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United States Department of Agriculture

Agricultural Research Service

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

Authors
item Mitcham, Elizabeth - UC DAVIS
item Tang, Juming - WASHINGTON STATE UNIVERS
item Hansen, James D
item Johnson, Judy
item Monzon, M.E. - UC DAVIS
item Simpson, T - UC DAVIS
item Bikoba, V. - UC DAVIS
item Biasi, W - UNIVERSITY OF CALIFORNIA
item Wang, Shaojin - WASHINGTON STATE UNIVERS
item Feng, X - UNIVERSITY OF CALIFORNIA

Submitted to: Postharvest International Symposium Proceedings
Publication Type: Abstract Only
Publication Acceptance Date: December 19, 2003
Publication Date: June 1, 2004
Citation: Mitcham, E.J., Tang, J., Hansen, J.D., Johnson, J.A., Monzon, M., Simpson, T.A., Bikoba, V., Biasi, W.V., Wang, S., Feng, X. 2004. Radio frequency heating of walnuts and sweet cherries to control insects after harvest. Postharvest International Symposium Proceedings. June 6-11, 2004. Verona, Italy. p. 112.

Interpretive Summary: 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. A 12 kW, 27 MHz pilot scale batch-type RF system was used. Walnuts were heated in a cylindrical polyethylene container until the coldest of 8 walnuts of which the kernel temperature was monitored reached target temperatures of 50 to 90C. Heating walnuts to 55C or higher resulted in 100% mortality of 5th instar navel orangeworm, the most resistant insect pest. Heating walnuts with RF energy to temperatures up to 80C had no effect on walnut quality, and may reduce the susceptibility of walnuts to becoming rancid in storage. Moisture content had a significant influence on the heating rate of the walnut kernels. In addition to the potential to use RF for postharvest insect control, it may also be useful for re-drying of washed, in-shell walnuts. For industrial applications, walnuts could move on a conveyor through one or more RF systems with mixing of nuts between systems. The bed of nuts could be as much as 20 cm deep and throughput would be determined by the size of the RF system. Freshly harvested 'Bing' sweet cherries were heated in a cylindrical polyethylene container with circulating saline (electrical conductivity 450- 470 'S/cm). 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 35C water for 6 minutes, then heated with RF energy to target temperatures between 50 and 54.5C and held for 0.5 to 6 min. before hydrocooling. Fruit were stored in perforated plastic bags for 1 day at 5C or 14 days at 0C to simulate air or sea shipment, respectively. Both storage periods were followed by 15 to 20 hours at 20C for shelf life simulation prior to quality evaluation. Stem browning, berry browning and pitting were increased by heat treatment and were significantly higher in sea shipped fruit as compared with air shipped fruit. Heated cherries were softer after sea shipment. Overall, shorter treatments at higher temperatures were better tolerated than longer treatments at lower temperatures. Cherry fruit infested with third instar codling moth larvae were subjected to the same treatments. Mortality was 100% in all treatments except those at 50C. However, fruit quality was unacceptable following sea shipment and marginal following air shipment. Treatment times would need to be significantly longer to provide for quarantine security required for export to Japan and therefore RF treatments do not appear promising for sweet cherry fruit.

Technical Abstract: 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. A 12 kW, 27 MHz pilot scale batch-type RF system was used. Walnuts were heated in a cylindrical polyethylene container until the coldest of 8 walnuts of which the kernel temperature was monitored reached target temperatures of 50 to 90C. Heating walnuts to 55C or higher resulted in 100% mortality of 5th instar navel orangeworm, the most resistant insect pest. Heating walnuts with RF energy to temperatures up to 80C had no effect on walnut quality, and may reduce the susceptibility of walnuts to becoming rancid in storage. Moisture content had a significant influence on the heating rate of the walnut kernels. In addition to the potential to use RF for postharvest insect control, it may also be useful for re-drying of washed, in-shell walnuts. For industrial applications, walnuts could move on a conveyor through one or more RF systems with mixing of nuts between systems. The bed of nuts could be as much as 20 cm deep and throughput would be determined by the size of the RF system. Freshly harvested 'Bing' sweet cherries were heated in a cylindrical polyethylene container with circulating saline (electrical conductivity 450- 470 'S/cm). 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 35C water for 6 minutes, then heated with RF energy to target temperatures between 50 and 54.5C and held for 0.5 to 6 min. before hydrocooling. Fruit were stored in perforated plastic bags for 1 day at 5C or 14 days at 0C to simulate air or sea shipment, respectively. Both storage periods were followed by 15 to 20 hours at 20C for shelf life simulation prior to quality evaluation. Stem browning, berry browning and pitting were increased by heat treatment and were significantly higher in sea shipped fruit as compared with air shipped fruit. Heated cherries were softer after sea shipment. Overall, shorter treatments at higher temperatures were better tolerated than longer treatments at lower temperatures. Cherry fruit infested with third instar codling moth larvae were subjected to the same treatments. Mortality was 100% in all treatments except those at 50C. However, fruit quality was unacceptable following sea shipment and marginal following air shipment. Treatment times would need to be significantly longer to provide for quarantine security required for export to Japan and therefore RF treatments do not appear promising for sweet cherry fruit.

Last Modified: 8/30/2014
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