## 3 Results
### The Metabolic Profiles in Different Fermentation Trials
It appeared that ethyl esters turned out to be the richest esters in  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Compounds**} & \multicolumn{4}{c}{**Concentration (mg/L) 1** } \\ \cline{2-5}  & **Sf** & **SP** & **SS** & **SSP** \\ \hline
**Alcohols** & & & & \\
2-Butanol & \(6.60\pm 0.80\)2  & \(-\)3 & \(0.17\pm 0.04\)3  & \(1.04\pm 0.01\)3  \\
3-Methyl-1-butanol & \(31.56\pm 1.15\)2  & \(13.82\pm 2.94\)3  & \(14.41\pm 1.18\)3  & \(2.69\pm 3.84\)3  \\
1-Pentanol & \(-\)3 & \(0.12\pm 0.02\)3  & \(0.15\pm 0.05\)3  & \(0.36\pm 0.06\)3  & \(0.14\pm 0.01\)3  & \(-\)3 & \(0.24\pm 0.04\)3  \\
1-Octene-3-ol & \(1.41\pm 0.33\)2  & \(0.25\pm 0.04\)3  & \(0.54\pm 0.11\)3  \\
2-Ethylcyclopentmeth methanol & \(0.46\pm 0.04\)3  & \(-\)3 & \(-\)3 \\
2-Octene-1-ol & \(-\)3 & \(-\)3 & \(-\)3 & \(-\)3 \\
2-Heptanol & \(-\)3 & \(-\)3 & \(-\)3 & \(-\)3 \\
1-Hexanol & \(0.46\pm 0.04\)3  & \(0.36\pm 0.05\)3  & \(0.87\pm 0.17\)3  \\
2-Octane-3 & \(-\)3 & \(-\)3 & \(-\)3 & \(-\)3 \\
1-Octene-3-ol & \(-\)3 & \(-\)3 & \(-\)3 & \(-\)3 \\
1-Octene-3- & \(-\)3 & \(-\)3 & \(-\)3 & \(-\)3 \\
2-Ethylcyclopentmeth methanol & \(0.49\pm 0.02\)3  & \(0.25\pm 0.04\)3  & \(0.98\pm 0.11\)3  \\
2-Octane-1 & \(-\)3 & \(-\)3 & \(-\)3 & \(-\)3 \\
2-Octane-3- & \(-\)3 & \(-\)3 & \(-\)3 & \(-\)3 \\
the fermentation cultures (Table 2). These compounds were mostly metabolites from yeasts, bacteria, or filamentous fungi, as well as formed through esterification of fatty acids with ethanol during the fermentation process [14], proving to be the great contributors to the pleasant floral, fruity, apple-, banana-, and pineapple-like flavors in Baijiu [15]. _S. fibuligera_ was also a strong producer of ethyl esters. It produced thirteen different ethyl esters with a combined concentration of 9.61 mg/L in Sf fermentations, such as ethyl hexanoate (fruity), ethyl octanoate (fruity), and ethyl acetate (pineapple), which were the strongest volatile components [16]. The mixed cultures showed significantly lower levels, and ethyl caprate, phenethyl acetate, and ethyl 3-phenylpropionate were only found in Sf fermentations. Some of the esters, such as ethyl lactate, were only found in mixed fermentations, which may be accounted for by high levels of lactate in the presence of _S. cerevisiae_ or _P. acidilactici_. Three aldehydes and six ketones were identified in the cultures. Among them, only 2,5-dimethyl benzaldehyde and 2-octanone were found in all fermentations, indicating that the production of aldehydes and ketones varied among different fermentations. Aldehydes and ketones, generally derived from the degradation of amino acids and lipids in the fermentation process, highly affect the aroma in food and increase the aftertaste of Chinese Baijiu [17]. Six aromatic compounds were identified in the cultures. These are important flavor compounds in various kinds of distilling spirits, which have the characteristics of low content, low threshold, and long retention time. Aromatic compounds mainly come from the synthesis of resolvents of tannin, lignin, protein, etc. in raw materials [18]. It appeared that _S. fibuligera_ produced all these components with a combined concentration of 46.09 mg/L in Sf fermentations. Among them, 4-ethyl-2-methoxyphenyl (27.75 mg/L), contributing to fruity, sweet, floral, smoky, and rubbery aromas, was highly produced in Sf fermentations. The mixed fermentations considerably reduced the contents of 4-ethyl-2-methoxyphenol as well as 4-ethylphenol, whereas higher production of 4-hydroxy-3-methoxyrene and 2,3-dihydrobenzofuran were combined with the presence of _S. cerevisiae_ considering their significant increase in SS and SSP fermentation cultures.