Metabolic Conversion and Quality Contribution of Tea-Derived <i>Aspergillus nidulans</i> 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 <i>Aspergillus nidulans</i>, isolated from dark tea, was investigated for its metabolic function, quality contribution, and potential safety risk. Sensory evaluation revealed that <i>A. nidulans</i> diminished the freshness and astringency of sun-dried tea leaves while imparting a stale, thick, and mellow taste during pile fermentation. <i>A. nidulans</i> demonstrates an exceptional capacity for producing theabrownins, achieving a remarkable yield of 167.93 g/kg. 74 differential metabolites were identified, with <i>N</i>-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 <i>A. nidulans</i>-fermented tea. This work advances our understanding of the functional role of tea-derived <i>A. nidulans</i> and provides a benchmark for precise inoculation for quality improvement.