## 3 Results and Discussion
### Characterization of Volatile Compounds of FAA Tea in Four Harvest Times
#### 3.4.3 Key Flavor Characteristics of FAA Tea
The R2Y and Q2 of the models were 0.995 and 0.877, respectively, indicating that they were effective in distinguishing FAA tea samples from different harvest periods. The cross-validation test (Figure 8b) showed that the results were accurate and there was no overfitting of the models. As described in Section 3.3.3, first, pairwise OPLS-DA comparisons were conducted for each harvest time pair, and metabolites with VIP > 1 and \(p\) < 0.05 (\(t\)-test) were chosen. Second, cross-group integration was carried out, prioritizing metabolites consistently altered across multiple comparisons as DAMs. And 75 differential volatile compounds were screened. A heatmap was drawn to visualize these differential volatiles, showing the variation among harvest times. As illustrated in Figure 8c, the compound abundances of FAA-3 and FAA-6 showed the greatest difference, while the abundances of FAA-4 and FAA-5 were closer. This is consistent with the PCA results, indicating that these differential compounds are the key to the flavor differences among FAA tea in different harvest periods. In contrast, FAA-3, FAA-4, and FAA-5 had higher contents of camphor, caryophyllene, lavandulol, and benzeneacetaldehyde, which contributed more to their woody, floral, and sweet aromas. In FAA-6, artemisinin alcohol, (\(-\))-carvone, pinocarvone, and (\(+\))-3-thujonewere volatile compounds with an \"herbal/sweet/fresh/green\" aroma. In addition to the differential metabolites discussed above, \(\alpha\)-thujene was also identified as a significant volatile compound in the HS-SPME/GC\(\times\)GC-TOFMS analysis (Figure 8c), although it was not detected in the HS-GC-IMS analysis. This discrepancy may be attributed to the differences in sensitivity and selectivity between the two analytical techniques. \(\alpha\)-Thujene is a well-known component of Artemisia essential oils and has been reported to exhibit various biological activities, including antimicrobial and anti-inflammatory properties [49]. Its presence in our HS-SPME/GC\(\times\)GC-TOFMS data further supports the complexity and diversity of volatile compounds in Artemisia species. Future studies could explore the specific roles of \(\alpha\)-thujene in the biological activities of Artemisia. Figure 8: (**a**) PCA score plots; (**b**) OPLS-DA permutation test; (**c**) differential molecular heat map; (**d**) sensory flavor characteristics and flavor compound network. The sensory flavor characteristics and flavor compound network diagram of FAA tea were constructed based on lgraph and Flavordb. As seen in Figure 8d, we selected 10 frequent flavors and 29 related differential compounds for the graph. The three differential compounds with ROAV \(\geq\) 1 are the key flavor compounds: 1-hexanol, benzeneacetaldehyde, and 2-undecanone. The content and ROAV of 1-hexanol in FAA-6 were both the highest, which was 10 times that in the lowest FAA-3, and contributed a strong green aroma to FAA-6. The high content of benzeneacetaldehyde in FAA-4 provided it with floral and honey aromas. The odor threshold of 2-undecanone was the lowest, and the ROAV value in FAA-6 was relatively high, emitting orange, fresh, and green aromas. In terms of the flavor composition, the sweet aroma contributed the most to the FAA tea flavor, followed by green, woody, and floral aromas. Fifteen volatile compounds were contributing to the sweet aroma, and nine compounds were related to the green. The woody and floral aromas were also formed by the six volatile compounds, respectively. Correlation Analyses of Antioxidant Power, Key Flavor Compounds, and Differential Non-Volatile Metabolites  
Correlation analyses were performed to investigate the relationships among antioxidant capacity, key flavor compounds, and non-volatile differential metabolites in FAA tea. As shown in Figure 9, compounds, such as D-xylitol, L-glutamic acid, honokiol, and costunolide, exhibited a highly positive correlation (r = 1) with the antioxidant activity of FAA tea.