Study on the flavor formation of fermented sweet tea and its relation with the polyphenol oxidase

Sweet tea, produced from the tender shoots of Lithocarpus litseifolius using the green tea process, usually exhibits an unacceptable flavor. In this study, the effect of fermentation on the sweet tea flavor and its mechanism had been comprehensively investigated. Results showed that fermentation could effectively reduce the sweetness of sweet tea by significantly lowering the level of trilobatin, and collectively weaken the bitterness and astringency by downregulating the contents of trilobatin and (+)-catechin. Fermentation could greatly develop the sweet fragrance through significantly altering the level and proportion of volatiles with different odors, leading to high flavor acceptability of sweet tea. Chemical fluctuations were closely associated with differential expression of the genes and the corresponding enzymatic reactions triggered by fermentation and its leading step rolling. The differential oxidation of trilobatin and phlorizin during fermentation was verified by catalytic characteristics of prokaryotically expressed polyphenol oxidase (LlPPO). The findings elucidated the potential mechanism underlying the quality improvement of the fermented sweet tea based on metabolite profiling, gene expression and catalytic property of key enzyme, which laid a solid foundation for developing high quality sweet tea.