POSTHARVEST CONTROL OF INSECT PESTS IN NUTS AND FRUITS BASED ON RADIO FREQUENCY ENERGY

Postharvest phytosanitation is essential for international and domestic commerce of tree fruits and nuts in the USA. Current methods used in the industry rely, however, on chemical fumigants that are either harmful to the environment or to human health. The multi-billion dollar US tree fruit and nut industries are facing a major challenge in meeting more stringent regulatory restrictions and in addressing ever-increasing public concern over health and environment. Developing thermal treatment protocols was our major focus based on radio frequency (RF) energy or in combination with conventional thermal methods such as water or air heating. To achieve a delicate balance between minimized thermal impact on product quality and complete kill of insect pests, information on thermal death kinetics of insects and commodity quality degradation kinetics is needed. In this paper, the general research strategy, principle of RF heating, and some main findings for post-harvest insect pest control in nuts are discussed. INTRODUCTION Many types of fresh commodities serve as hosts for insects that are categorized as quarantine pests because of their threat to local agriculture. These insects can be found on the surface or in the interior of harvested produce. Because infested produce is often not easily detectable by external inspection, regulatory agencies in many countries have established phytosanitary quarantine protocols intended to prevent the introduction of exotic pests. Quarantine protocols may include pre-harvest techniques, such as sterile fly release, non-host status, and pest-free growing periods, as well as postharvest measures, such as commodity treatments. Currently, the dried fruit and tree nut industries rely heavily on fumigation with methyl bromide (MeBr) and phosphine (hydrogen phosphide) for postharvest insect control. However, the Montreal Protocol (UNEP, 1992) has identified MeBr as an ozone depleter, which has resulted in a gradual annual reduction of most uses of MeBr and will result in its eventual elimination. Insect resistance to hydrogen phosphide has been documented in other commodities (Zettler et al., 1989), and the United States Environmental Protection Agency (USEPA) is considering increased restrictions on the use of hydrogen phosphide (USEPA, 1998). Although non-chemical treatments for postharvest dried fruits and nuts have been investigated in the past to some extent, little progress has been made and concerns over resistance and regulatory action have Proc. Postharvest Unltd Eds. B.E. Verlinden et al. Acta Hort. 599, ISHS 2003 176 generated a renewed interest in developing alternative treatments. Several alternative methods have been suggested, including ionizing radiation, cold storage, controlled atmospheres and combination treatments (Wang and Tang, 2001). All would require substantial capital investment and alteration of existing facilities. Cold storage and controlled atmospheres also require long treatment times for disinfestation, and there is concern over public acceptance of irradiated food. Heat treatments have been proposed to kill codling moth in different commodities using hot forced air or hot water dips (Yokoyama et al., 1991), but the lengthy exposure times needed may cause injury to the product (Lurie, 1998). Industrial radio frequency (RF) and microwave systems that are extensively used in the food processing, textile and wood processing industries may provide more rapid product heating (10-20°C/min) and have been suggested for control of postharvest insects (Tang et al., 2000; Wang et al., 2001). Knowledge of thermal death kinetics for targeted insects is essential in developing those thermal treatments. The objective of this study was to develop postharvest treatments using RF energy to control common insect pests in in-shell walnuts and cherries based on the thermal death kinetics studies. A pilot-scale 27 MHz RF system was used to study process parameters leading to a complete kill of those insect pests. The effects of selected thermal treatments and storage conditions on common quality were also examined. MATERIALS AND METHODS Strategy for Developing New Heat Treatment Method An important key to the development of successful thermal treatments is to identify a delicate balance between minimized thermal impact on product quality and complete killing of insects. It is possible to describe a 100% mortality curve and a safe quality curve as a function of temperatures for different agricultural products (Fig. 1). Because the activation energy for commodity quality changes in thermal treatments is generally smaller than that of insect mortality, the slope of this quality curve should be less than that of the insect mortality curve. The exact location and the slope of the quality curve depend on each commodity. The overlap between the lower region of the quality curve and upper region of the insect mortality curve defines the potential operation conditions for developing thermal quarantine treatments (Tang et al., 2000). A clear knowledge of the thermal death kinetics of insects is essential. Once the information for the thermal death kinetics of targeted insects becomes available, new treatment protocols can then be developed to deliver the desired lethal energy to the insects in a manner based on engineering principles that control the insects without damaging product quality. Description of the Heating Block System A heating block system was developed at Washington State University (WSU) and has been used to determine the thermal death kinetics of codling moth, Indianmeal moth and navel orangeworm (Johnson et al., 2002; Wang et al., 2002a;b). The system consists of top and bottom metal blocks, heating pads, an insect test chamber, controlled atmosphere circulating channels, and a data acquisition/control unit (Fig. 2). The heating blocks are made of aluminum alloys with low thermal capacitance and high thermal conductivity. The heating block system can treat up to 200 insects at a time at controlled heating rates between 0.1 and 20 °C/min. Calibrated type-T thermocouples inserted through sensor paths are used to monitor the temperatures of the top and bottom plates, and the air temperature in the chamber. The heating rate and the end-point temperature are controlled by the visual software WorkBench PC 2.0 (Strawberry Tree Inc., Sunnyvale, CA) via a solid state relay. The thermal capacitance of the blocks provides smooth temperature profiles over the heating period and precise control (±0.2°C) during the holding periods (Wang et al., 2002b). Principle and Treatment System of RF Heating The dielectric property data for the targeted insects, fruits and nuts provide