Investigation into the dynamic process of natural inoculation-enhanced fermentation of summer-autumn green tea with aspergillus cristatus.

The summer-autumn green tea is hard for consumers to accept because of its bitter taste and low quality. This study reveals the core mechanism by which inoculation with the single fungal strain Aspergillus cristatus in naturally enhanced fermentation improves the quality of summer-autumn green teas. Multi-omics (including microbiomics through Illumina MiSeq high-throughput sequencing, non-volatile metabolomics through high-performance liquid chromatography-mass spectrometry, and volatile metabolomics through gas chromatography-mass spectrometry) and sensory analysis were used to evaluate the metabolites and quality of summer-autumn green tea before and after fermentation. The results indicated that Aspergillus rapidly dominated the microbial community during the early stages of fermentation, maintaining an absolute dominance (accounting for over 99 % of total fungi). It significantly reduces the content of bitter compounds such as flavonoids, tea polyphenols, and catechins through key metabolic pathways (e.g., flavonoid biosynthesis, flavonoid and flavanol biosynthesis, and arginine biosynthesis), while optimizing the composition of free amino acids (Increasing in fresh and sweet-tasting amino acids and decreasing in salty and bitter-tasting ones). Furthermore, Aspergillus drove a significant increase in volatile organic compounds (Geraniol, Phenylethanol, Methyl salicylate, etc) through isoprenoid synthesis and phenylalanine metabolic pathways. This shift led to a transformation in aroma profile from esters to ketones, while creating a complex fragrance characterized by floral, fruity and sweet scents. Correlation analysis further indicated that Aspergillus metabolism directly regulated the conversion of characteristic non-volatile and volatile components. This research elucidates a complete pathway through which a single fungus reshapes the metabolic profile of summer-autumn green tea via directed metabolism, providing a theoretical foundation for optimizing natural fermentation processes.