Contact Ultrasound-Assisted Hot Air Drying of Sichuan Pepper: Drying Characteristics, Microstructure, and Physicochemical Quality Attributes

Sichuan Pepper is a high-value spice, but traditional drying often degrades its unique flavor and quality. This study investigates the applicability of contact ultrasound-assisted hot air drying (US-HAD) to address these issues. The effects of drying temperature (45, 50, 55 °C), ultrasonic power (48, 60, 72 W), and frequency (25, 28, 40 kHz) on drying kinetics, effective moisture diffusivity (Deff), and physicochemical quality were systematically evaluated. Results showed that US-HAD significantly reduced drying time by 20.00–33.33% compared to hot air drying (HAD). The Page model (R2 > 0.99) best described the drying kinetics. Ultrasound enhancement increased Deff (6.55 × 10−6 to 9.63 × 10−6 m2/s) by inducing micro-channel formation and stomatal opening, as evidenced by Scanning electron microscopy (SEM). Critically, US-HAD at 50 °C, 60 W, and 28 kHz minimized color degradation (∆E = 18.73), maximized the retention of total phenols and flavonoids, and increased antioxidant activity by 18.62%. GC-MS analysis confirmed better retention of volatile flavor compounds. However, the slight decrease in Deff at higher temperatures (55 °C) suggests potential surface hardening risks. This study confirms US-HAD as a promising technology for high-quality spice processing, though further research is still needed on the cost-effectiveness of industrial-scale expansion.