Mechanisms of phenolic conversion in anaerobic fermentation of tea leaves revealed by integrating microbiome and metabolome analysis.

To systematically investigate the phenolic transformation mechanisms during tea anaerobic fermentation, the changes in phenolics and microbial communities of pickled teas under varying extrusion degrees were analyzed by combining metabolomics and microbiomics. The changes in 118 differential phenolics (p < 0.05, variable importance in projection >1.0, and fold change >1.2 or < 1/1.2) revealed that anaerobic fermentation drives the bioconversion of bound phenolics into free phenolics and their derivatives in tea leaves. Additionally, the potential metabolic pathways of tea phenolics in anaerobic fermentation were comprehensively proposed, mainly including hydrolysis of polymerized/galloylated catechins, hydrolysis of flavonoid glycosides, and hydrolysis of galloylated phenolics to release gallic acid and its further derivatization. The degree of extrusion significantly influenced microbial community succession: high-extrusion enriched Enterobacter, Cladosporium, Setophoma, and Vishniacozyma, enhancing the hydrolysis of flavonoid glycosides and depsides, while light-extrusion promoted Candida, Cyberlindnera, Lactobacillus, and Pantoea, favoring the accumulation of free phenolics and their derivatives. These findings establish a mechanistic link between microbial ecology and phenolic biotransformation, providing a foundation for precision fermentation in tea processing.