Effect of high-voltage electrostatic field-assisted freeze-thaw pretreatment on the microwave freeze drying process of hawthorn

High-voltage electrostatic field assisted freezing (HVEF) and freeze-thaw (H-FT) pretreatment were conducted before the microwave freeze drying to improve the quality of freeze-dried hawthorn. The hawthorn samples were frozen at electric field intensities of 0, 1, 3, and 5 kV/cm (C, HVEF1, HVEF3, and HVEF5, respectively), and freeze-thawed in one to three cycles (H-FT1, H-FT2, and H-FT3, respectively) under the selected electric field intensity of 3 kV/cm. The control group was freeze-thawed in two cycles and without electric field treatment (CFT2). The results showed that the drying time of HVEF- and H-FT-treated hawthorn was reduced by 8-25% compared with that of the untreated hawthorn. The HVEF and H-FT pretreatments also affected the quality of hawthorn. Among the samples treated with HVEF, the total phenolic content of HVEF3 samples was the highest (46.31 mg GAE/g), followed by that of HVEF5 (41.87 mg GAE/g) and HVEF1 (37.25 mg GAE/g) samples. Similarly, among the samples treated with H-FT, the total anthocyanin content of H-FT2 samples was the highest (1.50 mg C-3-G eqiv/g), followed by that of H-FT1 (1.34 mg C-3-G eqiv/g) and H-FT3 (0.87 mg C-3-G eqiv/g) samples. Additionally, the H-FT2 treatment led to higher hardness (16.60 N), rehydration capacity (3.47 g/g), and total phenolic content (38.19 mg GAE/g) than the CFT2 treatment. In conclusion, H-FT2 improved the quality of hawthorn. This study lays a foundation for producing high-quality dried hawthorn using H-FT pretreatment, reducing the drying time and thereby improving drying efficiency.