Abstract:In order to solve the problems of freeze-dried (FD) apple chips, such as soft texture, hygroscopicity and high energy consumption, this study researched the effects of different pre-drying, i.e., hot air (AD-FD), microwave vacuum (MVD-FD), heat pump (HPD-FD) and medium short wave infrared (IRD-FD), combined freeze drying on color, texture (hardness, crispness), hygroscopicity, microstructure, sensory quality and nutritional functions (phenolic compounds, VC) of apple chips. And on this basis, the better heat pump drying was selected as the pre-drying method, the effects of heat pump drying at different water transfer points (moisture ratio: 0.2, 0.4, 0.6, 0.8) combined with vacuum freeze drying (H2F, H4F, H6F, H8F) on the quality of apple chips were studied. The results showed that combined drying could effectively improve the hardness of the apple chips. Compared with the FD products, the energy consumption decreased by 18.3%, the hardness of the H4F dried apple chips increased by 35%, crispness increased by 94%, hygroscopicity decreased by 12.2% (2 h), the microstructure remained good, the retention rate of major phenolic compounds such as chlorogenic acid was 70.8%, and the aroma of product was closer to that of the fresh samples. In general, the quality of apple chips can be effectively controlled by using heat pump drying to different moisture ratio and then using freeze drying to the final water content.