ABSTRACT
The objective of this research was to investigate the influence of freeze-thaw pretreatment on the physicochemical properties of microwave-dried beetroots. Fresh beetroots were subjected to different freeze-thaw pretreatments, that was FT0 (without freeze-thaw pretreatment), FT1 (freeze-thaw once), FT2 (freeze-thaw two times), FT3 (freeze-thaw three times). After freeze-thaw pretreatment, beetroots were dried by microwave drying. The drying time, rehydration ratio, color, betalains content and total flavonoids content were evaluated. Results showed that freeze-thaw pretreatment reduced drying time from 18.3% to 37.0%. Rehydration ratio of beetroots reduced significantly with the increase of freeze-thaw times. Compared to fresh beetroots, microwave-dried beetroots pretreated with freeze-thaw increased brightness, as well as decreased redness and yellowness. The unpretreated microwave-dried beetroots (FT0) showed the lowest total color change (ΔE), and ΔE increased with increasing freeze-thaw times. It was found that betacyanins, betaxanthins and total flavonoids content significantly decreased with the increase of freeze-thaw times.
