Effect of ultrasound-assisted drying on drying kinetics, color, total phenols content and antioxidant activity of pomegranate peel

The study investigated the effect of ultrasound (US) pre-treatment on drying kinetics, color, total phenols content and antioxidant activity of pomegranate peel (PP). The US pre-treatment was carried out at a frequency of 24 kHz at room temperature for two time periods of 5 and 10 min. The pretreated PP were then dried in a convective tray dryer at two different temperatures (70 and 80 °C). The US pre-treatment had a significant influence on reducing the drying time (approx. 45 % reduction) of PP. Five thin-layer drying models were fit to study the drying kinetics in which Midilli's model gave the best-fit curve. The effective moisture diffusivity (D_eff) of PP ranged from 5.44 × 10⁻⁰⁹ to 1.49 × 10⁻⁰⁸ m²/s. The PP activation energy (E_a) decreased with increased exposure time. The color values (ΔΕ^*) increased with increasing treatment time and temperature, however, no significant difference among color values was observed. The total phenol content and antioxidant activity were higher in US-treated peel powder than in untreated (UT) PP. The US pre-treatment of 10 min and dried at 70 °C successfully reduced degradation of total phenolic content and antioxidants and is recommended for PP drying pretreated with US