## 3 Results and Discussion
### Analysis of Volatile Flavor Substances in Fermentation Processes  
Volatile flavor substances in SY and KB were dissected through HS-SPME-GC/MS (Figure 2a). In total, 76 different volatile flavor substances were examined across both samples. Specifically, 68 volatile flavors were identified in the SY samples, comprising 25 esters, 14 alcohols, 9 acids, and 20 other flavor substances. In contrast, 64 flavors were identified among the KB samples, encompassing 22 esters, 13 alcohols, 9 aldehydes, 7 acids, and 13 other flavor substances. Of these, 56 volatile flavors were common to both groups. At the end of fermentation, the overall content of volatile flavor substances reached 337.50 mg/L in SY and 101.04 mg/L in KB. Esters, alcohols, and aldehydes, which together accounted for over 95% of the overall volatile flavor substances (in Figure 2a, the deeper the red color, the higher the content, while the deeper the blue color, the lower the content), were the predominant components in both groups. This finding was in accordance with previous studies that found that esters, alcohols, and aldehydes were the main flavors in chi-flavor and rice-flavor baijiu [39; 40]. Among these, the ester content in SY samples was 137.20 mg/L, significantly higher than the 49.51 mg/L found in KB at the end of fermentation. Esters can contribute fruity and floral aromas to baijiu [41]. Additionally, the alcohol content in SY samples was 178.55 mg/L, markedly higher than the 33.04 mg/L in KB samples at the end of fermentation. While alcohols primarily enhance the ester aroma, and some can exhibit floral notes, excessively high alcohol levels may lead to alcohol poisoning and brain paralysis [42]. Furthermore, the aldehyde content in SY samples was 9.12 mg/L, significantly surpassing the 1.44 mg/L in KB samples at the end of fermentation. Aldehydes can carry aromatic components from the baijiu and intensify the aroma release [43]. Consequently, it was evident that the flavor profile of SY was superior to that of KB, suggesting that the introduction of mixed strains may increase the concentration of CFB flavor compounds, particularly esters and alcohols. It is widely known that compounds with an OAV of 1 or higher contribute significantly to the overall flavor, with a higher OAV contributing more to the flavor profile of baijiu [44]. At the end of fermentation, 21 flavor compounds with OAVs of 1 or greater were identified, including ethyl acetate (B1), phenylethyl alcohol (A14), 1-octen-3-one (G2), ethyl caprylate (B8), and (E)-2-octenal (C3) (Figure 2b). Specifically, SY had 21 substances with OAVs greater than 1, comprising 12 esters, 4 alcohols, 2 aldehydes, and 2 other compounds. The top three flavor compounds by OAV were 1-octen-3-one (G2) (mushroom aroma), (E)-2-octenal (C3) (citrus aroma), and ethyl caprylate (B8) (brandy-like aroma). In comparison, KB had 17 substances with OAVs greater than 1, including 10 esters, 2 alcohols, 2 aldehydes, 1 ketone, and 2 other compounds, with the top three flavor compounds matching those of SY. Additionally, the only lactone flavor identified in this study was \(\gamma\)-nonanolactone, which enhances mouthfeel and provides a coconut scent with notable aroma stability [45]. Figure 2: Heatmap of volatile flavors (a) and volatile flavors with OAV \(>\) 1 (b) during CFB fermentation. At the end of fermentation, the concentration of (E)-2-octenal in SY was 0.7482 mg/L, nearly four times higher than in KB. This indicates that the mixed fermentation of CFB produces richer flavors and a more complex mouthfeel. Additionally, the ethyl acetate concentration in SY samples reached 34.81 mg/L, compared to just 13.50 mg/L in KB samples. The ethyl lactate content in SY was 15.86 mg/L, significantly higher than the 10.95 mg/L found in KB. These results demonstrate that mixed fermentation enhances the levels of ethyl acetate and ethyl lactate in CFB. Furthermore, the quality of CFB was assessed based on the concentrations of phenylethyl alcohol and diethyl succinate [46].