## 2 Results and Discussion
### Variation Pattern of Trace Aroma Compound Concentration During Aging of Nongxiangxing Baijiu  
The Baijiu samples aged 0, 5, and 15 years analyzed in this study were produced by the same manufacturer, with no alterations in core production processes or raw materials during this period. However, variations in and production instability may have partially influenced the observed discrepancies in final flavor quality. Variations in climatic conditions significantly influence the microbial community structure and flavor substance content within cellar soil [22]. Therefore, to minimize the confounding effects of climatic variability and production instability, samples produced from distinct fermentation pits representing different batches within the same vintage year were selected as parallel replicates. These samples underwent prolonged storage under identical conditions, facilitating focused analysis and discussion pertaining to the aging process. To further investigate the evolution of trace compounds--particularly those beyond common alcohols, acids, and esters--during storage, a total of 15 _Nongxiangxing_ Baijiu samples aged 0, 3, 5, 10, and 15 years were systematically evaluated. The concentration differences of 31 compounds across these samples were visualized using heatmaps, revealing distinct clustering patterns correlated with aging duration (Figure 2). Statistical analysis revealed that the concentration changes of 15 out of 31 trace compounds were significantly correlated with aging duration. Among these, ten compounds--vanillin, 4-methyl-2-methoxyphenol, 4-ethyl-2-methoxyphenol, benzaldehyde, \(\beta\)-phenylethanol, 3-(methylsulfonyl)propan-1-ol, Nonan-4-olide, 3-(Methylsulfanyl)propanal,  
Figure 2: Concentration distribution of 31 trace aroma compounds in 15 _Nongxiangxing_ Baijiu samples aged for different years. linalool, and 2,3,5,6-tetramethyllypyrazine--exhibited a positive correlation with increasing storage time. In contrast, five compounds, (_E,E_)-2,4-decadienal, 3-hydroxybutan-2-one, octan-3-ol, (_E_)-non-2-enal, and (_E_)-2-hept-2-enal, displayed a negative correlation with prolonged aging (Figure 3). Figure 3: Average concentrations of trace aroma compounds Vanillin (**a**), 4-Methyl-2-methoxyphenol (**b**), 4-Ethyl-2-methoxyphenol (**c**), Benzaldehyde (**d**), \(\beta\)-phenylethanol (**e**), 3-(methylsulfanyl)propan-1-ol (**f**), Nonan-4-olide (**g**), 3-(Methylsulfanyl)propanal (**h**), Linaloal (**i**), 2,3,5,6-Tetramethylpyrazine (**j**), (_E,E_)-2,4-Decadienal (**k**), 3-Hydroxybutan-2-one (**l**), Octan-3-ol (**m**), (_E_)-Non-2-enal (**n**), (_E_)-2-Hept-2-renal (**o**) in 0, 3, 5, 10, and 15 year old _Nongxiangxing_ Baijiu samples and their correlation with aging time. Bars labeled with differing superscript letters (a, b, c, ab, bc) denote significant differences (_p_ < 0.05) based on ANOVA with [PERSON]’s multiple range test. Shared letters indicate non-significant pairwise comparisons. * denotes significant correlation with time (_p_ < 0.05); ** denotes stronger correlation (_p_ < 0.01). Among the compounds analyzed, vanillin exhibited the strongest correlation with aging duration, a trend consistent with prior studies in wine [23], beer [24], and rice wine [25], where vanillin concentrations were reported to increase over time. The current findings align with this established pattern. However, the concentration of 4-vinylguaiacol remained below the detection limit. This observation suggests that while 4-vinylguaiacol may undergo conversion to vanillin, the resultant vanillin levels (significantly below its odor threshold of 438.52 ug/L) were insufficient to offset the depletion of 4-vinylguaiacol, leading to its progressive reduction during aging. According to [PERSON] et al. [24], 4-vinylguaiacol undergoes oxidative conversion to vanillin as its primary metabolic pathway. This transformation is modulated by environmental factors, including oxygen availability and pH. Their findings demonstrated maximal vanillin yield at pH 10.