Radio frequency selective heating of stored-grain insects at 27.12 MHz: A feasibility study

Abstract

The feasibility of radio frequency (RF) selective heating as a disinfestation technique was investigated, considering rusty grain beetle in bulk wheat samples at 27.12 MHz. Although pesticides and fumigants are the most commonly used methods for controlling insects in postharvest products such as grains, nuts, and fruits, they pose a direct threat to human health and the environment. In this work, power dissipation factors and the rate of increase of temperature in the insect bodies and wheat kernels were considered to assess the effectiveness of the proposed method. Dielectric and other physical properties of the insect and wheat were measured, and the electric field intensities in the system were estimated using an electromagnetic model. The host grain at three levels of moisture contents, 12%, 15%, 18% wet weight basis, were included to cover a wide range of grain storage scenarios, and the dielectric properties were measured at temperatures between 15 °C and 75 °C. The ratios of electric field intensity (Ei2/Eg2) and dielectric loss factor (εi″/εg″) within the insect bodies to that in grain kernels ranged from 0.13 to 0.76 and from 8.61 to 169.93 respectively. The latter had a significant effect on insect-to-wheat power absorption factor, Pig, which varied between 5 and 40. The relative rate of increase of temperature for insect-to-wheat was directly related to Pig, but inversely related to the product of insect/grain specific heat and density, which varied between 0.89 and 1.33, and was therefore almost identical to Pig, and ranged from 6.47 to 29.6. This large relative rate of increase of temperature in insect bodies compared to wheat may have potential benefits in heating the insects selectively with radio frequency energy to their lethal temperature while keeping the host grain at moderate temperature, and therefore minimising or eliminating adverse effects on its physicochemical properties.