Ultrasound-assisted freezing for enhanced freeze-drying of Codonopsis javanica : Effects of power and duty cycle during crystallization on kinetics and quality

Abstract

Freeze-drying is an effective preservation method for thermosensitive plant materials, although its industrial application is limited by long drying times and high energy consumption. Therefore, this study aimed to investigate the effects of ultrasound-assisted freezing applied during the crystallization stage (0 °C to −5 °C, 20 kHz) on the freeze-drying performance and quality attributes of Codonopsis javanica. The results showed that continuous ultrasound at 100 W reduced total freeze-drying time to 18.65 h, corresponding to a 7.5% reduction compared with conventional freeze-drying (20.17 h), while improving product quality (ΔE = 6.63; saponin retention = 95.35%). Increasing power to 150 W deteriorated product quality and prolonged crystallization, indicating a non-monotonic response to acoustic intensity. Further improvement was achieved using pulsed ultrasound during crystallization. The optimal duty cycle (t_on = 60 s, t_off = 90 s; A = 0.40) reduced the total process time to 13.89 h (31.13% reduction) while improving color and saponin retention (ΔE = 3.2; 97.92%). These results demonstrate that crystallization-stage ultrasound control is an effective approach for accelerating freeze-drying while preserving bioactive compounds in medicinal roots.