## 2 Results and Discussion
### Quantification of Aroma Compounds in Baijiu Samples and Trace Compounds OAV Analysis  
Trace compounds were defined as those with concentrations typically below 0.01% of the total sample mass (i.e., <100 ppm). While the contributions of higher-concentration compounds to the overall flavor profile of _Nongxiangxing_ Baijiu had been extensively characterized, the effects of most trace compounds on odor characteristics were found to differ markedly, often exerting greater influence on the sensory properties of alcoholic products compared to their higher-concentration counterparts [14]. To systematically evaluate the role of trace aroma compounds, the concept of Odor Activity Value (OAV) was introduced. It is defined as the ratio of the concentration of a specific odorant in a sample to its odor detection threshold concentration in an appropriate matrix, which quantifies their contributions to the holistic sensory experience based on both concentration and odor detection thresholds. A total of 85 compounds were accurately quantified, with their respective calibration curves, recovery rates, and linearity ranges summarized in Supplementary Table S1. Among them, 63 compounds were subsequently screened for Odor Activity Value (OAV) calculations. Comparative ratios of OAV data between G-15 (aged) and G-0 (young) samples were computed to assess the extent of changes in trace compounds during storage maturation and to determine their contributions to the aroma profiles of both young and aged Baijiu. As presented in Table 2, absolute quantitative concentrations of trace compounds exhibited minimal changes during Baijiu aging. However, when assessing odor contribution through Odor Activity Value (OAV) calculations incorporating detection thresholds, the impact of aging became pronounced. For instance, while the concentration of \(\beta\)-damascenone increased by 3.95-fold after 15 years of aging, its absolute concentration change was merely 10 \(\mu\)g/L. This demonstrates that sensory significance cannot be inferred solely from absolute concentration variations. These findings demonstrated that the use of OAV ratios provided a more objective and rational approach to assess the impact of trace compound variations on the overall flavor profiles of aged and young Baijiu, as this metric inherently integrates both quantitative changes and aroma olfactory sensitivity. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**No**} & \multirow{2}{*}{**Compound**} & \multirow{2}{*}{**RI \({}^{\bullet}\)**} & \multirow{2}{*}{**RI \({}^{\text{b}}\)**} & \multicolumn{2}{c}{**Odor**} & \multicolumn{2}{c}{**FD \({}^{\text{c}}\)**} & \multirow{2}{*}{**Identification \({}^{\text{d}}\)**} \\ \cline{5-6}  & & & & & **Descriptor** & & **15** & **5 years** & **0 years** & \\ \hline
91 & 4-Ethyl-2-methoxyphenol & 2022 & 2048 & Smoky, clove & 32 & 32 & 16 & RI, MS, Std, Aroma \\
92 & 4-Methylphenol & 2078 & 2089 & Phenolic & 256 & 128 & 128 & RI, MS, Std, Aroma \\
93 & 4-Ethylphenol & 2195 & 2195 & Smoky & 256 & 64 & 64 & RI, MS, Std, Aroma \\
94 & Vanillin & 2599 & 2600 & Vanilla & 256 & 128 & 128 & RI, MS, Std, Aroma \\ \hline \hline \end{tabular}  
* RI (retention index) is a standardized parameter in gas chromatography defining a compound’s retention behavior relative to n-alkane standards, calculated as a logarithmic function of retention times to enable consistent compound identification across different systems. \({}^{\text{b}}\) RIL (retention index library) is a comprehensive, curated database that stores experimentally determined or theoretically predicted retention index (RI) values of chemical compounds. \({}^{\text{c}}\) FD (flavor dilution factor), defined as the highest split ratio in the LLE fraction at which the odorant could be perceived by GC-0. \({}^{\text{d}}\) Identification based on RI (retention index), MS (mass spectrometry), Aroma (odor description by comparison to the reference standards by GC-0), and Std (standards).  
\end{table} 
Table 1: \(Cont.\)