Characterizing and decoding the effects of co-culture fermentation on dark tea infusion flavor development: elucidating microbial succession and metabolic dynamics mediated by Eurotium cristatum and water kefir

Water kefir (a probiotic-rich culture) and <i>Eurotium cristatum</i> (a key fungus in Fu brick tea) were co-fermented with dark tea infusion to investigate dynamic changes in volatile organic compounds (VOCs), microbial communities, and biochemical components. Co-fermentation dramatically altered microbial composition, enriching beneficial lactic acid bacteria (<i>Liquorilactobacillus</i>, <i>Lacticaseibacillus</i>) while inhibiting undesirable microbes (e.g., <i>Saccharomyces</i>). A total of 65 VOCs were identified, with co-fermentation promoting floral, fruity, minty, and stale aromas (e.g., linalool, methyl salicylate, acetophenone) while reducing off-flavors (e.g., 3-methylbutanoic acid). Multivariate analysis revealed core functional microbiota (<i>Gluconobacter</i>, <i>Acetobacter</i>, <i>Eurotium</i>, etc.) strongly associated with flavor formation. Additionally, co-fermentation modulated organic acids, free amino acids, and catechins, improving sensory attributes. Sensory evaluation confirmed superior aroma complexity (enhanced minty/fungal floral notes) and balanced taste (reduced sourness/bitter/astringency characteristics). These findings demonstrate that water kefir and <i>E. cristatum</i> co-fermentation synergistically improves dark tea flavor characteristics, offering a promising strategy for industrial-scale probiotic tea beverage development.