Elizabeth J. Mitcham

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.

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.

Effects of superatmospheric oxygen on strawberry fruit quality and decay.

The effects of elevated O2 alone or in combination with elevated CO2 atmospheres for postharvest decay control on strawberry fruit (Fragaria×ananassa Duch.) were assessed. In vitro and in vivo growth of Botrytis cinerea Pers.:Fr. and the effects on fruit quality were determined under eight atmospheres: air, 40, 60, 80, 90 and 100 kPa O2, 40 kPa O2+15 kPa CO2 and air+15 kPa CO2. The commercially-used CO2 level of 15 kPa in air and its combination with 40 kPa O2 were most effective in suppressing mycelial growth in vitro following 7 days at 5°C under the atmospheres. However, after 14 days of treatment at 5°C, 100 kPa O2 inhibited mycelial growth more than either of the CO2 treatments. In all treatments, the growth rate increased immediately upon removal from the atmosphere, indicating that there was no residual inhibitory effect. The 100 kPa O2 treatment was also the most effective in controlling decay on the fruit during 14 days of storage. Although the quality parameters of respiration, ethylene production, firmness, soluble solids, titratable acidity and external color were only mildly affected by the superatmospheric O2 treatments, volatile content (acetaldehyde, ethanol and ethyl acetate) increased greatly. While the fruit treated with 15 kPa CO2 had the highest volatile concentrations after 14 days at 5°C, after an additional 2 days in air at 20°C, volatile concentrations in fruit treated with 100 kPa O2 equaled or surpassed those of fruit treated with CO2. The volatile concentration in fruit treated with 15 kPa CO2 generally decreased during 2 days in air, while the volatile content increased in fruit treated with high O2, with greater increases with increasing O2 level. The fruit treated with 40 kPa O2+15 kPa CO2 achieved an intermediate level between the O2 and CO2 treatments. Although the 100 kPa O2 treatment reduced decay, both in vitro and in vivo, increased production of fermentative metabolites that impart a negative organoleptic property to the fruit makes this a doubtful alternative for decay control on strawberry.

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.

Postharvest physiology and handling of litchi (Litchi chinensis Sonn.)

Abstract The litchi is a stone fruit with a rough indehiscent pericarp surrounding the succulent, edible aril. During development and ripening the fruit exhibits a sigmoidal growth curve and a nonclimacteric respiratory pattern. Maturity is indicated by size and color, but a more accurate measure is the sugar:acid ratio. The major factors reducing the storage life and marketability of litchi are pathological decay and pericarp browning. Low temperature storage has been used to reduce pathological decay but has a limited role in reducing pericarp browning. Pericarp browning is induced by desiccation, and consequently methods to reduce desiccation help retain red color. Sulfur dioxide fumigation has been used to reduce pericarp browning, but there is increasing consumer and regulatory resistance to the use of this chemical. Insect disinfestation has become increasingly important with the expanding export market. Recent research has focussed on reducing these major postharvest problems to produce highly colored fruit free of disease and insects.

Effect of combinations of hot water dips, biological control and controlled atmospheres for control of gray mold on harvested strawberries

The efficacy of hot water, biological control and controlled atmospheres (CA), alone and in combinations, in controlling gray mold on harvested strawberry fruit was tested. All fruit were wound inoculated with Botrytis cinerea Pers.:Fr. Inoculated fruit were subsequently dipped in hot water at 63 8C for 12 s, inoculated with a biological control yeast, Pichia guilliermondii Wickerham, and/or immediately stored at 5 8C under air or 15 kPa CO2 for 5 and 14 days followed by 2 days at 20 8C to simulate market conditions. Fruit treated with the combination of heat, biocontrol, and CA had significantly less decay than those in all of the other treatments after 5 days at 5 8C plus 2 days at 20 8C. After 14 days at 5 8C and 14 days at 5 8C plus 2 days at 20 8C, the heat /biocontrol /CA treatment continued to control decay though not significantly more than CA alone, biocontrol /CA, or heat /CA treatments. Some damage occurred following heat treatment; however, quality parameters did not differ between treatments. Overall, the combination treatments did not provide better control than the current commercially used treatment of 15 kPa CO2. # 2002 Elsevier Science B.V. All rights reserved.

Postharvest heat treatments : introduction and workshop summary

Wide international interest in heat treatments for maintenance of postharvest quality, disease control, and as a quarantine technology, was reflected in the range of papers presented at this BARD Workshop on Postharvest Heat Treatments. Heat treatments are currently used commercially in several countries, for example as hot water dips, hot water brushing techniques, and hot air treatments. Research is continuing on these methods, on new techniques, and on the responses to high temperature treatments of fruit and vegetables, fungal pathogens, and insects. It was therefore timely that a workshop devoted to heat treatments should be held, and appropriate that it should be held in Israel, where much of the recent research has been conducted. This Special Issue of Posthar6est Biology & Technology comprises both review and research articles, based on papers given at the Workshop. Since substantial time was set aside for discussions, we have assembled this summary paper, covering some of the highlights, some of the gaps, and some speculation regarding the future.

Detection and monitoring of internal browning development in ‘Fuji’ apples using MRI

Abstract Magnetic resonance imaging (MRI) experiments were used to detect and monitor the progression of internal browning (IB) in Fuji apples ( Malus domestica Borkh. ). The apples were stored in two different controlled atmosphere (CA) conditions: 3% CO 2 at 0°C and 18% CO 2 at 20°C. The apples stored under 18% CO 2 , were imaged at 1, 3, 5, 7 and 9 days. Apples stored under 3% CO 2 were imaged once a week for the first 7 weeks and then every 2 weeks until the 12th week. Three distinct regions were identified in the MR images of apples with internal browning: normal, light brown and dark brown tissue. Light brown regions, more prominent in apples stored in 3% CO 2 at 0°C, had a lower signal intensity than normal tissue because of lower proton density (PD) and smaller transverse relaxation times ( T 2 ). Dark brown regions predominated in apples stored in 18% CO 2 at 20°C and displayed higher signal intensity than the normal apple tissue because of larger T 2 values. The speed of image acquisition was improved from 5.5 min to 20 s, by reducing the spatial resolution of the images. Both types of IB were detected by images acquired using coarse spatial resolution and correlated well to the IB detected by the images acquired using fine spatial resolution.

TEMPERATURE-DEPENDENT DIELECTRIC PROPERTIES OF SELECTED SUBTROPICAL AND TROPICAL FRUITS AND ASSOCIATED INSECT PESTS

Knowledge of the dielectric properties of commodities and insect pests is important in developing thermal treatments for postharvest insect control based on radio frequency (RF) and microwave energy. The dielectric properties of six subtropical and tropical fruits 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°C and 60°C. The dielectric loss factor of the fruits and the insects decreased with increasing frequency at constant temperatures. Especially over 10 to 300 MHz, the log of the dielectric loss factor decreased linearly with the log of increasing frequency. The loss factor of the fruits and the insects increased almost linearly with increasing temperature at 27.12 MHz radio frequency, but remained in a small range at 915 MHz microwave frequency. Both the dielectric constant and the loss factor were the highest in avocado fruit. The dielectric loss factors of insects were generally larger than that of the host fruit at all tested frequencies. But those values were within the same order of magnitude, suggesting that differential heating of insects in fresh fruits was unlikely to occur in RF and microwave systems. Direct measurement of electrical conductivity of fruit pulps can also provide a good estimation of the dielectric loss factor at 27.12 MHz. Matching the electrical conductivity of saline solution can be an effective and simple means to improve heating uniformity of the fruit when subjected to RF systems.

Effects of ethylene and 1-MCP on the quality and storage life of strawberries

Strawberry quality declines rapidly after harvest. Deterioration may be accelerated by ethylene and is potentially increased, decreased or unaffected by the ethylene inhibitor 1-MCP (1-methylcyclopropene). We have examined the effects of 0.01, 0.05, 0.1 and 1 m ll 1 of ethylene and 0.01, 0.1, and 1.0 m ll 1 1-MCP on the quality attributes and respiration rates of strawberries stored at 0 or 5 8C. Ethylene did not affect the rate of rot development. However, calyx quality was significantly reduced by exposure to 0.1 or 1.0 m ll 1 ethylene. Treatment with 1 m ll 1 1-MCP protected the calyx tissue from these effects. Exposure of strawberries to 0.01, 0.1 or 1.0 m ll 1 1-MCP did not affect overall fruit acceptability but did slightly increase the rate of rot development. 1-MCP treatment reduced ethylene production by the fruit. Increased production of CO2 by 1-MCP treated fruit was associated with the earlier onset of rots. Although the results suggest that blocking ethylene perception interferes with disease resistance in strawberries, there was only a small effect on total storage life. It was concluded that neither the removal of low levels of ethylene from the storage environment nor the treatment with 1-MCP are likely to be cost effective methods of extending strawberry storage life. It was concluded that neither removing low levels of ethylene from the storage environment nor treating with 1-MCP are likely to be cost-effective methods of extending storage life. # 2002 Elsevier Science B.V. All rights reserved.