Beetroot (Beta vulgaris L. var. conditiva Alef.) pretreated by freeze-thaw: influence of drying methods on the quality characteristics

This study aimed to investigate the impact of microwave drying (MD), microwave vacuum drying (MVD), freeze drying (FD), vacuum drying (VD), hot air drying (HAD), and sun drying (SD) on the quality attributes of freeze-thaw pretreated beetroots. The quality characteristics including physical properties, bioactive compounds, and antioxidant activities of dried beetroots were evaluated. The results showed that the freeze-thaw pretreated beetroots obtained using FD displayed the lowest shrinkage rate, hardness, and total color difference (ΔE), and the highest rehydration ratio of 6.49. FD, MD, and MVD resulted in porous structures of freeze-thaw pretreated beetroots. The highest betalains, ascorbic acid, total phenolic and total flavonoids contents were found in freeze-thaw pretreated beetroots prepared by MVD. Meanwhile, the largest FRAP value and ABTS free radical scavenging ability of freeze-thaw pretreated beetroots were obtained by MVD. These findings suggested that MVD is a more favorable method for drying freeze-thaw pretreated beetroots.