Radio frequency treatments for insect disinfestation of dried legumes

Dried legumes (chickpeas, green peas or lentils) are valuable export commodities in the US Pacific Northwest. A major problem in the marketing of these products is infestation by insect pests. Typically, chemical fumigants are used to disinfest product, but regulatory issues, insect resistance, environmental concerns and the increase of the organic market have forced the industry to explore non-chemical alternatives. One possible alternative is the use of radio frequency (RF) energy to rapidly heat product to insecticidal levels. To determine the potential of RF treatments to control insect pests in dried pulse products, the heat tolerance of the cowpea weevil (Callosobruchus maculatus F.) was evaluated and compared to the tolerance of previously studied insects, and the dielectric properties of both the insect and the products were compared. The most heat tolerant stage of the weevil was found to be the pupal stage, with adults being the most susceptible. Cowpea weevil pupae were fairly heat tolerant; to obtain rapid mortality (exposure < 10 min) temperatures of 56-58°C were needed. At frequencies commonly used by industry for RF heating, dielectric loss factors for both adult and larval cowpea weevil was higher than those for legumes, suggesting that cowpea weevils would heat at a faster rate than the product. Previous studies showed that suitable heating uniformity during RF treatments was obtained through the addition of hot air (60°C) and conveyor belt movement. These studies showed that chickpeas, green peas and lentils were able to tolerate RF treatments of 60°C for 10 min without adverse effects on quality. The results suggest that practical large scale RF treatments to disinfest pulses may be possible.Keywords: Heat treatments, Dried pulses, Cowpea weevil, Radio frequency, Disinfestation