Impact of co-fermentation with Debaryomyces hansenii and Bacillus subtilis on the flavor characteristic of dark tea infusion.

Debaryomyces and Bacillus are dominant microbial groups in dark teas such as Fu brick tea and play a key role in the formation of their characteristic flavors. In this study, dark tea infusion was co-fermented using Debaryomyces hansenii and Bacillus subtilis isolated from Fu brick tea. A comprehensive analytical approach was employed, including headspace solid-phase microextraction coupled with gas chromatography-mass spectrometry (HS-SPME-GC-MS), high-performance liquid chromatography (HPLC), relative odor activity value (rOAV) analysis, molecular docking, and sensory evaluation, to systematically investigate changes in tea polyphenols, catechins profile, and volatile organic compounds (VOCs) during fermentation and to identify key aroma-active substances. The results demonstrated that co-fermentation significantly reduced the contents of total tea polyphenols (by 45.12%) and ester catechins (by 92.63%), while promoting the accumulation of gallic acid. Volatile component analysis revealed that the co-fermentation integrated the aromatic profiles of both monocultures, retaining key aroma compounds such as linalool and nonanal, and introducing unique constituents such as α-cadinol, collectively forming a complex aroma profile dominated by floral, fruity, and woody notes. Molecular docking analysis confirmed strong binding affinity between key floral aroma compounds (e.g., linalool) and the olfactory receptor OR1A1. Sensory evaluation indicated that the co-fermented tea infusion achieved the highest overall scores in aroma and taste, exhibiting a mellow and sweet aftertaste along with a pure, bright, and refreshing aroma, highlighting the synergistic effect of co-culture fermentation. This study reveals that co-fermentation with D. hansenii and B. subtilis can effectively improve the flavor quality of dark tea infusion through synergistic regulation of polyphenol transformation and volatile compounds formation, providing a controllable bioprocess for the development of flavor-oriented, quality-stable fermented tea beverages with potential for industrial scale-up and commercial application.