Abstract: Sensory evaluation, gas chromatography-ion mobility spectrometry (GC-IMS), gas chromatography-mass spectrometry (GC-MS), an electronic nose, and an electronic tongue were used for multidimensional evaluation of the changes in flavor substances in different processing stages of concentrated apple juice (crushing/juicing, high-temperature sterilization, enzymatic hydrolysis, ultrafiltration and concentration). Key aroma components were then identified based on partial least squares-discriminant analysis (PLS-DA) and odor activity values (OAV). The results showed that high-temperature sterilization, ultrafiltration, and concentration treatment significantly reduced the sensory score of apple juice. Throughout the entire processing stages, a total of 36 volatiles were detected by GC-IMS. The variety of volatiles in apple juice samples remained stable after high-temperature sterilization and enzymatic hydrolysis; the concentration of volatiles such as 2-methyl butyl acetate decreased significantly after ultrafiltration, while more than 30 volatiles such as hexyl acetate disappeared completely after concentration. Across all samples from different processing stages, a total of 48 volatile substances were detected by GC-MS. Compared with fresh juice, the total concentration of volatile substances in apple juice was reduced by 34.1%, 37.1%, 62.8%, and 99.1% after high-temperature sterilization, enzymatic hydrolysis, ultrafiltration, and concentration, respectively, and the number of volatile substances in fresh juice was 35 and decreased to 8 after concentration, indicating severe aroma loss. Based on PLS-DA and OAV results, 11 compounds with variable importance in the projection (VIP) scores > 1 and OAV > 1, such as ethyl 2-methylbutyrate, were selected as the key aroma components of concentrated apple juice. The principal component analysis of the electronic nose and electronic tongue showed that the samples from each processing stage migrated in the first principal component direction, and the cumulative variance contribution was over 93%. The distribution area of the ultrafiltered and concentrated samples was the farthest from that of fresh juice, and the component discrimination was the most obvious. This study comprehensively revealed the changing pattern of flavor substances during the processing of concentrated apple juice, and proposed that the evaporation effect of heat treatment and the physical interception effect of ultrafiltration might be principally responsible for the flavor loss of concentrated apple juice, providing a theoretical basis for fruit juice manufacturers to optimize processing technology.

Key words: apple juice; flavor; sensory evaluation; gas chromatography-ion mobility spectrometry; gas chromatography-mass spectrometry; electronic nose; electronic tongue