Industrial-scale Radio Frequency Treatments for Insect Control in In-shell Walnuts

Abstract

Prior to the 2005 ban, in-shell walnuts were routinely fumigated with methyl bromide for insect control before export. Several pilot scale radio frequency (RF) studies have been reported as a new technology to completely control the most heat resistant insect, navel orangeworms, in in-shell walnuts without significant quality degradation. It is important to transfer pilot-scale or laboratory research results to large-scale industrial implementations. Heating uniformity is one of the most important considerations in scaling up of the established treatment protocol for walnuts. Temperature variations after RF heating may result from the different properties of walnuts and non-uniform electromagnetic field. It is desirable to determine the heating uniformity for a given RF unit and minimize the effect of walnut orientations and positions by stirring the walnuts during RF treatment. The required heating uniformity was achieved with one intermittent mixing at the electrode gap of 280 mm under the hot air temperature around 60°C for a continuous RF process. The average kernel and surface temperatures were around 55°C and 60°C after RF treatments, respectively. The efficacy treatments were conducted using a 25 kW, 27 MHz RF system at a conveyor belt speed of 57m/h. The results based on three replicates showed that 100% mortality of fifth-instar navel orangeworms was achieved with an acceptable walnut quality according to the industrial standard of peroxide, fatty acid and colour values. If this method could be economically integrated into the handling process, it would be an effective alternative to methyl bromide fumigation for disinfesting walnuts.