Metabolic Conversion and Quality Contribution of Tea-Derived Aspergillus nidulans and its Safety Assessment in Dark Tea Manufacture

The unique quality of dark tea is primarily shaped during pile fermentation by a group of functional microbes, yet their specific roles remain poorly understood. Therefore, the core fungus Aspergillus nidulans, isolated from dark tea, was investigated for its metabolic function, quality contribution, and potential safety risk. Sensory evaluation revealed that A. nidulans diminished the freshness and astringency of sun-dried tea leaves while imparting a stale, thick, and mellow taste during pile fermentation. A. nidulans demonstrates an exceptional capacity for producing theabrownins, achieving a remarkable yield of 167.93 g/kg. 74 differential metabolites were identified, with N-methylation of xanthine, hydrolysis/degradation/polymerization of catechins, and deglycosylation of flavonoid glycosides. No detectable level of aflatoxin B1/B2/G1/G2, fumonisin B1/B2, citrinin, ochratoxin A, deoxynivalenol, or zearalenone was found in A. nidulans-fermented tea. This work advances our understanding of the functional role of tea-derived A. nidulans and provides a benchmark for precise inoculation for quality improvement.