Eradication of storage insect pests in maize using microwave energy and the effects of the latter on grain quality

Abstract

To combat insect infestation of maize and maize products during storage without using chemical fumigants, a possible physical treatment method, microwave technology was investigated. Through its selective heating between cereals and insects, microwave technology is a possible physical treatment for eradication of insects and their eggs. Eradication of five insect species, namely Sitophilus zeamais, Rhizopertha dominica, Ephestia cautella, Cryptolestes ferrugineus and Tribolium Confusum was studied. Different microwave parameters such as power dosage, microwave mode, length of microwave cavity, maize exposure method and exposure time were investigated. The effective microwave treatment conditions were then selected, and used to treat maize kernels at laboratory scale. The effect of microwave treatment on the physicochemical properties of maize kernels was investigated. Microwave single exposures did not result in total insect mortality when maize was dropped through the microwave cavity (free falling) as the exposure times were too short. But utilising a pulley system, total insect mortality was achieved in a single exposure of 9 sec. A long microwave cavity (728 mm) resulted in maize kernel damage in terms of swelling, popping and discolouration. Redesigning the cavity by shortening it appeared to reduce these effects. The pulsed microwave mode was found to be better than continuous mode. The selected treatment that eradicated all five insect species with no visible kernel damage was pulsed mode at 2450 MHz frequency, using a 483 mm long microwave cavity, at a power level of 1.5 kW, with an exposure time of 9 sec. The selected conditions (normal treatment) and a more harsh treatment (2 kW power dosage, 18 sec exposure time) were applied to 4 kg samples of white and yellow maize kernels. The normal microwave treatment significantly decreased the moisture content and kernel weight of maize kernels but had no significant effect on test weight, stress cracks, germination and translucency. The harsh microwave treatment also had significant adverse effects on test weight, translucency, germinability, hardness and stress cracks. Additionally, reduced extractability of certain proteins was observed by 2D PAGE with the harsh microwave treatment. Both normal (power dosage of 1.5 kW, for 9 sec exposure time) and harsh (2 kW, 18 sec exposure time) treatment conditions eradicated adult insects and their eggs, but only the former maintains maize quality. The use of microwave technology has potential to be used as an insect control measure of maize products prior to packaging of the products. It is recommended that the effect of pulsed microwave disinfection on nutritional quality (starch and protein digestibility of maize products) be studied. Heat transfer phenomena should be studied and improved if possible to reduce the power usage and possibly shorten the exposure time from 9 sec.