Metabolomics Analysis of Variations in Chemical Components of Green Tea during Thermal Processing

In order to investigate the formation mechanism of the quality of baked green tea, we used metabolomics based on ultra-high performance liquid chromatography-quadrupole-Exactive mass spectrometry (UPLC-Q-Exactive/MS) to analyze the differences in the chemical compositions of green tea samples at different stages of thermal processing such as fixation, drying and roasting. A total of 125 compounds were identified, including 10 flavanols, 14 dimeric catechins, 19 flavanol-Oglycosides, 5 flavone-C-glycosides, 8 N-ethyl-2-pyrrolidinone-substituted flavan-3-ols (EPSF), 16 amino acids, 13 phenolic acids, 4 organic acids, 11 alkaloids, 13 lipids, 4 aroma glycosides and 8 other compounds. The results of partial least squaresdiscriminant analysis (PLS-DA) and heat map analysis showed that the chemical components obviously changed during the thermal processing of green tea, and 114 significantly differential compounds were selected (P < 0.05). In the three stages of thermal processing, the contents of most catechins and dimeric catechins decreased, and the contents of EPSFs, flavone-C-glycosides, and lipid compounds significantly increased. The contents of aroma glycosides (phenylethyl primeveroside, linalool primeveroside, and linalool oxide primeveroside) increased by 439% to 2497% during the fixation process. The content of N-(1-deoxy-D-fructosyl)theanine increased by 820% to 1290% during the drying process. The contents of flavanol-O-glycosides (myricetin 3-galactoside, kaempferol-3-galactoside, kaempferol 3-arabinoside, myricetin 3-glucoside and kaempferol-3-glucoside) decreased significantly during roasting. This study can provide a theoretical reference for future improvement of green tea quality.