Effects of pre-freezing on the drying characteristics and quality parameters of freeze drying apple slices

Vacuum freeze-drying food has become a commonly found product in worldwide, due to its low moisture content, long shelf life, reduced wrinkles, and less loss of nutrients. But the great challenge to produce vacuum freeze-drying fruits can be related to the long drying time, high energy consumption, and low crispness. Pre-freezing process is an important procedure of vacuum freeze drying, where the freezing rate usually determines the quality and drying characteristic of vacuum freeze-drying (FD) products. Freeze-thaw cycles (FTC) are generally used as a pretreatment method to control the drying efficiency and product quality. In this study, three freezing temperature (-20℃, -80℃, liquid nitrogen freezing) and 1-3times freeze-thaw pretreatment (FTC-1, FTC-2, FTC-3) were adopted in apple slices drying, in order to explore the influence of freezing rate and freeze-thaw treatment (FTC) on the drying characteristics and quality of FD apple chips. An investigation was also made to explore the effect of freeze treatment on the freezing and drying characteristics, microstructure, color, hardness, crispness, and nutritional quality, in the FD apple chips. The results showed that the freezing rate increased with the decrease of freezing temperature and the increase of FTC times. In drying characteristics, the Page model can better represent the drying process of apple chips with various temperatures and freeze-thaw cycles. The freeze-thaw treatment significantly increased the drying rate of apple slices, while reduced the time to reach the maximum drying rate. Compared with the fast freezing (-80℃), the drying time of apple slices was 5% shorter than that in the slow freezing (-20℃), and the crispness increased 50.1%. Compared with the -20℃ pre-freezing group, the apple crisps treated by FTC-1 showed a more uniform pore structure, with the 37.2% increase in pore volume, 15.3% reduction in FD drying time, 14.6% reduction in drying energy, and 117.6% increase in crispness. In phenolic compounds, there was no significant effect of pre-freezing temperatures on the phenolic compounds, whereas, the freeze-thaw treatment significantly affected the content of main phenolic compounds in apple chips. Specifically, the content of main phenolic compounds remained low, as the increasing time in the pretreated FTC. The total retention rate of phenol was 77.65% in freeze-dried apple chips, and that in the FTC-1 was 45.2%. There was no significant effect of pre-freezing temperature on the total content of soluble sugar and titratable acid. Although the freeze-thaw treatment can lead to the loss of some juice, the total content of soluble sugar and titratable acid decreased slightly. The sensory evaluation results showed that there were adverse effects of freeze-thaw treatment on the appearance and smell, with the decrease of smoke sensation and stickiness in the freeze-dried apple chips. Considering the quality of products and drying energy consumption, the FTC-1 treatment can be used as the optimal process to control the quality of vacuum freeze-dried apple chips. The freezing rate and freeze-thaw treatment can serve as the sensory quality control, to save the drying time and energy consumption for the production of vacuum freeze-dried fruit and vegetable crisp slices. © 2020, Editorial Department of the Transactions of the Chinese Society of Agricultural Engineering. All right reserved.