Screening of dominant microbial communities and their regulatory mechanisms on tea quality transformation during pile-fermentation of Tianjian dark tea

The final quality of Tianjian dark tea strongly depends on pile-fermentation and aging. However, poorly understood microbial mechanisms have led to inconsistent quality and lengthy cycles during traditional production. This study systematically investigated microbial regulatory mechanisms and quality transformation patterns. Tea leaves harvested from March to June were monitored for dynamic changes in polyphenols, catechins, and pigments during pile-fermentation (0–24 h) and aging (0–12 y). Key microorganisms were isolated using dilution plating and screened via decolorization assays, growth kinetics, and transformation capability evaluation. A microbial consortium was established to compare inoculated versus natural fermentation. Leaves harvested in April showed desirable quality balance. After 24 h of pile-fermentation, polyphenols decreased 41.3% while theabrownins increased 84.6%. Aging for 5–6 y yielded optimal quality with water extract > 42% and theabrownins reaching 4.5%. Three key functional strains were identified: Aspergillus C2 (D/d = 1.16), Aspergillus C1 (D/d = 1.12), and Klebsiella L2 (D/d = 1.11). Aspergillus dominated theabrownin formation while Klebsiella promoted thearubigin accumulation. The consortium (L2:C1:C2 = 1:1:2) reduced fermentation time by 20–30% and increased theabrownin formation rate by 27.6%, with advantages maintained after 5-year aging. PCA revealed 89.2% cumulative variance in first two components. A “time-microorganism-quality” three-dimensional regulatory model was established, providing theoretical basis and technical support for standardized production.