## 3 Results and Discussion
### Volatile Compounds Identified Using GC-MS  
Volatile compounds in different fermentation stages of Laobaign _Baijiu_ were identified by HP-SPME-GC-MS. As Table 2 reveals, 45 volatile compounds were identified totally in the samples, which consist of 10 alcohols, 17 esters, 7 aldehydes, 4 ketones, 3 acids, 1 furan, 1 aromatic, 1 lactone, and 1 sulfur-containing compound. Among them, ethyl acetate, ethyl lactate, and ethyl isovalerate, defined as the typical core volatile compounds of Laobaign _Baijiu_, have also been found in other aromatic flavors of _Baijiu_[24]. During the fermentation of Laobaign _Baijiu_, the overall quantity of volatile compounds exhibited a gradual increase over time, with specific categories of volatile components undergoing alterations at distinct stages. The main substances in Laobaign _Baijiu_ were alcohols (>10), with a mass fraction of 66.78% of the total volatile compounds. Among them, ethanol, 3-methylbutanol, 1-hexanol, and 1-propanol were the main alcohol compounds. The content of ethanol and 3-methylbutanol increased gradually and reached a peak at stage E-6. This was associated with Saccharomyces always being the main dominant genera. As fermentation proceeded, the content of ethanol was basically stabilized due to inhibition from other metabolites and consumption involved in other physiological metabolic activities, such as esterification [25]. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**No. **} & \multirow{2}{*}{**Compounds**} & \multicolumn{6}{c}{**Concentration (ug/L) 2**} \\ \cline{3-8}  & & **D-1** & **D-2** & **D-3** & **E-4** & **E-5** & **E-6** \\ \hline \multicolumn{8}{c}{Alcohols (10)} \\
1 & Ethanol & 3.341 \(\pm\) 0.045 a  & 29.893 \(\pm\) 0.104 a  & 38.780 \(\pm\) 0.034 a  & 44.657 \(\pm\) 0.141 a  & 47.785 \(\pm\) 0.036 a  & 53.451 \(\pm\) 0.007 a  \\
2 & 1-Propanol & ND 1 & 1.023 \(\pm\) 0.031 b  & 0.934 \(\pm\) 0.003 d  & 1.178 \(\pm\) 0.002 a  & 1.309 \(\pm\) 0.004 b  & 0.824 \(\pm\) 0.003 a  \\
3 & 2-Methylpropanol & ND & 0.102 \(\pm\) 0.009 d  & 0.092 \(\pm\) 0.006 d  & 0.131 \(\pm\) 0.002 d  & 0.182 \(\pm\) 0.002 a  & 0.114 \(\pm\) 0.001 a  \\
4 & 3-Methylbutanol & 2.012 \(\pm\) 0.014 a  & 20.438 \(\pm\) 0.122 a  & 22.091 \(\pm\) 0.051 b  & 23.598 \(\pm\) 0.084 c  & 24.781 \(\pm\) 0.006 d  & 26.900 \(\pm\) 0.002 a  \\
5 & Pentanol & ND & 0.0934 \(\pm\) 0.011 a  & 0.101 \(\pm\) 0.004 d  & 0.112 \(\pm\) 0.009 d  & 0.131 \(\pm\) 0.003 d  & 0.113 \(\pm\) 0.001 d  \\
6 & 1-Hexanol & 0.207 \(\pm\) 0.011 b  & 0.903 \(\pm\) 0.016 a  & 1.238 \(\pm\) 0.013 a  & 1.301 \(\pm\) 0.003 a  & 1.526 \(\pm\) 0.003 a  & 1.290 \(\pm\) 0.005 a  \\
7 & 1-Octanol & ND & 0.112 \(\pm\) 0.073 d  & 0.320 \(\pm\) 0.005 d  & 0.368 \(\pm\) 0.005 b  & 0.520 \(\pm\) 0.003 a  & 0.392 \(\pm\) 0.002 a  \\
8 & 1-Nonanol & ND & 0.089 \(\pm\) 0.016 d  & 0.134 \(\pm\) 0.002 d  & 0.147 \(\pm\) 0.006 d  & 0.187 \(\pm\) 0.000 d  & 0.161 \(\pm\) 0.002 c  \\
9 & Benzyl alcohol & ND & 0.045 \(\pm\) 0.003 c [FOOTNOEsters were the most important volatile compounds, which had a great influence on the quality of Laobaign _Baijiu_, and a total of 17 ester compounds were quantified. The content of these esters accumulated dramatically at stage D-3 and increased gradually up to the highest (42.452 ug/L) at stage E-6. This was associated with the relative abundances of Candida, Aspergillus, Pichia, and other fungal increasing dramatically at stage E-3. There was a small decrease at stage E-4 due to the fact that the addition of _Daqu_ at the beginning of _Ercha_ fermentation had an effect on the composition of the fermentation substrate. Candida and Pichia have been reported to have efficient ester production abilities. A large quantity of ester compounds was produced via condensation reaction in the presence of esterase [26]. Usually, the ester compounds have the characteristics of a fruity aroma. It is the most important volatile substance in _Baijiu_, which determines its aromatic flavor type and quality [27].