Developing Hot Air-Assisted Radio Frequency Drying for In-shell Macadamia Nuts

Abstract

Dehydration reduces water activity and extends shelf life of perishable agricultural products. The purpose of this research was to study the application of radio frequency (RF) energy in dehydration of in-shell Macadamia nuts and shorten the lengthy process times needed in conventional hot air drying operations. A pilot scale 27.12-MHz and 6- kW RF system was used to determine the operational parameters, the drying curve, and the quality attributes of the processed nuts. The results showed that an electrode gap of 15.5 cm and a hot air temperature of 50 °C provided an acceptable heating rate and stable sample temperatures, and were used for further drying tests. The drying curves showed an exponential decay and required 750 and 360 min to achieve the final moisture content of 0.030 kg water/kg dry solid (3.0 % dry basis) in whole nuts in hot air drying and RF heating/hot air combined drying, respective- ly. The drying kinetics of the nuts were described well by the Page model for hot air drying, but a logarithmic model was more suited for RF/hot air drying. Peroxide value and free fatty acid increased with the drying time both for hot air and RF drying but remained within acceptable range re- quired by the nut industry. The RF process shows potential to provide rapid, uniform, and quality-acceptable drying technology for the nut industry.