# 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 isotopic distribution of pyrroles further confirms the diverse transformations of sugar metabolites. 2-Pyrrolecarboxaldehyde's characteristic \(\text{[M}+4]\text{:[M}+5]\approx 1\text{:}1\) in the fully labeled system, combined with the complex distribution of \(\text{[M]:[M+1]:[M+2]:[M+3]:[M+4]}\approx 2\text{:}2\text{:}1\text{:}3\text{:}1\) in the half-labeled system, suggests that it may be generated through a cyclization reaction involving the C-3 position of glucose. In contrast, the 100% fully labeled \(\text{[M+7]}\) versus half-labeled \(\text{[M+5]:[M+7]}\approx 2\text{:}1\) characterization of 2-acetylpyrrole reveals that its acetyl group may originate from a specific cleavage product of the glucose C-5 site. Particularly noteworthy is the 100% fully labeled \(\text{[M+4]}\) versus half-labeled complex distribution pattern (\(\text{[M]:[M+1]:[M+2]:[M+3]:[M+4]}\approx 3\text{:}1\text{:}1\text{:}1 \text{:}3\)) of 2,3-butanedione, an isotopic distribution feature that not only confirms that its carbon source is exclusively from glucose, but also suggests the existence of two distinct metabolic pathways: the primary pathway is a single glucose molecule degradation and the secondary pathway may involve the interaction between two glucose molecules. This dual-path metabolic mechanism provides important clues for understanding the complex transformation of sugar precursors. The results show that the formation of aroma compounds in the Maillard system is characterized by a remarkable diversity of metabolic pathways, including glucose alone (e.g., 2,3-butanedione), sugar-amino acid interactions (e.g., hexanal), and multi-pathway transformations of precursors (e.g., furfural). Isotopic tracer studies of thermally reactive systems of deuterium-labeled theanine, combined with the experimental data in Tables 2 and 3, reveal the specific contribution of theanine in the formation of aroma compounds and its metabolic transformation mechanism. In a previous study, [PERSON] [27] produced amides by heating theanine alone and compared them with the aroma-active compounds detected, highlighting that theanine may be a precursor for the volatiles that contribute to the aroma of tea during preparation. Among furans and aldehydes, furfuryl alcohol, hexanal, and furfural all showed 100% [M] in the fully labeled system, a phenomenon that suggests that the main carbon skeleton of these compounds originates from unlabeled glucose metabolites. It is noteworthy that theanine may be involved in its generation through two indirect pathways: first, theanine undergoes Strecker degradation in a thermal reaction to produce ethylamine and glutamic acid, and these degradation products form the target compounds with glucose derivatives via Maillard reaction; second, glutamic acid is directly involved in the synthesis of hexanal as an amino acid precursor. The isotopic distribution pattern of the pyrrole analogs reveals a more refined mechanism of precursor assignment. 1-Ethyl-1H-pyrrole exhibits a 100% [M + 5] signature in both the half-labeled and fully labeled systems, and this unique isotopic distribution suggests that the molecular structure has a clear dichotomous origin: the carbon skeleton of the pyrrole ring is derived exclusively from glucose metabolites (corresponding to the unlabeled carbon source). In a previous study, [PERSON] [28] further demonstrated that pyrroles can be produced by constructing a Maillard reaction model system for glucose and lysine. The nitrogen atom and some carbon atoms in the ethyl substituent, on the other hand, are derived from the ethylamine component of theanine. This finding confirms that theanine can be involved in the construction of heterocyclic compounds through the directional transfer of the N-ethylamine group in a thermally reactive system, which is manifested in the specific condensation reaction of the ethylamine component with the five-membered ring structure derived from glucose.