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BACKGROUND: Roasting plays an important role in the formation of flavor of roasted green tea; however, the changes in chemicals during this process have not been systematically studied until now. To reveal the dynamic changes in chemicals in green tea during roasting, non‐targeted metabolomics, coupled with chemometrics, was employed. RESULTS: A total of 101 non‐volatile metabolites were identified in tea samples, and 29 metabolites were identified as characteristic metabolites of roasting. A significant increase in catechins and their derivatives, organic acids, and flavonoid glycosides was observed, while the content of some amino acids and their derivatives decreased over 50% during roasting. The content of theanine glucoside increased dramatically (by 21.23‐fold at the roasting stage), and Maillard‐derived compounds also increased to varying degrees. CONCLUSION: Glycosylation, oxidative polymerization, and pyrolysis were important reactions responsible for the formation and transformation of flavor compounds in roasted green tea during roasting. © 2022 Society of Chemical Industry. [ABSTRACT FROM AUTHOR]

[Display omitted] • Oxygen-enriched fermentation significantly improves the quality of black tea. • Decreased catechins, flavonoid glycosides and phenolic acids can reduce astringency. • Oxygen facilitated the oxidation of catechins, flavonoid glycosides and phenolic acids. • Catechins oxidation accelerates the degradation of amino acids to form volatile aldehydes. Oxygen involved fermentation is generally recognized as the critical process for the formation of quality of black tea. However, the specific role of oxygen plays in taste-related metabolites' alteration has not been illustrated clearly. In the present work, a series of fermentation systems with different oxygen concentrations were used to investigate the mechanism of the effects of oxygen on the quality and nonvolatile metabolites in black tea. The results showed that oxygen-enriched fermentation significantly improved the taste of black tea. And sixty-six metabolites, including catechins, theaflavins (TFs), proanthocyanidins, amino acids, flavonoid glycosides, and phenolic acids, were significantly different in the black teas fermented by three oxygen concentrations. Meanwhile, a 10–30% decrease in catechins, flavonoid glycosides and phenolic acids and a 5% increase in TFs, glutamate and glutamine in oxygen-enriched group, when compared to the control group, reduced astringency and bitterness and enhanced umami intensity. Furthermore, increased oxygen concentrations promoted the oxidation of catechins, flavonoid glycosides and some phenolic acids. And catechins oxidation in turn could accelerate the degradation of amino acids to form volatile aldehydes and also promote phenolic acids oxidation. Our results reveal the potential role of oxygen plays in the metabolites' alteration in black tea during fermentation, which gives a new insight into understanding the quality formation of black tea. [ABSTRACT FROM AUTHOR]