Temperature-dependent Dielectric Properties of Selected Subtropical and Tropical Fruits and Associated Insect Pests

Knowledge of the dielectric properties of commodities and insect pests is important in developing thermal treatments for postharvest insect control based on radio frequency (RF) and microwave energy. The dielectric properties of six subtropical and tropical fruits along with four associated insect pests were measured between 1 and 1800 MHz using an open-ended coaxial-line probe technique and at temperatures between 20°C and 60°C. The dielectric loss factor of the fruits and the insects decreased with increasing frequency at constant temperatures. Especially over 10 to 300 MHz, the log of the dielectric loss factor decreased linearly with the log of increasing frequency. The loss factor of the fruits and the insects increased almost linearly with increasing temperature at 27.12 MHz radio frequency, but remained in a small range at 915 MHz microwave frequency. Both the dielectric constant and the loss factor were the highest in avocado fruit. The dielectric loss factors of insects were generally larger than that of the host fruit at all tested frequencies. But those values were within the same order of magnitude, suggesting that differential heating of insects in fresh fruits was unlikely to occur in RF and microwave systems. Direct measurement of electrical conductivity of fruit pulps can also provide a good estimation of the dielectric loss factor at 27.12 MHz. Matching the electrical conductivity of saline solution can be an effective and simple means to improve heating uniformity of the fruit when subjected to RF systems.