Effects of freeze–thaw pre-treatment with different freezing methods on the microwave freeze drying of carrots

Abstract Three freeze–thaw (FT) pre-treatments using different freezing methods that freezing at −80 °C (FT-80), freezing at −25 °C (FT-25) and vacuum freezing (VFT) were used for carrot slices, the effects and mechanisms of FT pre-treatment on the drying characteristics and product quality of carrots during microwave freeze drying (MFD) were studied. The results showed that FT pre-treatment under different freezing methods affected the water state of carrots, resulting in varying degrees of moisture loss and shortening drying time by 14.3% to 42.8%. VFT treatment caused the greatest water loss (35.33%) during FT and significantly increased the effective moisture diffusivity in the MFD process. FT treatments did not cause any significant degradation in the colour of the carrots. The VFT and FT-80 treatments resulted in an elevated rehydration ratio, increased crispiness of MFD carrots and products with a porous microstructure and higher porosity. In contrast, the FT-25 treatment led to a decrease in porosity due to destructive damage to the carrot cell structure. However, this damage favoured the extractability of carotenoids, resulting in a higher carotenoid content than the other treatments. This study provides a theoretical foundation for the practical implementation of FT pre-treatment using various freezing methods on fresh foods prior to drying. It is recommended to employ VFT pre-treatment before MFD due to its efficiency and effectiveness.