## 3 Results and Discussion
### Correlation Analysis between Aroma-Active Compounds and Sensory Attributes  
Separating undivided compounds using a second column is one of the important advantages of GC \(\times\) GC [36], but this leads to a very large number of compounds being identified. Aside from that, the olfactometry system is essential and more accurate for aroma analysis [37], but it requires a significant amount of human resources and time. Therefore, it is too difficult to quickly find indicators to distinguish between the varieties of tea samples. Thus, multivariate statistical analysis should be considered. The orthogonal partial least squares-discriminant analysis (OPLS-DA) was employed to analyze the correlation between aroma compounds and sensory attributes (Figure 6). The permutation test shown in Figure 6C demonstrated that the model validation succeeded due to both Q2 and R2 being over 0.5. Figure 6A shows that three kinds of tea were clearly distinguishable. It can be observed from Figure 6B that green tea was more related to the fresh and chest-like aroma, oolong tea was more correlated with the floral notes, and black tea was close to the fruity, sweet, and fermented notes. The Variable Importance for the Projection (VIP) reflects the intensity and explanatory power of the expression pattern of each metabolite that can be used for the classification and discrimination of each sample group, thereby assisting in the screening of marker metabolites (a VIP value > 1.0 is usually used as the screening criterion). VIP values indicate a correlation. The aroma compounds with VIP values > 1.0 were elaborated in Figure 6D, including no.109, linalool; no. 204, nerol; no. 284, trans-nerolidol; no. 148, phenylethanol; no. 245, geraniol; no. 199, methyl salicate; no. 101, benzaldehyde; no. 318, indole; no. 83, (_Z_)-linalo oxide; no. 262, 2-phenylethanol; no. 78, (_E_)-2,4-heptalional; no. 30, \(\beta\)-myrcene; no. 50, ocimene; no. 88, (_E_)-linalo oxide; no. 67, 6-methyl-5-hepten-2-one; no. 197, (_E_)-linalo oxide (pyranoid); no. 265, \(\beta\)-Lonone; no. 19, hexanal; no. 36, (_E_)-2-hexenal; no. 249, benzyl alcohol; no. 158, dehydrolinalool; no. 35, (+)-limonene; no. 121, (_E_)-3,5-octadian-2-one; no. 125, \(\beta\)-cyclodextrin; no. 155, 3-hexenyl hexanoate; no. 188, (_E_)-citral; no. 162, \(\beta\)-farnesene; no. 116, 5-methylfurfural; no. 223, nerylacetone; no. 314, creamy lactone; no. 108, 3,5-octadian-2-one; no. 132, hexyl hexanoate; no. 46, 2-pentylfuran; no. 70, 1-hexanol; no. 131, safranal; no. 90, furfural; no. 72, nonanal; no. 15, toluene; no. 91, cis-3-hexenyl butyrate; no. 171, (_Z_)-3,7-dimethylocta-2,6-dienal; no. 139, 1-ethyl-1H-pyrrole-2-carboxaldehyde; no. 200, \(\alpha\)-farnesene; no. 167, (_E_)-2-hexen-1-yl hexanoate; no. 69, (_Z_)-3-hexenyl-2-methylbutanoate; no. 74, (_Z_)-3-hexen-1-ol; no. 8, 2-methylbutanal; no. 7,3-methylbutanal; no. 87, (_E_)-2-octenal; and no. 66, 2,2,6-trimethyl-cyclohexanone. These aroma compounds were commonly reported in various tea leaves [38; 39; 40]. Fortunately, most of them could be smelled using olfactometry. A circular heatmap illustrated in Figure 7 is based on the peak areas of aroma compounds with VIP values > 1.0. It shows that no.109 linalool, which has been widely emphasized in tea aroma compounds [3; 10; 13], had the highest VIP value and delivered the most floral notes, making it the most important compound among all of the samples. Four black teas showed the highest content of linalool. This is mainly because the thorough  
Figure 6: OPLS-DA plots based on the result of the HS-SPME-GC \(\times\) GC-MS data. (**A**) was the predicted score scatter plot. The green, blue, and red colored signals represented by the green teas, oolong teas, and black teas samples, respectively; (**B**) represented the biplot, with different ellipse areas depicted as 95% confidence regions. The red frame areas (a–d) were enlarged as below respectively; (**C**) represented the validated model; (**D**) represented the variable importance in projection (VIP).