# Discovery and Flavor Characterization of High-Grade Markers in Baked Green Tea
## 2 Results and Discussion
### Odor Composition Correlations Analysis and Grade A Odor Markers' Identification  
The visualization results of the odor compounds in the different grades of baked green tea were obtained via cluster analysis of the relative quantification results of the odor compounds in the tea infusion of the three different kinds of H tea, T tea, and S tea (details shown in Figure 2). The odor compounds of the tea samples with different grades were very similar, but their odor concentrations were different. By comparing the color intensity variation of the heatmap in Figure 2, the Grade A tea (i.e., H-1, H-2, T-1, and S-1 tea) can be clearly distinguished from the other grades of tea. Through cluster analysis, it was determined that the Grade A tea had some similarities: D-limonene, linalool, and \(\beta\)-myrcene exhibited a pleasant odor and were found at nearly higher concentrations in the Grade A teas [28]; (E,E)-2,4-Heptdienal was found in lower concentrations in the Grade A tea, and it exuded a rather fatty odor [26; 29]. The three kinds of teas also have their own characteristics of correlation in terms of odor. In the Huangshan Maofeng at Grade A and Grade B teas, (E)-2-pentenal and cis-3-hexenyl hexanoate were detected and characterized as pleasant odor compounds, while the difference between the Grade A tea and Grade B tea is that the Grade B tea also contained some unpleasant ingredients at higher concentrations, such as 2-methyl butyraldehyde, 1-penten-3-one, and 3-octanol. The presence of these compounds renders the odor of the Grade B tea infusion impure. Butyl hexanoate, which is unique in the type T tea, was higher in the Grade C tea and showed a negative correlation with grade. In Shucheng Xiaolanhua, ethyl acetate with a fruity odor was present at a higher concentration in the Grade A tea, while it was not identified in Huangshan Maofeng or Taiping Houkui.  
OPLS-DA can eliminate data irrelevant to a category's information through orthogonalization and screen out the characteristic variables of samples [30; 31]. Figure 3a,b shows that the Grade A tea and other tea grades were clearly separated. R\({}^{2}\) and Q\({}^{2}\) values were used as indicators of model fitting and predictability, respectively [32]. The parameters of the OPLS-DA model were as follows: R\({}^{2}\)X = 0.994, R\({}^{2}\)Y = 0.926, and Q\({}^{2}\) = 0.654 in the Grade A and B green tea samples; R\({}^{2}\)X = 0.815, R\({}^{2}\)Y = 0.949, and Q\({}^{2}\) = 0.892 in the Grade A and C green tea samples; and R\({}^{2}\) and Q\({}^{2}\) over 0.5 indicate acceptable model fit results. After 200 permutation tests (results shown in Figure 3c,d), the intersection of the Q\({}^{2}\) regression line with the vertical axis was less than zero, indicating that there was no overfitting of the model and, consequently, that the model was successfully validated. The results were considered to be applicable to the grade identification analysis of baked green tea odor compounds.  
Figure 2: Heat-map-clustering analysis of odor compounds according to their concentrations in the baked green teas. Each square represents an odor compound. If a compound is present at a higher concentration, the color of the square becomes redder; otherwise, the color is bluer. (**a**) [PERSON]; (**b**) [PERSON]; (**c**) [PERSON].  
In conclusion, D-limonene and linalool were both important odor compounds in the three different kinds of baked green tea, and their concentrations in the grade A tea were much higher than those in the teas of grades B and C. In the sniffing experiment, D-limonene and linalool showed citrus-like and flowery aromas, respectively, which were both considered pleasant aromas. These two odor compounds were identified as odor markers of Grade A baked green tea.