Improving heating uniformity of fresh fruit in radio frequency treatments for pest control

Abstract

Non-uniform heating of fresh fruit caused by variations in radio frequency (RF) fields is a major obstacle in developing postharvest insect control treatments based on RF energy. A fruit mover was developed to evaluate possibilities to improve RF heating uniformity of large fruit, such as oranges and apples, in a 12 kW batch type RF system. This fruit mover provided a means to rotate and move fruit in water when subjected to RF heating. Parameters for moving and rotating fruit in the mover were selected based on consideration of vortex formation, foaming, damage to the fruit, and volume occupied by the fruit in water. RF heating uniformity of oranges and apples in the mover was assessed using an infrared imaging technique and direct temperature measurement with fiber-optic sensors and thermocouples. The results showed that, with rotation and movement of fruit, temperature uniformity in oranges and apples was significantly improved with less than 2.8 and 3.1• C standard deviations, respectively, after an average temperature rise of about 30• C in 7.8 min. The fruit mover can be used to develop a treatment protocol for disinfecting fresh fruit and to simulate industrial scale and continuous treatment systems.

Computer simulation of radio frequency heating of model fruit immersed in water

“…The core temperature of centrally placed fruit was recorded and logged during the RF heating experiment using a fiber optic sensor (UMI�8, FISO Technology, Quebec, Can� ada). In additional experiments, five model fruits were RF heated in a fruit mover developed by Birla et al (2004) to study the effect of rotation and movement of fruits during RF heating. The fruit mover had twelve numbers of water spray nozzles mounted on the periph� ery of a square container to rotate fruits on their axes and in a circular path.…”

“…The fruit mover had twelve numbers of water spray nozzles mounted on the periph� ery of a square container to rotate fruits on their axes and in a circular path. Details of the fruit mover can be found in our earlier publication (Birla et al, 2004). The RF heated model fruit was immediately bisected ver� tically and a thermal image was recorded with the (Stogryn, 1971) v (von Hippel, 1995, p. 327) infrared camera for one of the cut surfaces within 10 s to avoid the surface cooling.…”

“…It has been demonstrated experimentally that rota� tion and movement of fruit during RF heating improves the heating uniformity (Birla et al, 2004). However, sim� ulation of RF heating of rotating and moving object is very difficult because of the associated moving bound� ary conditions.…”

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

“…When fresh fruits are heated by RF energy in air, fruits may suffer thermal damage (burn) at the points of contact with the container or with other fruit because the contact points form the route of least resistance for RF energy to pass (Ikediala et al, 2002;Birla et al, 2004). A water immersion technique was used to avoid non-uniform RF heating in fresh fruits (Ikediala et al, 2002;Birla et al, 2004).…”

Continuous combined microwave and hot air treatment of apples for fruit fly (Bactrocera tryoni and B. jarvisi) disinfestation

“…Studies conducted by Gazit, Rossler, Wang, Tang, and Lurie (2004) recorded the z values for Mediterranean fruit fly (Diptera: Tephritidae; Ceratitis capitata, Weidemann) eggs and 3rd instar larvae of 4.1 and 3.6°C, respectively. Given the similarity between the Mediterranean fruit fly and the B. tryoni (Birla et al, 2004;Ekman & Pristijono, 2010) a z value of 4°C was selected to represent both eggs and larvae for this study.…”