Untargeted Metabolomics Reveals the Metabolic Consequences of Lactobacillus plantarum Fermentation on Apple Juice

Lactic acid bacteria (LAB) fermentation is widely used to improve the flavor, stability, and bioactivity of fruit products. However, the global metabolic consequences of LAB fermentation on apple juice and the underlying metabolite-level changes remain poorly defined. Here, untargeted UHPLC-MS metabolomics coupled with multivariate analysis was used to compare apple juice before (UFAJ) and after 24 h fermentation with Lactobacillus plantarum (FAJ). Among 974 detected metabolites, 219 were differential (VIP > 1, p < 0.05), with 169 upregulated and 50 downregulated, and these were dominated by phenylpropanoids and polyketides, lipids, organoheterocyclic compounds, and organic acids. Fermentation enriched a panel of functional constituents—4-trimethylammoniobutanoic acid showed the largest increase, fraxetin was markedly retained, and the flavonoids chrysoeriol and tricetin, hypoxanthine, and folinic acid were upregulated—while galactinol and several glycosylated precursors declined. KEGG analysis identified 55 enriched pathways (p < 0.05), led by nucleotide metabolism, ABC transporters, phenylalanine and purine metabolism, and C5-branched dibasic acid metabolism. Overall, fermentation was associated with marked changes in metabolite abundance in apple juice, with relative enrichment of several bioactive metabolites, offering a metabolomic basis for functional fermented apple juice.