APPLICATION OF RADIO FREQUENCY TREATMENTS TO CONTOL INSECTS IN IN-SHELL WALNUTS

Authors: VELTMAN, R - UNIVERSITY OF CALIFORNIA; FENG, X - UNIVERSITY OF CALIFORNIA; DE CASTRO, E - UNIVERSITY OF CALIFORNIA; Johnson, Judy; SIMPSON, T - UNIVERSITY OF CALIFORNIA; BIASI, W - UNIVERSITY OF CALIFORNIA; TANG, J - WASHINGTON UNIVERSITY; MITCHAM, E - UNIVERSITY OF CALIFORNIA

Submitted to: Postharvest Biology and Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/12/2004
Publication Date: 6/1/2004
Citation: Veltman, R.H., Feng, X., De Castro, E., Johnson, J.A., Simpson, T.L., Biasi, W.V., Tang, J., Mitcham, E.J. 2004. Application of radio frequency treatments to contol insects in in-shell walnuts. Postharvest Biology and Technology. 33(1):93-100.

Interpretive Summary: Methyl bromide fumigation is the current treatment applied to most in-shell walnuts to meet quarantine and phytosanitary requirements before shipment to domestic and international markets. Because increased restrictions have increased the cost of the fumigant, and will reduce its availability in the future, there is an interest in developing a non-chemical alternative. One suggested alternative is radio frequency heating, consisting of direct interactions between dielectric materials, such as fruits and nuts, with electromagnetic waves of 10 to 100 MHz. Understanding the factors that control the heating uniformity in walnuts treated with radio frequency is critical to translating laboratory experiments to commercial applications. Our tests have shown that heating walnuts with radio frequency energy to temperatures lethal to insect pests have no negative effect on walnut quality, and may even reduce the susceptibility of walnuts to rancidity in storage. Radio frequency treatments provided acceptable heating rates (5-6 degrees C /minute) of in-shell walnuts to insecticidal temperatures. Heating walnuts to 55 degrees C or higher resulted in 100 percent mortality of navel orangeworm larvae, which is the most heat tolerant pest under consideration. The relationship between average moisture content and average heating rate was linear, and the variability in moisture content and heating rate was higher at higher average moisture contents. Radio frequency treatments decrease the moisture content of walnuts. A combined system of radio frequency heating with hot air has the potential to accelerate or even replace batch drying of walnuts in the future.

Technical Abstract: Codling moth (Cydia pomonella [L.]), navel orangeworm (Amyelois transitella [Walker]), and Indianmeal moth (Plodia interpunctella [Hübner]) are common insect pests in walnuts (Juglans regia [L.]). Currently, exported in-shell walnuts are disinfested using methyl bromide fumigation. Restrictions on methyl bromide use have increased interest in developing alternative postharvest treatments. Radio frequency heating is such an alternative. Our tests have shown that heating walnuts with radio frequency energy to temperatures lethal to these important insect pests have no negative effect on walnut quality, and may even reduce the susceptibility of walnuts to becoming rancid in storage. Radio frequency treatments provided acceptable rates of heating (5-6 degrees C/minute) for in-shell walnuts to lethal temperatures. Heating walnuts to 55 degrees C or higher resulted in 100 percent mortality of fifth instar navel orangeworm, which is the most resistant of the three insect pests. Moisture content of walnuts is an important factor affecting heating rates during radio frequency treatments. The relationship between average moisture content and average heating rate was linear, and the variability in moisture content and heating rates was higher at higher average moisture contents. Radio frequency treatments decrease the moisture content of walnuts. A combined system of radio frequency heating with hot air has the potential to accelerate or even replace batch drying of walnuts in the future.