Industrial-scale radio frequency treatments for insect control in lentils

Authors: JIAO, S - Washington State University; Johnson, Judy; TANG, J - Washington State University; WANG, S - Washington State University

Submitted to: Journal of Stored Products Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/5/2011
Publication Date: 1/30/2012
Citation: Jiao, S., Johnson, J.A., Tang, J., Wang, S. 2012. Industrial-scale radio frequency treatments for insect control in lentils. Journal of Stored Products Research. 48:143-148.

Interpretive Summary: Disinfestation treatments for U.S. grown lentils targeting insect pests are necessary to meet postharvest phytosanitary regulations before export to international markets. Currently, fumigants are the most effective disinfestation method, but regulatory, environmental, and insect resistance issues have created interest in non-chemical alternatives. One such alternative is rapid heating of the product using a combination of radio frequency (RF) and hot air, which has been shown to disinfest lentils of internal seed pests such as the cowpea weevil without significant quality degradation. A customized, laboratory-scale RF unit with a hot air blower was used to determine if the temperature uniformity and product throughput needed for commercial uses can be achieved. The treatment protocol selected was designed to provide complete kill of cowpea weevil, combining RF with forced hot air to heat product to 60°C for 10 minutes, followed by forced ambient air cooling for 20 minutes. The electrode gap and conveyor belt speed were selected based on the electric current and heating time in order to optimize throughput. Heating uniformity was evaluated by measuring both surface and internal product temperatures after treatment. Changes in moisture content, color and germination were used to evaluate treatment effects on product quality. Finally, the RF system heating efficiency and throughput were calculated. Heating uniformity and quality of lentils in continuous RF treatment with hot air and movement were acceptable, while the average heating efficiency of the RF system (76.5%) and throughput (208.7 kg/h) was sufficient for commercial purposes. This information will be used to develop an industrial-scale RF process as an alternative to chemical fumigation, allowing processors to keep international markets while avoiding the use of environmentally damaging fumigants.

Technical Abstract: Radio frequency (RF) treatments are considered to be a potential postharvest technology for disinfesting legumes of internal seed pests such as the cowpea weevil. After treatment protocols are shown to control postharvest insects without significant quality degradation, it is important to scale-up laboratory RF treatments to industrial level applications. A 27.12 MHz, 6 kW radio frequency (RF) unit with a built-in forced hot air system was used to conduct industrial scale-up studies. A treatment protocol designed to provide 100% cowpea weevil mortality combined RF with forced hot air to heat product to 60°C for 10 min, followed by forced ambient air cooling for 20 min. An electrode gap (14 cm) was chosen based on the electric current and heating time, and conveyor belt speed was set to 7.5 m/h. Heating uniformity was evaluated by measuring post-treatment surface temperatures with a thermal image camera and interior temperatures with thermocouples. Changes in moisture content, color and germination were used to evaluate treatment effects on product quality. Finally, the RF system heating efficiency and throughput were calculated. Results showed that heating uniformity and quality of lentils in continuous RF treatment with hot air and movement were acceptable, the average heating efficiency of the RF system was 76.5% and throughput was 208.7 kg/h. The average energy efficiency and throughput of the RF system provided sufficient data to develop an industrial-scale RF process as an alternative to chemical fumigation.