Radio Frequency Heating of Walnuts and Sweet Cherries to Control Insects after Harvest

Radio frequency (RF) heating has been explored as a potential non-chemical method to control insects in harvested walnuts and as a quarantine treatment for ‘Bing’ sweet cherries. Walnuts were heated to 50 to 90°C. Heating walnuts to 55°C or higher resulted in 100% mortality of fifth instar navel orangeworm, and heating to 80°C had no effect on walnut quality. Moisture content had a significant influence on the heating rate of the walnut kernels. For industrial applications, walnuts could move on a conveyor through one or more RF systems with mixing of nuts between systems. ‘Bing’ sweet cherries were heated in a polyethylene container holding 10 liters of circulating distilled water with 2.3 g of NaCl. Fresh fruit must be treated in a saline solution to prevent burning at fruit contact points, and circulation improves heating uniformity within the RF field. Cherries were equilibrated in 38°C water for 6 minutes, then heated with RF energy to target temperatures between 50 and 54.5°C and held for 0.5 to 6 min before hydrocooling. Fruit were stored for 1 day at 5°C or 14 days at 0°C to simulate air or sea shipment, respectively. Shorter treatments at higher temperatures were better tolerated than longer treatments at lower temperatures. Cherry fruit infested with codling moth larvae were subjected to the same treatments. Mortality was 100% in all treatments except those at 50°C. However, fruit quality was unacceptable following sea shipment and marginal following air shipment. Treatment times would be significantly longer to provide for Probit 9 security (99.9968% mortality) required for export to Japan and therefore RF treatments do not appear promising for sweet cherry fruit. INTRODUCTION Methyl bromide (MeBr) fumigation is the current treatment applied to most inshell walnuts to meet quarantine and phytosanitary requirements before shipment to domestic and international markets. The three most economically significant pests in walnuts are codling moth (Cydia pomonella [L.]), navel orangeworm (Amyelois transitella [Walker]), and Indianmeal moth (Plodia interpunctella [Hübner]). Methyl bromide fumigation is currently required for export of California sweet cherry fruit to Japan to control potential codling moth infestations. Under the Montreal Protocol of the United Nations, MeBr will be banned from use for purposes other than pre-shipment and quarantine by 2005 (Anonymous, 2001). In addition, increased MeBr restrictions have increased the cost of the fumigant three-fold, and may reduce its availability in the future. For these reasons there is interest to develop alternative methods. Industrial radio frequency (RF) heating has been successfully used in the food processing and textile industries. Direct interactions between dielectric materials, such as fruits and nuts, with electromagnetic waves generate heat throughout the product avoiding 2133 Proc. 5 Int. Postharvest Symp. Eds. F. Mencarelli and P. Tonutti Acta Hort. 682, ISHS 2005 limitations experienced with conventional surface heating with air or water due to airspaces or product bulkiness. Because of their dielectric properties, RF may also heat insects faster than some fresh fruit and especially nuts (Wang et al., 2003). RF is classified as “non-ionizing” radiation because these frequencies produce insufficient energy to ionize water molecules. In developing a postharvest treatment, tests must be done on the most tolerant life stage and species of the target insects. To determine the most tolerant species and life stage, insect mortality data must be developed over a range of temperatures. Washington State University has developed a thermal block heating system to heat insects at various rates comparable to those achieved with RF heating and to a range of different temperatures (Wang et al., 2002). The experiments done by Wang et al. led to a “thermal death curve” for the different life stages of codling moth, navel orangeworm, and Indianmeal moth. These data were used to establish the optimal temperatures and exposure times for insect control with RF. Hot water heating has been tested as a potential quarantine treatment for sweet cherries. Fruit heated to 122°F in approximately 5 minutes. However, there was only a narrow window where fruit tolerance and codling moth mortality were both acceptable (Feng et al., 2004). Radiofrequency (RF) heating allows cherry fruit to be heated to insecticidal temperatures in as little as 2 minutes as compared with 5 minutes or longer by hot water heating (Ikediala et al., 2002). Radio frequency treatments are shorter, the heating rate is stable and faster, and fruit heat fairly evenly from inside to outside resulting in a lower total heat exposure on the fruit surface where most damage occurs. Quarantine treatments against insects commonly require Probit 9 mortality (99.9986%). Treatments to control pests in storage or pre-shipment require a high level of mortality, but not Probit 9 efficacy. It is important to examine the effects on product quality of treatments (times and temperatures) that control the target pests. MATERIALS AND METHODS Walnuts Walnuts (Juglans regia L. ‘Hartley’) were obtained from a local processing plant and stored at 0°C in 23 kg raffia bags until use. About 2.5 kg of nuts (ca. 500 nuts) were heated in a cylindrical polyethylene sample container with a diameter of 33.5 cm and a height of 20 cm until the coldest of 8 walnuts of which the kernel temperature was monitored reached target temperatures of 47 to 55°C. In every experiment, the target temperature was the minimum temperature. The container was filled completely in all experiments. A 27 MHz, 12 kW batch RF machine (Strayfield Inter. Ltd., Workingham UK) was used for these experiments. The standard settings of the RF unit were: a minimum gap of 20.5 cm between the upper and the lower electrode and a voltage of 12 kW. The Ainitial is the heating power (amps) just after beginning RF heating. During RF heating, the kernel temperatures of eight nuts were monitored using fiber-optic probes (Fiso Technologies Inc., Quebec, Canada). Four probes were randomly placed in walnuts located in the lower part of the container; the remaining probes were placed in walnuts in the top layer. The moisture content of in-shell walnuts was adjusted by immersing them in tap water for approximately 8 hours. After immersion, the nuts were dried in a room at 25°C or 35°C. Nuts were heated in the RF unit on day 1, 2, 4, 7 and 16 during drying and moisture content was determined. To determine the upper limit of walnut tolerance to heating, walnuts were heated with RF to 50, 60, 70, 80 or 90°C. Accelerated storage life tests were conducted in which walnuts were stored for 20 days at 35°C to simulate storage at 4°C for 2 years (Taoukis et al., 1997), or samples were analyzed directly after RF treatment without storage. Samples were stored at 0°C until oil analyses. Oil was pressed from the walnuts at room temperature using a Carver Laboratory Press; model K (Fred S. Carver Inc., Summit, NJ, USA), and milliequivalents of peroxide (PV; AOCS, 1998a) and percent free fatty acid