## 2 Results and Discussion
### Aroma Recombination and Omission
Notably, although vanillin exhibited a strong correlation with aging duration, its concentration remained far below the odor detection threshold, rendering it imperceptible. Based on these findings, six compounds--4-methyl-2-methoxyphenyl, 3-(methylsulfanyl)propanal-ol, 3-(methylsulfanyl)propanal, \(\beta\)-phenylethanol, benzaldehyde, and linalool--were identified as key trace aroma components defining the maturation-driven sensory characteristics of _Nongxiangxing_ Baijiu. Phenolic compounds are primarily responsible for imparting woody and smoky notes in alcoholic beverages. Previous studies have suggested that their formation may be derived from the thermal degradation of lignin or the aromatization of aliphatic compounds under high-temperature conditions [26; 27]. In aged Gujinggong Baijiu, two phenolic derivatives--4-methyl-2-methoxyphenyl and 4-ethyl-2-methoxyphenyl--were identified as trace aroma compounds exhibiting significant correlations with aging duration. Notably, 4-methyl-2-methoxy2 phenol, while unreported in alcoholic beverages, has been widely documented as a key aroma contributor in smoked foods (e.g., ham) due to its smoky nuances [28]. Conversely, 4-ethyl-2-methoxyphenyl is recognized as a metabolic by-product of _Brettanomyces yeast_ in wine, enhancing complexity at moderate levels but imparting barnyard-like odors at elevated concentrations [29]. In the context of _Nongxiangxing_ Baijiu, \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**No** & **Compounds** & **Odor Descriptor** & **N/20** & **Significance 1 ** \\ \hline
1 & Vanillin & Vanilla, sweet & 3 & - \\
2 & 4-Methyl-2-methoxyphenol & Smoky & 18 & *** \\
3 & 4-Ethyl-2-methoxyphenol & Smoky & 6 & - \\
4 & Benzaldehyde & Almond & 11 & ** \\
5 & \(\beta\)-phenylethanol & Rose, floral & 13 & ** \\
6 & 3-(Methylsulfanyl)propanal-ol & Roasted meat & 16 & *** \\
7 & Nonan-4-olide & Coconut & 4 & - \\
8 & 3-(Methylsulfanyl)propanal & Musty tomato & 18 & *** \\
9 & Linalool & Floral, citrus & 11 & ** \\
10 & 2,3,5,6-Tetramethylpyrazine & Nut & 1 & - \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Omission test from complete reconstitution of aroma compounds. these phenolic compounds may shift sensory perceptions in aged samples: panelists predominantly associated them with grain-like nuances rather than the undesirable burnt notes characteristic of younger samples. This divergence underscores their role in refining the aroma profile during maturation. The concentrations of benzaldehyde and \(\beta\)-phenylethanol, two aromatic compounds, were observed to increase significantly with extended aging duration, exhibiting almond-like and rose-like aromas, respectively. This elevation in their levels was found to further enhance the maturation characteristics of Baijiu, potentially amplifying the sweet and floral flavor profile of aged samples. Notably, the role of aromatic compounds in oxidative reactions has been documented in wine studies; for instance, [PERSON] demonstrated that such compounds are closely associated with oxidation-driven transformations during wine aging [30], a mechanism that may also underpin their dynamics in Baijiu systems. Sulfur-containing compounds have been recognized as critical aroma contributors in numerous Baijiu varieties due to their exceptionally low odor thresholds, which enable them to impart intense characteristic aromas even at trace levels. Among these, 3-(methylsulfonyl)propanan-1-ol and 3-(methylsulfonyl)propanan were characterized by distinct roasted meat-like and musty tomato-like notes, respectively. These compounds were identified as key trace aroma components shaping the flavor profile of aged _Nongxiangxing_ Baijiu. Notably, their roles in alcoholic beverages parallel findings in wine studies, where sulfur compounds (e.g., methionol) have been extensively linked to methionine degradation pathways during fermentation and aging [31, 32].