Abstract: In this study, the freezing concentration and vacuum evaporation concentration of apple juice were compared and analyzed by using a prototype of a multi-pass intelligent freeze concentrator and a rotary vacuum evaporator. The freeze concentrator works in suspension crystallization mode and integrates the traditional separated unit operations: scraper heat exchanger, crystallizer and washing column to form a trinity compact structure. Its concentration ratio in a single pass was about 1.8. After three-pass freeze concentration, the concentration of apple juice increased from 10.5 to 34.5 °Brix. The retention rates of vitamin C and butyl acetate as an aromatic component were above 90%. At the same time, it had little effect on reducing sugar damage and the color of the juice. In addition, the distribution coefficients of soluble solids in ice crystals and mother liquor ranged from 0.008 1 to 0.067 0. The recovery yield of soluble solids was 98% and the loss of soluble solids was about 2% after three-pass freeze concentration. Parallel comparative experiments of apple juice concentration by vacuum evaporation showed that when the concentration of apple juice was 33.6 °Brix, the retention rates of vitamin C and butyl acetate were 84.5% and 43.9%, respectively. Therefore, freeze condensation was much better than evaporation concentration in terms of nutrition and flavor preservation.

Key words: apple juice, freeze concentration, evaporating concentration, vitamin C, aromatic substance, retention rate, comparative study