Influence of Different Lactic Acid Bacteria Fermentations on the Nutritional Substance and Volatile Flavor Compounds of Concentrated Pear Juice

In this study, strains of Lactobacillus plantarum (LP), Lactobacillus acidophilus (LA), Lactobacillus brevis (LGG), and Leuconostoc mesenteroides (LM) were used as fermentation strains to produce acidifiers from the concentrated pear juice residue in the distilled pear industry, the changes in active ingredients and antioxidant capacity during fermentation were investigated, and volatile flavour substances were analysed by headspace solid-phase microextraction (SPME)-gas chromatography-mass spectrometry (GC-MS), with a view to improving the nutritional properties and flavour quality. The results showed that among four single bacteria strains in the fermentation process of concentrated pear juice, LP bacteria strain had the highest viable count (9.7 lg CFU/mL), making it the most suitable strain for the fermentation of concentrated pear juice. During mixed bacteria fermentation, the total acid content gradually increased with the extension of fermentation time. On the fourth day of fermentation, the PMF group reached its peak (1.22 g/100 mL), while the LPF group had the lowest level (0.82 g/100 mL). Reducing sugar, total phenolics, total flavonoids, and protein all decreased gradually during fermentation. At the end of fermentation, the PAF group had the highest levels of all indicators, which were 28 g/L, 13.54 mg GAE/L, 17.46 mg/L, and 35.39 mg/L, respectively. Antioxidant indicators such as DPPH radical scavenging activity, ABTS+ radical scavenging activity, and iron ion reduction capacity (FRAP) first increased and then decreased during both single-strain and mixed-strain fermentation, reaching the maximum on the second day of fermentation. A total of 94 volatile flavor compounds were detected in mixed-strain fermentation, of which 20 had an OAV value greater than 1. There were 15 flavor substances with OAV greater than 1 in PAF group, and produced more important flavor substances (such as 2-heptanone, geraniol, and linalool), which had the most positive impact on the acidification agent for concentrated pear juice fermentation. In conclusion, this study examine the lactic acid bacteria combinations for fermenting acidification agents in concentrated pear juice, providing a theoretical basis for promoting efficient processing and utilization of pear industry by-products.