Study on Kinetics and Moisture Migration Characteristics of Freeze–Thaw Pretreated Solar Hot-Air Drying of Mongolian Astragalus Slices

This study investigated the effects of freeze–thaw pretreatment on the solar hot-air drying behavior, moisture migration, and microstructure of Mongolian Astragalus (Astragalus membranaceus var. mongholicus) slices. An L9 orthogonal design with slice thickness, diameter, air velocity, and drying temperature was used; drying kinetics, water-state distribution, and surface morphology were assessed by thin-layer models, apparent effective moisture diffusivity, LF-NMR, and SEM. The drying process showed no obvious constant-rate period and was mainly characterized by a falling-rate stage, indicating that dehydration was controlled by internal moisture migration. Freeze–thaw pretreatment redistributed the initial water fractions but did not uniformly accelerate drying; the longest drying time decreased from 130 to 100 min, showing a condition-dependent effect. Slice thickness was the dominant factor affecting the average drying rate. The preferred conditions were 1–3 mm thickness, 8–11 mm diameter, 1.0 m·s−1 air velocity, and 50 °C for the control group, and 1–3 mm thickness, 11–14 mm diameter, 1.5 m·s−1 air velocity, and 50 °C after freeze–thaw pretreatment. The Midilli model best fit the moisture-ratio data, and the apparent effective moisture diffusivity remained on the order of 10−9 m2·s−1. LF-NMR showed that endpoint residual moisture was mainly bound water, with free water almost completely removed. SEM observations showed a looser surface with more visible pores and cracks after freeze–thaw pretreatment. Overall, freeze–thaw pretreatment mainly affected solar hot-air drying by regulating moisture migration, with effects depending on process conditions.