least 5 min resulted in 100% mortality of the fifth-instar navel orangeworm. Rancidity, sensory qualities and shell characteristics were not affected by the treatments. The process slightly reduced the moisture content of the walnut kernels, which could prove an additional benefit by providing even nut moisture content and reducing the growth of microorganisms. If this method can be economically integrated into the handling process, it should have excellent potential as a disinfestation method for in-shell walnuts.

DIFFERENTIAL HEATING OF INSECTS IN DRIED NUTS AND FRUITS ASSOCIATED WITH RADIO FREQUENCY AND MICROWAVE TREATMENTS

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/m2 .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.

Modeling fruit internal heating rates for hot air and hot water treatments

Hot air and hot water heating methods have been extensively studied as thermal treatments to control insect pests in fruits to replace chemical fumigation. An inherent difficulty in using these methods is that slow heating rates may result in long treatment times and possible damage to fruit quality. Many factors influence heating time. A systematic analysis of those influences is desirable to help in designing effective treatment protocols. A simulation model based on heat transfer theory was developed to study the effect of fruit thermal property, fruit size, heating medium and heating medium speed on heat transfer rates within spherical fruits. The simulation demonstrated that the small variation in thermal diffusivity among fruits had little effect on heating time. Fruit internal heat transfer rate was significantly influenced by fruit size and by heating medium. Water was a more efficient medium than air and increasing air speed increased heating rates. Water circulation speeds had little influence on heat transfer rate. The Biot number showed that internal energy transfer by conduction was a heating rate limiting factor. Combining low frequency electromagnetic energy with hot air or hot water eliminated conduction as a major rate-limiting factor because the energy was directly delivered to the fruit interior.

Experimental and numerical studies on the heterogeneity of crop transpiration in a plastic tunnel

Abstract The heterogeneity of crop transpiration is important to clearly understand the microclimate mechanisms and to efficiently handle the water resource in greenhouses. A computational fluid dynamic software ( cfd2000 ) was used to study the climate and crop transpiration distributions in a 22×8 m 2 plastic tunnel situated in Avignon, France, together with a global solar radiation model and a crop heat exchange model. The distribution of solar radiation within the greenhouse tunnel was determined based on the path of the sun, the greenhouse geometry, the cover transmittance and the sky conditions. The crop transpiration was deduced by assimilating the crop to a porous medium exchanging latent and sensible heats with its environment. The radiative and convective heterogeneity in two vertical sections of the tunnel predicted by the CFD model was validated against the experimental results obtained by a group of solar cells and sonic anemometers. The validated model was finally used to predict the transpiration flux of a mature lettuce crop in the tunnel. The crop transpiration strongly varied (up to 30%) according to the place in the tunnel. The predicted crop transpiration was in close agreement with the measured value both along the sections with the side openings and between the consecutive openings.

Development of a saline water immersion technique with RF energy as a postharvest treatment against codling moth in cherries

‘Sweetheart’ sweet cherries (Prunus aium L.) immersed in 0.15% saline water were treated with radio frequency (RF) energy. The dielectric and ionic conductivity properties of the immersion water and that of fruit were matched to obtain a relatively uniform temperature distribution within and among fruits during RF heating. With immersion in saline water of 0.15% NaCl, the mean temperature of the water and that of the cherries differed by 0.6 °C, while the maximum temperature variation within and among fruits determined within 1 min after RF treatment completion was 1.0 °C of the set temperatures of 48 and 50 °C. The saline water immersion technique helped overcome the markedly high temperature differential problem within and among fruits, normally associated with treatments in air (without immersion) during RF heating. More than 99% mortality of the 200–400 codling moth larvae or 589–624 eggs was obtained at 50 °C when treated for between 7 and 10 min (heating 2–5 min and holding 5 min). Most quality parameters analyzed were better, or are comparable with methyl bromide fumigated fruit. Saline–water-immersion treatment in RF may be used to overcome the problem of slow conventional hot air or water heating, as well as the non-uniformity of temperature associated with electromagnetic heating in air, for developing alternative quarantine treatment for fruits.

Thermal-death kinetics of fifth-instar Amyelois transitella (Walker) (Lepidoptera: Pyralidae)

Information on kinetics for thermal mortality of navel orangeworm, Amyelois transitella (Walker) (Lepidoptera: Pyralidae), is needed for developing post-harvest phytosanitation thermal treatments of walnuts. Thermal-death kinetics for fifth-instar navel orangeworms were determined at temperatures between 461C and 541C at a heating rate of 181C min � 1 using a heating block system. Thermal-death curves for fifth-instar navel orangeworms followed a 0.5th-order of kinetic reaction. The time required to achieve 100% mortality (N0 ¼ 600) decreased with increasing temperature in a logarithmic manner. Complete kill of 600 insects required a minimum exposure time of 140, 50, 15, 6, and 1 min at 461C, 481C, 501C, 521C, and 541C, respectively. The reaction rate (k) was affected by treatment temperatures following an Arrhenius relationship. The activation energy for thermal kill of fifth-instar navel orangeworms was

Thermal death kinetics of fifth-instar Plodia interpunctella (Lepidoptera: Pyralidae).

Heat treatments have been suggested as alternatives to chemical fumigants for control of postharvest insects in dried fruits and nuts. Conventional forced hot air treatments heat product too slowly to be practical, but radio frequency treatments are capable of more rapid product heating. While developing radio frequency heat treatments for dried fruits and nuts, the heat tolerance of nondiapausing and diapausing fifth-instar larvae of the Indianmeal moth, Plodia interpunctella (Hübner), was determined using a heating block system developed by Washington State University. Both a 0.5th order kinetic model and a classical empirical model were used to estimate lethal exposure times for temperatures of 44-52 degrees C for nondiapausing fifth-instar larvae. We obtained 95% mortality at exposures suitable for practical radio frequency treatments (< or = 5 min) with temperatures of 50 and 52 degrees C. Diapausing larvae were significantly more tolerant than nondiapausing larvae at the lowest treatment temperature and shortest exposure, but differences were not significant at more extreme temperature-time combinations. Previous studies showed that fifth-instar larvae of the navel orangeworm, Amyelois transitella (Walker), were more heat tolerant than either diapausing or nondiapausing Indianmeal moth larvae. Consequently, efficacious treatments for navel orangeworm would also control Indianmeal moth.