Investigation of hot-air assisted radio frequency (HARF) dielectric heating for improving drying efficiency and ensuring quality of dried hazelnuts (Corylus avellana L.)

Abstract

Hot-air assisted radio frequency (HARF) dielectric heating was utilized to dry inshell hazelnuts (Barcelona and Jefferson) in order to achieve rapid, uniform drying and better nut quality. The process parameters studied were electrode gap (EG) (14, 15 and 16 cm) and sample thickness (ST) (3, 5 and 7 cm) for HARF drying, 2-step hot-air (HA)-HARF drying at different intermediate moisture contents (MC) (16, 14 and 12%, wet basis), and two HARF modes (continuous and intermittent). The optimal intermittent HARF operations were identified as 15 cm EG and 5 cm or 7 cm ST, resulting in drying time of 100.5 min and 180.9 min for Barcelona and Jefferson, respectively, vs. 22 h using HA drying. The 2-step HA-HARF drying at 12% intermediate MC reduced shell cracking ratio <15%, provided fast drying rate and improved heating uniformity, whereas a longer HA pre-drying time resulted in undesirable high lipid oxidation of dried kernels. The HARF and 2-step HA-HARF in the continuous mode increased drying rate and better retention of bioactive compounds in comparison with the intermittent mode. This study generated important information of HARF technology for drying hazelnuts with improved drying performance and nut quality compared with conventional HA drying.