TEMPERATURE-DEPENDENT DIELECTRIC PROPERTIES OF SELECTED SUBTROPICAL AND TROPICAL FRUITS AND ASSOCIATED INSECT PESTS

Abstract

Knowledge of the dielectric properties of commodities and insect pests is important in developing thermaltreatments for postharvest insect control based on radio frequency (RF) and microwave energy. The dielectric properties ofsix subtropical and tropical fruits along with four associated insect pests were measured between 1 and 1800 MHz using anopen-ended coaxial-line probe technique and at temperatures between 20C and 60C. The dielectric loss factor of the fruitsand the insects decreased with increasing frequency at constant temperatures. Especially over 10 to 300 MHz, the log of thedielectric loss factor decreased linearly with the log of increasing frequency. The loss factor of the fruits and the insectsincreased almost linearly with increasing temperature at 27.12 MHz radio frequency, but remained in a small range at915 MHz microwave frequency. Both the dielectric constant and the loss factor were the highest in avocado fruit. Thedielectric loss factors of insects were generally larger than that of the host fruit at all tested frequencies. But those valueswere within the same order of magnitude, suggesting that differential heating of insects in fresh fruits was unlikely to occurin RF and microwave systems. Direct measurement of electrical conductivity of fruit pulps can also provide a good estimationof the dielectric loss factor at 27.12 MHz. Matching the electrical conductivity of saline solution can be an effective and simplemeans to improve heating uniformity of the fruit when subjected to RF systems.