Optimization of the pulsed vacuum drying process of green walnut husk through temperature adaptive regulation

This study aimed to optimize the temperature adaptive conditions of pulsed vacuum drying (PVD) for green walnut husk (GWH) to tackle the issues of severe environmental pollution and limited utilization of GWH. The results of the single-factor experiment revealed that the optimal drying temperature for PVD of GWH was 65°C, with a pulsed ratio of 9 min: 3 min. The drying time decreased from 10.87 to 6.32 h with increasing drying temperature and from 8.83 to 6.23 kW·h/kg with increasing pulsed ratio. Energy consumption also decreased with shorter drying time and shorter vacuum time. Under this optimal variable temperature drying condition, GWH exhibited the highest total active substance content, with respective values of 9.43 mg/g for total triterpenes, 35.68 mg/g for flavonoids, 9.51 mg/g for polyphenols, and 9.55 mg/g for quinones. The experimental drying data of GWH were best fitted by a logarithmic model, with R2 values ranging from 0.9927 to 0.9943. Furthermore, the observed microstructure of GWH corresponded to the variations in total active substance content. This study provided valuable theoretical guidance for addressing environmental pollution associated with GWH and facilitating the industrialization and refinement of GWH drying processes. PRACTICAL APPLICATION: There is a growing interest in harnessing the potential value of agricultural waste to transform low-cost raw materials into high-value products while mitigating environmental pollution. In this study, for the first time, the effects of variable temperature pulsed vacuum drying on the content of active substances, drying time, and energy consumption of green walnut husk (GWH) were investigated. The findings serve as a theoretical foundation for addressing environmental pollution issues associated with GWH and enabling the industrialization and precision drying of GWH.

Drying sea buckthorn berries (Hippophae rhamnoides L.): Effects of different drying methods on drying kinetics, physicochemical properties, and microstructure

Sea buckthorn berries are important ingredients in Chinese medicine and food processing; however, their high moisture content can reduce their shelf life. Effective drying is crucial for extending their shelf life. In the present study, we investigated the effects of hot-air drying (HAD), infrared drying (IRD), infrared-assisted hot-air drying (IR-HAD), pulsed-vacuum drying (PVD), and vacuum freeze-drying (VFD) on the drying kinetics, microstructure, physicochemical properties (color, non-enzyme browning index, and rehydration ratio), and total phenol, total flavonoid, and ascorbic acid contents of sea buckthorn berries. The results showed that the IR-HAD time was the shortest, followed by the HAD, IRD, and PVD times, whereas the VFD time was the longest. The value of the color parameter L* decreased from 53.44 in fresh sea buckthorn berries to 44.18 (VFD), 42.60 (PVD), 37.58 (IRD), 36.39 (HAD), and 36.00 (IR-HAD) in dried berries. The browning index also showed the same trend as the color change. Vacuum freeze-dried berries had the lowest browning index (0.24 Abs/g d.m.) followed by that of pulsed-vacuum-(0.28 Abs/g d.m.), infrared- (0.35 Abs/g d.m.), hot-air-(0.42 Abs/g d.m.), and infrared-assisted hot-air-dried berries (0.59 Abs/g d.m.). The ascorbic acid content of sea buckthorn berries decreased by 45.39, 53.81, 74.23, 77.09, and 79.93% after VFD, PVD, IRD, IR-HAD, and HAD, respectively. The vacuum freeze-dried and pulsed-vacuum-dried sea buckthorn berries had better physicochemical properties than those dried by HAD, IRD, and IR-HAD. Overall, VFD and PVD had the highest ascorbic acid and total phenolic contents, good rehydration ability, and bright color. Nonetheless, considering the high cost of VFD, we suggest that PVD is an optimal drying technology for sea buckthorn berries, with the potential for industrial application.