GC-MS-based metabolomic study reveals dynamic changes of chemical compositions during black tea processing

The chemical composition of black tea during tea processing is in a state of flux. However, the dynamic changes of this sophisticated metabolic process are far from clear. GC-MS-based metabolomic analyses were performed to examine changes in volatile and non-volatile compounds throughout the five stages of tea processing. The results showed that the most striking differences were observed at the withering and rolling stages, during which 62 non-volatile and 47 volatile compounds were significantly changed. The levels of most monosaccharides decreased at the withering stage and increased in subsequent stages while di-saccharides decreased as the process progressed. Free amino acids increased sharply at the withering stage, and most kept increasing or remained stable afterwards. However, levels of catechin, epicatechin, epigallocatechin, and epigallocatechin gallate decreased after withering and remained at low levels afterwards. Among the 47 volatile compounds with altered levels, phenylpropanoids/benzenoids and carotenoid-derived volatiles, which contribute to the honey-like and rose-like fragrances and quality of Danxia2 tea, kept increasing during the processing, among them eight were newly produced. Furthermore, 19 volatiles with a grassy odor decreased during processing. This study provides a comprehensive profile of metabolic changes during black tea processing, which is potentially important for both quality control and improvement of the flavor of black teas.