Targeted metabolomics and DIA proteomics-based analyses of proteinaceous amino acids and driving proteins in black tea during withering

Withering is a necessary step to improve the flavors during the manufacturing of black tea. Fresh tea leaves of Fuding Dabai were subjected to withering (kept for 0–28 h at room temperature of 25 °C) to observe corresponding morphological changes. In addition, dynamic changes in proteinaceous amino acids, especially taste-related proteinaceous amino acids and their corresponding driving proteins, were investigated using targeted metabolomics and data-independent acquisition (DIA) proteomics. The results indicated that tea leaves gradually withered and shrunk, and their color deepened with the extension of withering time. Furthermore, the contents of umami amino acids were always higher during the withering process. Sweet and bitter amino acids exhibited a significant cumulative effect at the middle and late withering stages. A total of 31 driving proteins were detected that were involved in the metabolic regulation of Ser, Thr, Phe, Trp, and His amino acids. The functional analysis of 776 differential expressed proteins revealed that the tea leaf cells maintained the maximum possible cellular activity during the withering process, and the toxins were decomposed at the end of withering. These findings can provide the metabolic basis for room temperature withering during black tea processing to further enhance the quality of tea.