No abstract

BACKGROUND: Red light withering significantly improves the sensory flavor qualities of tea, although changes in metabolites during this process have not been systematically studied until now. The present study comprehensively analyzes metabolites in withered tea leaves at 2-h intervals up to 12 h under red light (630 nm) and dark conditions using ultra performance liquid chromatography-high resolution mass spectrometry (untargeted metabolomics).RESULTS: Ninety-four non-volatile compounds are identified and relatively quantified, including amino acids, catechins, dimeric catechins, flavonol glycosides, glycosidically-bound volatiles, phenolic acids and nucleosides. The results show that amino acids, catechins and dimeric catechins are most affected by red light treatment. Ten free amino acids, theaflavins and theasinensin A increase after red light irradiation, whereas epigallocatechin gallate and catechin fall.CONCLUSION: The present study provides a comprehensive and systematic profile of the dynamic effects of red light on withering tea and a rationale for its use in tea processing quality control.

Combined with the unique processing technology of oolong tea, oolong tea with stem processing has a better flavor compared to oolong tea without stem processing. However, there is currently no available evidence to support the contribution of stems to the taste quality of oolong tea. In this study, the electronic tongue, sensory evaluation method combined with liquid chromatography, and gas chromatography–mass spectrometry were used to explore the influence of the presence of stems on the flavor substances and aroma of oolong tea during processing. The results showed that the presence of stems significantly increased the umami taste of oolong tea (p < 0.05), and the content of seven free amino acids (p < 0.05), including theanine (53.165 μg/mL) and aspartic acid (3.190 μg/mL), two umami-related amino acids, significantly increased. Moreover, the content of nerolidol (2.598 μg/g) in aroma components was significantly increased. This study identifies the contribution of stems to oolong tea quality during processing.