Effects of various radio frequencies on combined drying and disinfestation treatments for in-shell walnuts

Abstract

Four radio frequency (RF) heating systems with a power of 2 kW and four frequencies of 6.78, 13.56, 27.12, and 40.68 MHz were used to compare the effect of different frequencies on drying characteristics, disinfestation efficacy and product quality of in-shell walnuts. The results showed that with the increase of frequency, the average walnut temperature rose from 70.87 ± 5.59 °C to 76.16 ± 5.83 °C and the heating uniformity index dropped significantly (P < 0.05), and the treatment time was reduced to achieve 100% mortality of pests. The frequency had no significant effect on walnut cracking ratio (CR) and color (L*) (P > 0.05), but significant differences were observed in peroxide value (PV) and free fatty acid (FFA) (P < 0.05), which still met the industrial standard (PV < 1.0 meq/kg, FFA < 0.6%). Overall, the shortest drying time (150 min) and the highest energy efficiency (21.50%) were achieved when the 27.12 MHz RF system was used to dry walnuts from a MC of 20% (d.b.) to 8% (d.b.) at the similar heating rate (1.40 °C/min) and temperature. Therefore, the 27.12 MHz RF system could substantially shorten the drying time and improve the energy efficiency, and hold potential for industrial applications.