## 3 Results and Discussion
### Volatile Compounds Identified by GC-IMS
#### 3.1.2 Differences of Volatile Compounds in Teas by GC-IMS
Changes in the volatile compounds in the processed teas were analyzed by the gallery plot analysis, which revealed that each sample had its own characteristic aroma components (the different colored boxes in Figure 3). After the FTLs were processed into BT, the total content of the volatile compounds increased; 57 individual compounds increased, and only 18 decreased. The compounds affected the flavor and quality of BT, producing a sweet taste, and floral and fruit aromas. Although the content of dimethyl sulfide was significantly reduced in BT (from 4.67% to 0.67%), it generated a delicate scent at low concentration [17], thus exerting a positive effect on the quality of BT. The main aroma compounds in BT, including (E)-2-hexenol, linalool and its oxides, benzaldehyde, hexanal, benzecetaldehyde, methyl salicate, and furfural, showed increased content (shown in Table 1), which is consistent with previous research results [25]. One of the important compounds that determines the tender aroma of BT is (E)-2-hexenol, with green, fruit, and vegetable odors [23], which increased in content more than tenfold, \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**No.**} & \multirow{2}{*}{**Volatile Compounds**} & \multirow{2}{*}{**RI**} & \multicolumn{3}{c}{**Relative Content (\%)**} & \multirow{2}{*}{**Odor Description \#**} \\  & & & **FTL** & & **BT** & & **GT** \\ \hline
4 & 2-Acetylfuran-D & 912.4 & 0.15 & 0.36 & 0.17 & Nutty, sweet, roasted, \\  & & & & & & & baked \\
5 & 2-Ethylfuran & 693.1 & 0.21 & 0.30 & 0.19 & Bean, fruity, earthy, \\ sum & & & **1.21** & **2.20** & **1.52** & green, vegetable \\
**Alkenes** & & & & & & \\
1 & \(\beta\)-Pinene & 928.7 & 0.08 & 0.20 & 0.09 & Turpentine, resin \\
**sum** & & & **0.08** & **0.20** & **0.09** & \\
**Others** & & & & & & \\
1 & Dimethyl sulfide & 488 & 4.67 & 0.67 & 2.38 &
\begin{tabular}{c} Delicate at low concentrations. \\ \end{tabular} \\
**sum** & & & **4.67** & **0.67** & **2.38** & \\ \hline \hline \end{tabular}  
* M, monomers; D, dimers. RI, retention index, which was calculated with reference to the retention time of C4–C9 n-ketones under the same conditions. # Odor description found in the literature ([PERSON]; The LRI and Odour Database); --, no odor description information was found in the literature. The bold part represents the kind of the compound and the total relative content of each class.  
\end{table} 
Table 1: _Cont._  
Figure 2: The proportion of volatiles of different categories identified by GC–IMS in three tea samples. and the relative content was high. Owing to withering, the content of (E)-2-hexenol can increase more than tenfold than that in fresh leaves [6]. As an important source of flower and fruit aromas in BT, linalool showed an obviously increased level. In BT, the linalool content increased from 0.69% to 1.67%. Linalool is the characteristic aroma compound of Keemun black tea (containing 2.54%), and is mainly formed from the hydrolysis of glucoside during fermentation [26]. Therefore, the degree of glucoside hydrolysis can be increased by increasing the fermentation time or temperature. This approach increases linalool content. Phenylacetaldehyde has a floral odor, and benzaldehyde may contribute to the burnt sugar odor in tea. In spring tea, phenylacetaldehyde and benzaldehyde are mainly produced by the degradation and oxidation of amino acids and reducing sugars [27]. These two flavor compounds were detected in summer tea, producing a floral and sweet-caramel aroma, and were also detected at high concentrations in Keemun black tea [26]. After the FTLs were processed into GT, the content of total volatile compounds increased. Compared with the content of FTL, the content of 50 volatile compounds increased, whereas the content of 25 volatile compounds decreased in GT. These compounds affect the flavor and quality characteristics of green tea through their combined effects, contributing to the unique odor of GT.