Effects of ultrasound-assisted freezing on drying kinetics, mass transfer and quality of freeze-dried Abelmoschus sagittifolius root

Abstract

Freeze-drying is widely recognized as an effective technique for preserving thermolabile medicinal materials. In practice, the long drying period can significantly increase both energy consumption and operating expenses. This study investigated the effects of ultrasound-assisted freezing (UAF) on the drying kinetics, mass transfer characteristics, and quality attributes of freeze-dried Abelmoschus sagittifolius (Kurz) Merr. roots. Ultrasound power levels of 0, 25, 50, 75, and 100 W were applied during freezing. The results indicated that UAF markedly improved drying efficiency. Drying time was reduced by up to 37.5%, while the specific energy consumption (SEC) decreased by approximately 37% at 50 W compared with the control. The Midilli model best described the experimental data compared to the other models (R 2 > 0.997). The effective moisture diffusivity ( D eff ) increased with ultrasound treatment and reached its highest value at 75 W. Microstructural observations revealed a more homogeneous porous structure, which may have facilitated moisture migration during drying. Moreover, the highest reported RGSFI, corresponding to 96% of the fresh-sample dry-basis value, was observed at 75 W. Although 50 W provided the shortest drying time and the lowest specific energy consumption (SEC), 75 W produced the highest reported RGSFI and was identified by the AHP–TOPSIS analysis as the best overall quality-oriented condition. The improved freeze-drying performance may be associated with ultrasound-induced modifications in ice crystal formation and pore structure, which may reduce mass transfer resistance and enhance moisture migration during drying.