Effects of Re-processing and Freezing Methods on the Efficiency of Vacuum Freeze-drying and Quality of Freeze-dried Apple Slices
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Abstract:
Pre-treatment, cooling, and solidification are key processes in the freeze-drying of fruits and vegetables. In order to improve the efficiency of vacuum freeze-drying and ensure the quality of freeze-dried products, apple slices were subjected to different pre-processing and freezing methods including ultrasonication, vacuum freezing, and ultrasound-vacuum freezing. The apple slices were randomly divided into 4 groups: conventional group, ultrasound-conventional group, vacuum-freezing group, and ultrasound-vacuum-freezing group. The drying times and quality indicators for these groups were compared. The results showed that the ultrasound-conventional group, which was subjected to ultrasound followed by vacuum freezing in the refrigerator, showed the best drying efficiency. The drying time reduced from about 16.00 h to 11.83 h. The rehydration rate of dried apple slices was also the highest in this group, which increased by about 62.16% compared to the conventional group. The b* value of dried apple slices reduced from 28.24 to 19.46, and the color was whiter. The retention rate of vitamin C was also the highest, reaching 83.90%. The hardness was the lowest, giving crisp apple slices, and the sensory evaluation gave the best results. The vacuum-freezing group, which was subjected to vacuum freezing during the process of pre-freezing, showed a reduction in the drying time of the apple slices from 16.00 h to 14.17 h and an increase in the rehydration rate of the dried products by 17.99%, compared to the conventional group. The b* value of dried apple slices decreased from 28.24 to 22.98, and the color became white. The dried apple slices acquired moderate crispness and good chewiness, and the sensory evaluation was good. This study provides a reference for optimizing the parameters for the vacuum freeze-drying of apple slices and the development of new fruit slices.
