## 3 Results and Discussion
### Effects of Fermentation on the Composition of Aroma-Active Compounds in MT and FMT  
The flavor characteristic of tea is determined by the combination of different aroma compounds with different odor characteristics and different aroma contribution degrees [32]. In order to evaluate the contribution of VOCs to the flavor characteristic, the ROAV of the compounds were calculated. ROAV is a measure of the minimum threshold at which humans perceive taste, and the ROAV can be used to assess the contribution of each volatile component to the overall flavor, and larger values of ROAV were considered to contribute more [27]. The compound with ROAV \(\geq 1\) was considered as a key aroma compound [33]. We used the Odor database and FlavorDB jointly to analyze the odor characteristics of the aroma compounds [26]. These databases related the chemical features of the VOCs to odors and flavors, providing a comprehensive understanding of the flavor attributes. Table 1 summarizes the key aroma compounds in MT and FMT. A total of 33 aroma-active compounds were identified in MT and FMT, including ketones, aldehydes, alcohols, heterocyclic compounds, benzenoids, lipids and lipid-like molecules, and esters. It is worth noting that ketones, alcohols, heterocyclic compounds, lipids, and lipid-like molecules become more abundant after fermentation. The number of aldehydes was highest in the aroma-active compounds of MT, while ketones were the most abundant substances in the aroma-active compounds of FMT. In MT, 2-pentylfuran showed the highest ROAV (10,601.94), followed by heptanal (2878.87) and trimethyl-pyrazine (1311.26). The high ROAV of these compounds indicated that these three compounds might be the most important contributors to  
Figure 3: Results of the multivariate statistical analysis of volatile compounds. **(A)** PCA score plot, R2X = 59.4%; **(B)** OPLS-DA score plot, R2X = 58.6%, R2Y = 100%, Q2 = 98.8%. MT1–6 represent 6 parallel samples of MT, and FMT1–6 represent 6 parallel samples of FMT. the flavor characteristics formation to MT, and there was a total of 25 aroma compounds with ROAV \(\geq\) 1 in MT (Table 1). The compound with the highest ROAV in FMT was 1-octen-3-one (3950.26), while compounds with ROAV \(\geq\) 1000 included trimethyl-pyrazine (3026.34) and (E)-2-nonenal (1756.96), and there were 26 aroma compounds with ROAV \(\geq\) 1 in FMT (Table 1). However, some of the key aroma compounds (octanal, hexanal, and 6-methyl-5-hepten-2-one) of MT were not the key aroma compounds in FMT due to their ROAV \(<\) 1 in FMT. This was attributed to the fermentation of _E. cristatum_[30]. In addition to this, trimethyl-pyrazine, (E)-2-octenal, 2-ethylhexanol, acetophenone, isophorone, naphthalene, phenol, and a-terpineol had higher ROAV values in FMT compared to MT, suggesting that these aroma compounds contributed more to the flavor characteristics of FMT than MT. Based on changes in the relative contents of VOCs, OPLS-DA, and ROAV (ROAV \(>\) 1000, FC \(>\) 2 or \(<\)0.5, and VIP \(>\) 1) [21], the potential markers that contribute to aroma differences in MT and FMT were screened. 1-octen-3-one, (E)-2-nonenal, trimethyl-pyrazine, 2-pentylfuran, and heptanal were the potential aroma markers between MT and FMT. Figure 4: Heatmap analysis of the critical volatile compounds in the tea samples. A color-coded scale from blue to red corresponds to the content of critical metabolites shifting from low to high. MT1–6 represent 6 parallel samples of MT, and FMT1–6 represent 6 parallel samples of FMT. \begin{table}
\begin{tabular}{c c c c c c c c c c c} \hline \hline
**Compounds** & **Class** & **CAS** & **Range of Odor** & **Range of Odor** & **ROAV of MT** & **ROAV of PMT** & **Flavor Description** & \begin{tabular}{c} **Fold Change** \\ **(MFT vs. MT)** \\ \end{tabular}