Abstract: In this study, changes in optical density (OD) during the fermentation of apple juice with pure and mixed cultures of lactic acid bacteria were monitored by a UV-Vis spectrophotometer. The aroma components of nine fermented apple juices were identified by head-space solid phase micro-extraction (HS-SPME) combined with gas chromatography-mass spectrometry (GC-MS), and the characteristic aroma components were evaluated by odor activity value (OAV) and odor threshold value (OTV). Sensory evaluation was also carried out on the samples. The results showed that the OD value of apple juice increased logarithmically with time. Forty eight aroma components were identified from nine fermented apple juices, mainly including alcohols, esters, aldehydes, ketones and phenols. Esters were the most abundant components with the highest species diversity, followed by alcohols. In addition, according to their aroma values, butyric acid ethyl ester, ethyl-2-methylbutyrate, 2-methylbutyl acetate and hexyl acetate were the main aroma components in all the fermented samples. The contents of aroma components and aroma values from single and mixed culture fermentation of apple juice were significantly different (P < 0.05). It was shown that the total contents of alcohols, esters and other aroma components with mixed culture fermentation were significantly higher than those of single culture fermentation and apple juice fermented with single cultures had a stronger fruity, green and flowery aroma. The overall sensory score of apple juice fermented with mixed cultures was higher than that of single culture fermentation. A mixed culture of Lactobacillus paracasei 20241, Bifidobacterium animalis 6165, Streptococcus thermophilus 6063 and Lactobacillus acidophilus 6005 at a ratio of 1:1:1:1 was the optimal combination in sensory acceptance. These results can provide a theoretical basis for the research and application of lactic acid bacteria in fermented fruit and vegetable juice.

Key words: apple juice, lactic acid bacteria, mixed-strain fermentation, aroma components, head-space solid phase microextraction (HS-SPME), gas chromatography-mass spectrometry (GC-MS)