Application of vacuum-cooling pretreatment to microwave freeze drying of carrot slices

In order to discuss the feasibility of vacuum-cooling pretreatment applied to microwave freeze drying process, the experiment of vacuum-cooling pretreatment process and three different processing techniques of microwave freeze drying were conducted on WDG-5 type microwave freeze dryer using the material of carrot slices, which involved in group A: vacuum cooling substituting the process of freezing in refrigerator, group B: vacuum cooling followed by freezing in refrigerator, and group C: no vacuum cooling pretreatment. After 22 minutes of vacuum cooling with full capacity of the vacuum pump and cold trap, the temperature of carrot slices declined to freezing point as well as 16.5% water content removed. 60 minutes of vacuum cooling pretreatment removed 20% water content and decreased surface temperature and core temperature of the carrots to -41.2℃ and -29.5℃, respectively. Compared with group C, group A and B increased microwave power in initial drying stage, shortened the drying time by 2 h, and respectively reduced 17.3% and 19.9% of the total power consumption. There was no significant difference among three groups’ dried carrot slices in moisture content, rehydration rate and color indices. The retention rates of Vitamin C in group A and B were both more than 80%, higher than the 68% retention rate in group C. It is concluded that vacuum-cooling pretreatment applied to microwave freeze drying can alleviate the problem of glow discharge in initial drying stage, reduce the drying time and power consumption and improve the retention rate of Vitamin C.