## 3 Results and Discussion
### Alterations of Phytochemical Compositions of Green and Black Tea after the Decaffeination
#### 3.1.2 Volatile Components  
The changes in the volatile component in green and black tea samples were shown in Figures 3 and 4, respectively. A total of 26 and 64 volatile compounds were identified in green and black teas, respectively. In the green tea, 4,6-dimethyldodecane and \(\beta\)-ionone were enriched in DeCAF-GT, whereas the decaffeination process decreased 3,8-dimethylundecane, 2,3,6,7-tetramethyloctane, and 2-bromo dodecane. Notably, five volatile compounds (e.g., octanal and D-limonene) were removed entirely in DeCAF-GT, but eleven volatile compounds were generated during the decaffeination process. Similar to our results, [PERSON], [PERSON] and [PERSON] [24] found that octanal and limonene were removed in green tea after the SCD. trans,trans-2,4-Heptdiagnenal, methyl salicate, \(\beta\)-cyclodextrin, and safranal contributed to tea's flower/fruit-like aroma, but they were hardly detected in green tea samples [30]. In our study, these volatile compounds were generated after decaffeination, indicating the potential aroma enrichment in DeCAF-GT. [PERSON], [PERSON], [PERSON] and [PERSON] [14] studied the effect of supercritical carbon dioxide decaffeination on volatile components of green teas and found that 3-methyl-1-butanol, 3-methyl-2-pentanol, N,N-diethylbutanamine, and 4-methoxybutanoic acid were generated after the decaffeination process. Another study found that 2-ethyl-1-hexanol, (3\(Z\))-3-hexen-1-yl hexanoate, and 2-pentylfuran were generated in green teas after the supercritical carbon dioxide decaffeination [31]. These results may be explained by the hydrolysis, volatilization, Maillard reaction, and redox reaction of aroma precursors, but the molecular mechanisms need to be revealed in further study [30]. The formation of \(\beta\)-cyclodextrin and safranal may be due to the non-enzymatic degradation (auto-oxidation or thermal degradation) of carotenoids during tea processing [32]. In the BT, 2,6,11-trimethyldodecane, trans,trans-2,4-heptadienal, benzeneacetaldehyde, methyl salicate, 6-methyl-5-hepten-2-one, and g  
Figure 2: Contents of (**A**) total theaflavins, thearubigins, and theabrownins; (**B**) theaflavins, (**C**) catechins, (**D**) amino acids, and (**E**) organic acids of the green tea (GT), decaffeinated green tea (DeCAF-GT), black tea (BT), and decaffeinated black tea (DeCAF-BT). Abbreviations: theaflavins (TF); theaflavin-3-gallate (TF-3G); theaflavin 3\({}^{\circ}\)-O-gallate (TF-3\({}^{\circ}\)-G); theaflavin 3,3\({}^{\circ}\)-digallate (TFDG); epigallocatechin gallate (EGCG); gallocatechin (GC); epigallocatechin (EGC); catechchin (**C**); epicatechin (EC); gallocatechin gallate (GCG); epicatechin gallate (ECG); gallic acid (GA). Different letters for the same component indicate significant differences (\(p<0.05\)). in the DeCAF-BT, but 3,5-ocudation-2-one, linalool, \(\alpha\)-terpineol, and \(\beta\)-citral were increased after the decaffeination process. Of note, 17 volatile compounds were removed completely during the decaffeination process, whereas 26 volatile compounds were generated in the DeCAF-BT. 3,5-Octadien-2-one and \(\alpha\)-terpineol exhibited sweet smelling and mint-like aroma, and they were enriched in the DeCAF-BT [33]. Linalool is a vital aroma contributor in black teas, which exhibits citrus-like and flowery aroma [34; 35]. The decaffeination process significantly enhanced it, which may be explained by the degradation of trans,trans-2,4-heptadieneal during the SCD [36]. The decaffeination process increased the levels of \(\beta\)-citral, which may contribute to the fruity-like aroma of the DeCAF-BT [34]. Acetic acid can provide a vinegar-like aroma and sour taste to tea infusion. In our study, the DeCAF-BT contained higher acetic acid than the BT, suggesting that DeCAF-BT may exhibit a stronger sour taste and strong vinegar-like aroma [33]. Ocimene and limonene, fruity-smelling compounds, were generated after the decaffeination.