Vacuum freeze and infrared-assisted hot air segmented combined drying of fig slices: drying characteristics, physicochemical properties, and moisture migration changes

Figs are rich in bioactive compounds but have a short postharvest shelf life, which limits their commercial utilization. This study compared hot air drying (HAD), infrared-assisted hot air drying (IR), vacuum freeze drying (VFD), and combined drying methods to evaluate their effects on drying behavior, moisture migration, physicochemical properties, and rehydration performance of fig slices. The combined methods included infrared followed by vacuum freeze drying (IF) and vacuum freeze drying followed by infrared drying (FI). In the combined drying treatments, samples were switched between VFD and IR at specific moisture conversion rates (e.g., FI60, switching at 60%). IR and combined drying markedly improved drying efficiency. FI60 reduced drying time by 64.3% compared with VFD. Low-field nuclear magnetic resonance (LF-NMR) showed rapid transformation of moisture states during drying, while VFD caused minimal disruption to water binding. Magnetic resonance imaging (MRI) revealed that VFD and FI-treated samples had more uniform moisture distribution due to porous network structures, which contributed to improved rehydration. The highest rehydration ratios were obtained for VFD (2.50) and FI30 (2.39). In terms of quality, VFD samples exhibited the highest brightness, the lowest browning index, and the highest nutritional component content. FI30 achieved a favorable balance between drying efficiency and quality preservation. The highest total phenolic content (4.47 mg GAE/g, gallic acid equivalents) and total flavonoid content (2.68 mg RE/g, rutin equivalents) indicated strong antioxidant potential, while IF50 showed the highest DPPH radical scavenging activity (1.97 mg TE/g, Trolox equivalents). Overall, FI combined drying integrates the efficiency of infrared drying with the quality advantages of vacuum freeze drying and shows strong potential for producing high-quality dried fig products.