Non-Targeted Metabolomic Characterization of Metabolites during the Fermentation of ‘Golden Flower’ Eucommia ulmoides Leaf Tea

In order to investigate the changes of metabolite profile during the fermentation of ‘Golden Flower’ Eucommia ulmoides leaf tea, quantitative, qualitative and differential analyses of small molecule metabolites in tea samples at different fermentation times (0, 2, 4, 6, 8 and 10 days) were carried out using ultra-high performance liquid chromatography-quadrupole time of flight mass spectrometry (UPLC-Q-TOF-MS). The results showed that 172 significantly differential metabolites, belonging to 13 chemical classes, were identified during the entire fermentation process, including 46 flavonoids, 44 fatty acids, 16 amino acids and 11 organic acids. Furthermore, by selecting significantly differential metabolites at different fermentation times, it was found that the bioactive substances whose contents significantly increased after fermentation included flavonoids, fatty acids and organic acids. These metabolites played an important role in enhancing the taste and aroma of the tea. After fermentation, the contents of the differential metabolites L-glutamine, 20-hydroxyeicosatetraenoic acid, glucuronic acid, trans-ferulic acid, norvaline and auriculoside increased. The fermented tea had health functions such as antioxidant, hypotensive, detoxicating, anti-inflammatory and immunoenhancing effects compared with the unfermented tea. Therefore, fermentation can significantly alter the metabolite profile of E. ulmoides leaf tea, which plays an important role in the formation of the taste and aroma quality and antioxidants of fermented E. ulmoides leaf tea.