Influence of different pretreatments on the quality attributes of heat pump dried beetroot slices

This research investigates the effects of different pretreatments, including ultrasound (US), steam blanching (SB), osmotic dehydration (OD) and freeze-thaw (FT), on the quality characteristics of heat pump dried beetroot slices. The physicochemical properties, phytochemical compositions, and antioxidant capacity of heat pump dried beetroot slices were analyzed. In terms of physicochemical properties, the dried beetroot slices pretreated by FT showed the lowest drying time of 215 min and the highest rehydration ratio of 7.31. The dried beetroot slices pretreated by SB exhibited the highest hardness, shrinkage rate, and bulk density, namely 1574.9 g, 93.38%, and 1.29 g/mL, respectively. Additionally, the samples pretreated by US effectively preserved the original microstructure of beetroot slices and showed the most attractive red color with a* value of 31.36. Regarding phytochemical compositions, SB resulted in the higher preservation of betalains, TPC, and TFC than other pretreatments. Compared to CK, the four pretreatments all significantly improved the ascorbic acid content of dried beetroot slices (p < 0.05), and SB led to the highest ascorbic acid content of 342.28 mg/100 g dm. Concerning antioxidant capacity, SB displayed higher retention of FRAP value and ABTS radical scavenging activity of dried beetroot slices compared to other pretreatments. Our findings indicate that SB is a promising pretreatment to improve drying process and quality characteristics of heat pump dried beetroot slices.