# Discovery and Flavor Characterization of High-Grade Markers in Baked Green Tea
## 2 Results and Discussion
### Identification of Grade A Taste Markers by PCA and OPLS-DA  
PCA is a multivariate, unsupervised statistical analysis method used in metabolomics studies that can achieve dimensionality reduction and visually reflect some characteristics of samples [33]. As shown in Figure 5, the first two principal components (PC1 and PC2) accounted for 81.67% of the total variance, which indicated they were sufficient to explain most of the information of the sample. The separation of the different grades of tea was mainly based on PC1. A closer distribution of the tea samples shows more similar taste qualities. Therefore, these three different grades of baked green teas were distinguished via PCA. Upon comparison of the distances between samples, it is evident that the difference between grade A and grade C is large, while the difference between grade A and grade B is small. The OPLS-DA model was constructed to further investigate the holistic characteristics and change correlations among the samples and identify the taste markers of Grade A  
Figure 4: Heat-map-clustering analysis of taste compounds according to their content in the baked green tea. Every square represents a taste compound. If a compound has a higher concentration, the color of the square is redder; otherwise, the color is closer to blue. (**a**) [PERSON]; (**b**) [PERSON]; (**c**) [PERSON]. Figure 5: Forty-variable-based PCA of Grade A, B, and C baked green tea. The tea samples are located at the distinct positions in two-dimensional space described by two vectors of PC1 = 76% and PC2 = 5.67%. Green color represents Grade A tea, blue color represents Grade B tea, and red color represents Grade C tea. green tea. Forty taste compounds in the tea infusion were used as dependent variables and different grades were used as independent variables to build an OPLS-DA model. The parameters of the OPLS-DA model were as follows: R\({}^{2}\)X = 0.945, R\({}^{2}\)Y = 0.999, and Q\({}^{2}\) = 0.965 in Grade A and B green tea samples; R\({}^{2}\)X = 0.816, R\({}^{2}\)Y = 0.996, and Q\({}^{2}\) = 0.981 in Grade A and C green tea samples; and R\({}^{2}\) and Q\({}^{2}\) over 0.5 indicate acceptable model fit results. The results of 200 permutation test was shown in Figure 6c,d, the model validation was effective. The results were considered to be applicable to the grade identification analysis of baked green tea taste compounds. The OPLS-DA score plot revealed that Grade A and other grades were clearly separated, as shown in Figure 6a,b. In Figure 7 shows the S-plot results of the differential metabolites in the different grades of the baked green tea samples. The dots at two ends of the \"S\" pattern indicate potential chemical markers with high confidence levels. In the S-plot of the A-B grades and A-C grades, it is obvious that there are five common taste compounds that were positively correlated with the Grade A tea at the bottom-left end of the \"S\" pattern, specifically, gallic acid, galloylglucose, digalloyglucose, trigalloyglucose, and strictinin. Gallic acid, galloylglucose, digalloyglucose, trigalloyglucose, and strictinin are all phenolic compounds, and the concentrations of each of these compounds were significantly high in the Grade A tea. Therefore, combined with the results of the clustering analysis and OPLS-DA, they were identified as taste markers of Grade A Huangshan Maofeng, Taiping Houkui, and Shucheng Xiaolanhua tea. It is worth noting that galloylglucose, digalloyglucose, trigalloyglucose, and strictinin belong to the group of hydrolyzable tannins, and their content can be used as a potential indicator for high-grade green tea [34]. Figure 6: The OPLS-DA score plot of Grade A, B, and C baked green tea with taste compounds. (**a**) OPLS-DA score plots between Grade A and B tea: green color and blue color represent Grade A tea and Grade B tea, respectively. (**b**) OPLS-DA score plots between Grade A and C tea: green color and red color represent Grade A tea and Grade C tea, respectively. (**c**) Permutation test of OPLS-DA model between Grade A and B tea.