Synergistic Drying Solutions: Combining Vacuum and Infrared Techniques for Optimal Persimmon Preservation

This study evaluated vacuum drying (VD) and infrared drying (IRD) methods for persimmon, including individual and two-step sequential processes (VD followed by IRD and vice versa). The combined drying strategies were selected to harness the rapid surface heating of IRD and the low-temperature, low-oxygen benefits of VD, aiming to overcome limitations of single drying methods such as extended drying times and nutrient degradation. Drying experiments were conducted using laboratory-scale equipment at 50-70°C, for VD, with a vacuum pressure of 50 mbar (absolute pressure) and a pump speed of 2 L/s. Results showed a significant effect of drying combination strategies on drying rate, duration, effective moisture diffusivity, shrinkage, activation energy, color characteristics, microstructure, and phytochemical constituents of persimmon. The shortest drying times were recorded for IRD (240 min), followed by VD + IRD (343 min) and IRD + VD (376 min), whereas VD required the longest (520 min). Effective moisture diffusivity ranged from 1.42 × 10^-9 m²/s for VD at 50°C (VD-50) to 7.83 × 10^-9 m²/s for IRD at 70°C (IRD-70), with both individual IRD-70 and a combination of IRD + VD demonstrating improved moisture transfer performance. The IRD + VD combination resulted in the best microstructure preservation and showed lower shrinkage compared to other drying strategies. Moreover, this combination best preserved the persimmon color with the lowest total color change (ΔE = 5.591), whereas VD showed the highest (ΔE = 35.875). Activation energy was lowest in IRD + VD (13.98 kJ/mol), followed by VD + IRD (18.61 kJ/mol), with higher values in VD (34.08 kJ/mol) and IRD (22.75 kJ/mol). Phytochemical analysis showed IRD (total phenolic content [TPC] = 35.79 mgGAE/g, total flavonoid content [TFC] = 54.83 mgQE/g) and IRD + VD (TPC = 17.02 mgGAE/g, TFC = 58.52 mgQE/g) retaining the highest bioactive compounds. This study contributes to optimizing drying techniques for persimmon, enhancing energy efficiency, preserving nutritional quality, and supporting sustainable food processing, making it relevant for the food industry, food engineering, and food science fields.