Fixation is crucial for green tea quality; previous studies were conducted on finished teas and focused on the volatile metabolites of green tea. However, non-volatile metabolites’ (NVMs) evolution and formation mechanisms during fixation remain unclear. Here, fixated tea samples were collected at seven different time points during the green tea fixation stage, the dynamic changes and formation mechanisms of NVMs investigated via ultra-performance liquid chromatography-tandem mass spectrometry and enzymatic activity analyses. The results showed that fixation contained enzymatic activity thermal and non-enzymatic thermal stages. Moreover, 1196 NVMs (10 classes) were detected, and significant NVM variations were observed during the "enzymatic activity thermal" fixation stages, with increasing trends in lipids and amino acids, except alkaloids. Furthermore, 160 common differential NVMs were identified. KEGG analysis revealed that unsaturated fatty acid, linoleic acid phenylpropanoid, and flavonoid metabolism pathways were essential during fixation and influenced green tea aroma, taste, and color formation. These results provide a theoretical basis to regulate green tea fixation technology and enrich the chemical theory of green tea processing.
Oolong tea is a partially fermented tea with distinct tastes and aromas. However, the dynamic biochemical changes during oolong tea processing are not well understood. In this study, we performed metabolomics-based profiling of non-volatile and volatile constituents of oolong tea during its entire processing procedures by UPLC-QTOF MS and GC-TOF MS. A step-wise change of tea metabolome was observed, where catechins and oxidized products, flavonol glycosides and amino acids were identified as key discriminate metabolites. The ZuoQing process comprising alternating YaoQing and TanQing steps was deemed most critical for key metabolic transformation. Extensive YaoQing facilitated the oxidative polymerizations of catechins into theaflavins and proanthocyanidins, lowering the astringency in raw tea. Two direct terpene precursors farnesyl pyrophosphate and geranyl pyrophosphate accumulated to high levels during ZuoQing, which provided more substrates for the synthesis of downstream volatile terpenes. Moreover, both YaoQing and prolonged TanQing facilitated the formation of terpenes as well as fatty acid and benzenoid-derived volatiles, which contributed to the fruity and floral fragrances in oolong tea. The fixation step not only converted amino acids into aromatic compounds, but also lowered the amounts of flavonol glycosides, potentially improving the flavor quality of the final tea product. This study provides a comprehensive profile of flavor-related metabolic changes during oolong tea processing and will contribute to better quality control and flavor improvement of oolong tea.
危险品化学品经营许可证（带存储）