## 3 Results
### The Metabolic Profiles in Different Fermentation Trials  
In this study, headspace solid phase microextraction combined with gas chromatography-mass spectrometry (HS-SPME-GC-MS) was used for identification and quantification of the volatile components in different fermentation trials. The grouping of these detected compounds depended on the chemical structures, which consisted of alcohols, esters, acids, aldehydes, ketones, and aromatic compounds (see Table 2). It was shown that alcohols as the largest class of aroma components were found in all fermentation cultures. They could be produced mainly from sugars in aerobic conditions, as well as by yeasts from amino acids in anaerobic conditions or by chemical reduction of corresponding aldehydes [11]. _S. fibuligera_ turned out to be a high generator of higher alcohols. Twelve different higher alcohols were produced, such as isobutyl alcohol, phenethyl alcohol, 1-butanol, and isoamyl alcohol, and the combined concentration reached 71.91 mg/L in Sf fermentations (Table 2). It is known that these by-products are of great importance to the aroma and taste of Baijiu, increasing the sweetness and aftertaste of the liquor [12]. It appeared that higher alcohols were significantly reduced in the mixed fermentations with the presence of _S. cerevisiae_ or _P. acidilactici_; although _S. cerevisiae_ was expected to produce more alcohol, this difference was mainly accounted for by the contents of phenethyl alcohol, isoamyl alcohol, and 2-butanol. In addition, 1-pentanol was only undetected in Sf fermentation. Furthermore, 2-heptanol and 2-octanol were only detected in SS and SSP fermentation cultures. An interesting result was the higher production of alcohols in triple culture when compared to double culture. Two acids, including isobutyric acid and caproic acid, were identified in Sf and SP fermentation cultures with low levels. A slight increase was observed in the contents of acids with the presence of _S. cerevisiae_, and butyric acid was found in SS and SSP fermentation cultures. Volatile acids play important roles in the aroma of Baijiu. Components such as caproic acid and octanoic acid increase cheesy, sweaty, and rancid odors; isobutyric acid and butyric acid increase sweaty and acid odors. At the appropriate concentrations, the acids and other aroma compounds jointly form the unique flavor of Baijiu. Moreover, acids as the precursors of esters could significantly affect their production [13]. Figure 3: The contents of acetic acid and lactic acid in different fermentation trials. The results shown are means of three replicates. The bars correspond to the estimated average levels. The error bars show standard errors of the mean values. Different letters represent significant difference (\(p<0.05\)) of acetic acid or lactic acid content. Sf = _Saccharomyces fibuligera_ 12, SS = _Saccharomyces cerevisiae fibuligera_ 12/_Saccharomyces cerevisiae_ 3, SP = _Saccharomyces cerevisiae_ 12/_Pediococcus acidilactici_ 4, SSP = _Saccharomyces cerevisiae_ 12/_Saccharomyces cerevisiae_ 3/_Pediococcus acidilactici_ 4. Esters, as another large group of volatile compounds, were observed in different fermentation cultures. They play important roles in the aroma of not only natural foods (such as fresh fruits) but also fermented foods (like Baijiu).