Lactobacillus casei Fermentation Enhances the Sensory Quality of Shoumei Tea Infusion: Based on Flavor and Metabolic Analysis.

Shoumei white tea is widely consumed, yet its limited product diversity constrains its economic value. This study used Shoumei tea infusion as a fermentation substrate to examine how Lactobacillus casei modulates flavor and underlying biochemical pathways. During 72 h of fermentation, the pH decreased from 5.0 to 4.1, accompanied by a 95.5% reduction in soluble sugars (from 10,056 to 453 mg/L). The accumulation of organic acids, particularly lactic acid (rising from 873.90 to 1268.17 µg/mL) and malic acid (rising from 150.83 to 725.18 µg/mL), drove the increase in sourness. Concurrently, substantial reductions in L-theanine (by 47.66%), catechins, flavonoids, and polyphenolic pigments resulted in markedly decreased bitterness and astringency. Headspace solid-phase microextraction-gas chromatography-mass spectrometry (HS-SPME-GC-MS) combined with partial least squares discriminant analysis (PLS-DA) identified 51 volatile flavor compounds (VFCs), among which phenylethyl alcohol, linalool, 1-heptanol, 1-hexanol, ethyl hexanoate, and ethyl octanoate (variable importance in projection [VIP] > 1) were established as major contributors to aroma enhancement. Integrated metabolomics and enzyme activity analyses indicated that elevated β-glucosidase and polyphenol oxidase activities, together with shifts in amino acid biosynthesis, 2-oxocarboxylic acid metabolism, and galactose metabolism, drove the formation of characteristic fermentation aromas. These findings provide mechanistic insight and technical support for developing value-added fermented Shoumei tea beverages.