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Developing hot air-assisted radio frequency drying for in-shell walnuts

Bo Zhang, Ajuan Zheng, Liyang Zhou, Zhi Huang, Shaojin Wang.




Abstract

Radio frequency (RF) drying is a rapid dehydration technique that can be applied to perishable agricultural products. The objective of this research was to develop hot air-assisted RF drying protocols for in-shell walnuts. A pilot-scale, 27.12 MHz, 6 kW RF unit was used to study the drying process, including drying curves, drying kinetic models, and quality changes during RF drying and storage. Results showed that an electrode gap of 18.0 cm combined with hot air temperature of 50 °C provided an acceptable heating rate and stable sample temperatures during RF drying. Total drying times to reduce the whole walnut moisture content from 20 to 8.0% on dry basis required 240 and 100 min using hot air drying and RF/hot air combined drying, respectively. The drying curve for RF/hot air heating was fitted to five different kinetic models, and the best one was the Page model with R2 = 0.998. Quality analysis showed that there were no significant differences in peroxide value, free fatty acid, and colour between RF treatments and untreated controls during drying and storage periods (P > 0.05). RF heating can, therefore, provide a practical and effective method for drying in-shell walnuts with acceptable product quality.

Key words: Walnuts; Radio frequency; Drying curve; Kinetic models; Quality






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