Metabolomics of Non-volatiles in Typical Chinese Brick Dark Tea

To investigate the metabolite profiles of typical Chinese brick dark tea and the relationship between the metabolite types of brick dark tea and fungi isolated from it, this study used non-targeted metabolomics based on liquid chromatography-mass spectrometry (LC-MS) and feature-based molecular networking (FBMN) for metabolomic analysis of typical brick dark tea in China (Hubei Qingzhuan brick tea, Sichuan Kangzhuan brick tea, dark brick tea, Huazhuan brick tea and Fuzhuan brick tea from Hunan, and Fuzhuan brick tea from Shaanxi), fresh tea leaves and various fungal strains isolated from dark tea. The results indicated that the non-volatile metabolite profiles of brick dark tea tended to be classified into two groups according to production regions: one group included brick dark tea from Hunan and Shaanxi, and the other group included brick dark tea from Sichuan and Hubei. The major factor determining the metabolite profile was the stalk percentage of brick tea, and the stalk percentage of brick dark tea from Sichuan and Hubei was significantly higher than that of the other group. The major reason for the difference in metabolite profile between the two groups was the higher contents of flavan-3-ols as well as most amino acids and their derivatives in brick dark tea from Hunan and Shaanxi. The molecular network analysis of the metabolites of brick dark tea and fungi showed that linear and cyclic dipeptides derived from fungi were widely distributed in brick dark tea, especially Kangzhuan brick tea. Isopentenyl diketopiperazine compounds of diverse structural types were found to be abundant in Fuzhuan brick tea, which were derived from Eurotium cristatum. Therefore, fungi contribute to the rich variety of metabolites in brick dark tea. This research can provide a theoretical reference for a comprehensive understanding and knowledge of the chemical basis of quality in typical brick dark tea in China.