Untargeted Metabolomics Revealed the Effects of Fresh Tea Leaves Plucked in Different Seasons on the Flavor Quality of Huhong Congou Black Tea

To explore the influence of fresh tea leaves harvested in different seasons on the flavor quality of Huhong Congou black tea, we selected young tea shoots with one bud and two leaves from three typical clonal varieties in Changsha plucked in spring and summer to produce Huhong Congou black tea using the same processing technology. The differences in flavor quality between black tea made from spring and summer tea leaves were evaluated by sensory evaluation. Non-targeted metabolomics analysis based on ultra-high performance liquid chromatography-Q Exactive-mass spectrometry (UPLC-Q Exactive-MS) and multivariate statistical analysis were integrated to analyze the key differential non-volatile constituents between the spring and summer tea leaves and their influence on the quality of Congou black tea. The results of sensory evaluation showed that the flavor quality of Huhong Congou black tea produced in spring was better than that in summer, especially in taste quality. Metabolomics analyses showed differences in the metabolic profiles of tea leaves plucked in spring and summer. Forty key differential metabolites were selected based on P < 0.05 and variable importance in projection (VIP) value > 1, mainly involved in flavonoid biosynthesis, flavone and flavonol biosynthesis and amino acid metabolism pathways. The contents of sweet and umami amino acids (theanine, glutamic acid, and glutamine) as well as phenolic acids (digalloylglucose, trigalloyl glucopyranose, and galloylglucose) with umami-enhancing effect were significantly higher in the spring tea, whereas the contents of bitter amino acids (leucine, lysine, tyrosine and phenylalanine) were higher in the summer tea. In addition, flavonols and flavone/flavonol glycosides showed different distribution trends in the spring and summer teas; flavone-C-glycosides, myricetin-O-glycosides, and quercetin-O-glycosides were more abundant in the summer tea, whereas kaempferol-O-glycosides were more abundant in the spring tea. This study provides theoretical support for the quality enhancement and regulation of Huhong Congou black tea.