# Formation Mechanisms and Kinetic Modeling of Key Aroma Compounds During Qidan Tea Roasting
## 3 Results and Discussion
### Labeling Glucose to Infer the Source of Arona Compounds  
The study of the metabolic pathways of glucose and theanine precursors by stable isotope labeling technique revealed the carbon source specificity and biosynthetic mechanisms of different aroma compounds (Tables 2 and 3). Analysis of the isotopic distribution patterns of various aroma compounds showed that there were significant differences in their precursors. \begin{table}
\begin{tabular}{c c c c c c c c c c c c} \hline \hline \multirow{2}{*}{**No. **} & \multirow{2}{*}{**Compounds**} & \multirow{2}{*}{**m/z**} & \multirow{2}{*}{**Molecular**} & \multirow{2}{*}{**System**} & \multicolumn{4}{c}{**\({}^{13}\)C-**} & \multicolumn{4}{c}{**Relative Distributions of Isotopologues (\%)**\({}^{\text{c}}\)} \\  & & & **mal** & & **b** & **Labeled** & **M** & **M + 1** & **M + 2** & **M + 3** & **M + 4** & **M + 5** & **M + 6** & **M + 7** \\ \hline
1  
Among the aldehydes, 3-methylbutylaldehyde and benzaldehyde both showed 100% unlabeled [M] in the fully labeled/half-labeled system, suggesting that their carbon skeletons may be entirely derived from unlabeled sources or that there are specific metabolic bypasses, implying that these two aldehydes may not be involved in the conventional metabolic pathways of labeled precursors. In contrast, hexanal was close to a 1:1 ratio of [M + 3]:[M + 5] in the fully labeled system and close to a [M]:[M + 3]:[M + 5] = 1:1:1 distribution in the half-labeled system, confirming that it can be generated via the Maillard reaction involving glutamic acid/theanine and contains at least one carbon atom of amino acid origin. Furfuryl compounds exhibit a complex carbon source profile. Furfuryl alcohol did not detect [M] in the fully labeled system, and the half-labeled system presented [M]:[M + 5] = 1:1, which, combined with its fully labeled [M + 4]:[M + 5] = 2:3, suggested the existence of a dual pathway of glucose metabolism and amino acid alcoholization. The distribution pattern of furfural with the predominance of the fully labeled system [M + 5] (100%) and the half-labeled system [M]:[M + 4]:[M + 5] \(\approx\) 1:1:1 confirms that it can be formed through multiple pathways such as dehydration of pentose sugar, decomposition of hydroxymethylfurfural, and cyclization of Strecker's degradation products, which is consistent with the dual pathway mechanism of generation proposed by [PERSON] et al. (2015). The isotopic distribution pattern of pyrazines reveals their unique carbon and nitrogen source mechanisms. The basic pyrazine structure presents 100% [M + 4] in the fully labeled system and [M]:[M + 2]:[M + 4] = 2:5:3 in the half-labeled system, which, combined with the structural features of its nitrogen-containing aromatic ring, suggests that the carbon skeleton originates exclusively from the C-2 position of glucose while the nitrogen atom is derived from the amino acid ([PERSON] et al., 2015). Notably, the pyrazine derivatives with different substituents show specific cleavage patterns: the cleavage feature at the C-2/3 position of 2-methylpyrazine,