Impact of Various Drying Technologies on the Drying Characteristics, Physico-chemical Properties, and Antioxidant Capacity of Walnut Green Husk

Walnut green husk (WGH) is a common agricultural by-product with high yield and serious pollution problem. The present study aimed to evaluate the impacts of sun drying (SD), pulsed vacuum drying (PVD), short and medium infrared radiation drying (SMIR), hot air drying based on temperature and humidity control (TH-HAD), and heat pump drying (HPD) on the drying characteristics, energy consumption, physico-chemical properties, bioactive compounds content and antioxidant capacity of WGH. Drying characteristics and drying kinetics showed that artificial drying could significantly improve drying efficiency. Compared with SD, the drying time of PVD, SMIR, TH-HAD, and HPD reduced by 63.6, 78.7, 66.3, and 54.0%, respectively. Specific energy consumption analysis demonstrated that HPD showed the lowest specific energy consumption of 2.2 kW<middle dot>h/kg. Hydration properties analysis revealed that PVD resulted in sample of best water-holding capacity and the HPD sample had the highest water solubility index (43.44%). Structural analysis showed that the cell wall structure of WGH was damaged by different drying methods. Generally, among the five drying methods, HPD was found to be in the middle range in terms of active substance content and antioxidant capacity of dried samples, but it had the least energy consumption. Therefore, HPD has great potential for commercial-scale drying of WGH, offering a new approach for low-energy food processing.