Dynamic Changes in the Microbial Community and Metabolite Profile during the Pile Fermentation Process of Fuzhuan Brick Tea

The pile fermentation process of Fuzhuan brick tea is unique in that it involves preheating without the use of starter cultures. The detailed metabolite changes and their drivers during this procedure are not known. Characterizing these unknown changes that occur in the metabolites and microbes during pile fermentation of Fuzhuan brick tea is important for industrial modernization of this traditional fermented food. Using microbial DNA amplicon sequencing, mass spectrometry-based untargeted metabolomics, and feature-based molecular networking, we herein reveal that significant changes in the microbial community occur before changes in the metabolite profile. These changes were characterized by a decrease in Klebsiella and Aspergillus, alongside an increase in Bacillus and Eurotium. The decrease in lysophosphatidylcholines, unsaturated fatty acids, and some astringent flavan-3-ols and bitter amino acids, as well as the increase in some less astringent flavan-3-ols and sweet or umami amino acids, contributed importantly to the overall changes observed in the metabolite profile. The majority of these changes was caused by bacterial metabolism and the corresponding heat generated by it.

Feature-Based Molecular Networking Analysis of the Metabolites Produced by In Vitro Solid-State Fermentation Reveals Pathways for the Bioconversion of Epigallocatechin Gallate

Dark teas are prepared by a microbial fermentation process. Flavan-3-ol B-ring fission analogues (FBRFAs) are some of the key bioactive constituents that characterize dark teas. The precursors and the synthetic mechanism involved in the formation of FBRFAs are not known. Using a unique solid-state fermentation system with β-cyclodextrin inclusion complexation as well as targeted chromatographic isolation, spectroscopic identification, and Feature-based Molecular Networking on the Global Natural Products Social Molecular Networking web platform, we reveal that dihydromyricetin and the FBRFAs, including teadenol A and fuzhuanin A, are derived from epigallocatechin gallate upon exposure to fungal strains isolated from Fuzhuan brick tea. In particular, the strains from subphylum Pezizomycotina were key drivers for these B-/C-ring oxidation transformations. These are the same transformations seen during the fermentation process of dark teas. These discoveries set the stage to enrich dark teas and other food products for these health-promoting constituents.

Fuzhuanins A and B: the B-ring fission lactones of flavan-3-ols from Fuzhuan brick-tea

Fuzhuan brick-tea is a special dark tea prepared from the leaves of Camellia sinensis var. sinensis. Its production involves a fungal fermentation stage, which forms the unique flavors and functions by a series of biochemical reactions. Our phytochemical research of the material led to the isolation of two new B-ring fission lactones of flavan-3-ols, fuzhuanins A (1) and B (2). In addition, three other flavan-3-ol derivatives (3-5), three flavone C-glycosides (6-8), eight flavonoid O-glycosides (10-17), five simple phenolics (19-23), two norisoprenoid glycosides (24, 25), two sesquiterpenoids (26, 27), and theobromine (28), as well as two flavonoid anions (9 and 18), were also identified. The structures of these compounds were determined by spectroscopic methods. Compounds 4, 19, 20, 22-24, 26, and 27 were reported for the first time in Camellia spp. and tea. Furthermore, HPLC analysis method was performed to compare the chemical constituents of the before/after fungal fermentation Fuzhuan brick-teas. Compound 1 was indicated as one of the major characteristic constituents generated in the fungal fermentation process. The IC50 value of the antiproliferative activity of 2 on HeLa cells was assayed as 4.48 μM. None of the isolated compounds showed any inhibition activity against the enteric pathogenic microbes at 800 μg/mL by the hole plate diffusion method.