Metabolites of epigallocatechin gallate and changes in antioxidant activity through biotransformation with Eurotium cristatum during liquid-state fermentation

Eurotium cristatum, the dominant fungus in Fu brick tea (FBT) production, critically determines its characteristic quality. Epigallocatechin gallate (EGCG)-the predominant and bioactive tea catechins-undergoes substantial modification during FBT processing, though its microbial-driven biotransformation pathways remain poorly understood. This study comprehensively analyzed E. cristatum-mediated EGCG metabolism during liquid-state fermentation, characterizing the metabolite profiles and their functional implications. Fermentation greatly increased total phenolics (40.16%) and flavonoids (3.2-fold) contents through fungal enzymatic activities (tannase, polyphenol oxidase, catalase, cellulase, and α-amylase). These enzymes catalyzed EGCG's sequential conversion into primary intermediates (gallic acid and epigallocatechin), followed by structural diversification via sulfation, glycosylation, methylation, and polymerization, significantly enhancing antioxidant capacity. Untargeted metabolomics revealed 128 differential metabolites, with nine major EGCG derivatives characterized and demonstrating potent antioxidant activity. Notably, EGCG displayed dual roles: as a bioconversion substrate and a growth stimulant, increasing E. cristatum biomass by 61.54% through pH and carbon-to‑nitrogen (C/N) ratio modulation. Our findings elucidate the enzymatic mechanisms and metabolic network of EGCG biotransformation by E. cristatum, providing insights for potential FBT quality enhancement.