Multi-frequency ultrasound-assisted hot-air drying of Cistanche deserticola slices: Effects on drying efficiency, energy consumption, bioactive compounds, and microstructure.

This study systematically investigated the effects of single-frequency (SFU), dual-frequency (DFU), and triple-frequency ultrasound (TFU) combined with hot-air drying on the drying performance, energy consumption, physicochemical quality, characteristic bioactive compounds, and microstructure of Cistanche deserticola slices. All ultrasound treatments were conducted at 55 °C with a constant total ultrasonic power of 120 W using 25, 28, and 40 kHz individually or in combination. Ultrasound accelerated dehydration, with DFU and TFU reducing the drying time by up to 43.75% compared with conventional hot-air drying. Among the tested modes, DFU2 (25 + 40 kHz) showed the lowest specific energy consumption of 76.15 kWh/kg water, corresponding to a 32.28% reduction relative to hot-air drying. TFU was characterized by higher measured contents of phenolics, flavonoids, and polysaccharides, stronger antioxidant activity, lower color deviation, and generally higher contents of most characteristic phenylethanoid and iridoid glycosides. SEM observations showed that ultrasound produced a more open and porous tissue structure, consistent with the improved drying behavior. Multivariate analysis further distinguished the treatments, showing that TFU was more closely associated with most quality indicators, whereas DFU2 was more strongly associated with energy efficiency. These findings indicate that increasing the number of ultrasound frequencies did not result in uniform improvement across all responses; instead, different frequency modes favored different processing objectives. Overall, ultrasound-assisted HA improved drying performance while maintaining favorable quality attributes.