Radio Frequency Heating Patterns as influenced by Dielectric Properties of Constituent Parts of Fruit

Abstract

Current quarantine method, fumigation with methyl bromide, for fresh fruit is under international pressure to phase out because methyl bromide damages the ozone layer. To develop alternative methods, many researchers have explored use of non-chemical treatment methods. But all these methods have their own limitations. For example, conventional hot water treatment takes long time which results in undesirable quality changes in treated fruit. Radio frequency (RF) heating has been looked upon as a means to shorten thermal treatment time. In this study, RF heating patterns in orange, apple, grapefruit, peach, and avocado were investigated. The selected fruits were placed in a fruit mover and RF heated in a 12 kW parallel plate RF unit operating at 27.12 MHz. Dielectric properties of each fruit constitutional part were measured by open-ended coaxial cable probe method. The study showed that peel dielectric properties along with peel thickness greatly influenced the RF heating behavior of an orange and a grapefruit. Core heating was prominent in apple, peeled orange and grapefruit; whereas in whole oranges the subsurface was at higher temperature than the core. The study helped in better understanding of the complex RF heating characteristics of fruits for designing practical product specific treatment protocol.