Hot Air-Assisted Radio Frequency Heating for Rapid Drying and Roasting of Fresh In-Shell Peanuts

This study investigated hot air-assisted radio frequency (HARF) heating for drying and roasting fresh in-shell peanuts in a single operation. In a 10 kW, 40.68 MHz HARF system with 100 °C hot air, the estimated RF output power increased with sample loading and decreased slightly with increasing electrode gap. At a 17 cm gap, 1 kg of peanuts (42% moisture, wet basis) was dried to below 10% moisture in 17 min, and one further minute of heating gave roasted peanuts with a kernel moisture content of 3.69% and a water activity of 0.35; the corresponding peanut-bed temperatures (not kernel temperatures) were approximately 120 °C and 136 °C. Moisture content decreased approximately linearly with time (R2 = 0.993), suggesting a nearly constant drying rate. Compared with 45 °C cold-air drying, the combined HARF process increased the drying rate approximately 23.8-fold (19.31 vs. 0.81 g/min), shortened the drying time by 95%, and reduced the specific energy consumption from 21.29 to 2.18 kWh/kg (by about 90%) at laboratory scale. Roasting made the kernels darker, less hard, and more brittle. Total aerobic counts and yeast and mold counts fell from about 106 CFU/g to below the detection limit (<10 CFU/g) immediately after treatment, aflatoxins were not detected in any sample, including the raw material, and acid and peroxide values remained low during 20 days of accelerated storage (vacuum-packaged, 37 °C). HARF heating is a promising rapid process for fresh in-shell peanuts, but hot-air-only controls, kernel temperature measurement, and scale-up remain to be studied.