Chemical Composition Profiling of Sun-Dried Black Tea of Different Ages Based on Metabolomics Approach

In order to investigate the changes in the chemical composition of black tea during storage, we analyzed sun-dried black tea stored for 0, 1, 3 and 4 years by metabolomics based on ultra-high performance liquid chromatography-quadrupole orbitrap mass spectrometry (UPLC-Q-Orbitrap/MS). A total of 82 compounds were identified, including six catechins, nine dimeric catechins, five amino acids, eight alkaloids, eight phenolic acids, three organic acids, two aroma precursors, six N-ethyl-2-pyrrolidinone-substituted flavan-3-ols (EPSF), twenty flavonol/flavone glycosides, ten lipids and five other compounds. Partial least squares discriminant analysis (PLS-DA) and heatmap analysis showed that the chemical composition of sun-dried black tea of different ages was greatly different, with 66 significantly differential compounds being identified between groups (P < 0.05). The contents of catechins, dimeric catechins, amino acids and most flavonol-O-glycosides decreased after storage, while the contents of caffeine, flavone glycosides and six EPSF compounds increased. The Pearson correlation coefficients of the contents of EPSF compounds with storage time were 0.802–0.986, indicating that the contents of EPSF compounds increased in a linear manner during storage. This study can provide a theoretical basis for the elucidation of the chemical composition of sun-dried black tea and for its scientific storage.