Pest control by microwave and radio frequency energy: dielectric properties of stone fruit

Abstract

Current pesticides such as methyl bromide are progressively removed from the market due to harmful residues in food. The stone fruit industries are thus seeking alternatives for postharvest control of insect pests. Microwave and radio frequency methods hold potential for postharvest thermal disinfestations of stone fruits to replace chemical fumigation. Knowledge of dielectric properties is essential for understanding the interaction between the electromagnetic fields and the target stone fruits and designing treatment beds in industrial applications. Here, we determined the dielectric properties of nectarine, peach, and plum between 10 and 1,800 MHz over a temperature range of 20–60 °C using an impedance analyzer. Our results show that the dielectric constant generally varied between 60 and 75, accounting for changes of 8–10 % due to temperature effect. But, the loss factor decreased linearly with frequency on the log scale at all temperatures for three stone fruits. The loss factor of Mediterranean fruit fly, nectarine, peach, and plum increased about 106, 108, 110, and 64 %, respectively, when the sample temperature increased from 20 to 60 °C. The penetration depth in all stone fruits decreased with increasing frequency and temperature. The loss factor ratio at 27 MHz of Mediterranean fruit fly to nectarine, peach, and plum was 1.65, 1.66, and 1.87 at 20 °C, respectively, suggesting potential differential heating between insects and host stone fruits in radio frequency treatments.