## 3 Results and Discussion
### Volatile Compounds Identified in Soybean Protein Hydrolysate Samples via GC-MS
[21] pointed out that the decreases in the concentrations of aldehydes in fermented mung bean might be beneficial for the overall aroma of the fermented foods, as high concentrations of aldehydes could cause undesirable flavors. Acids are known to account for the sour flavor of foods. In this study, all the fermented soybean hydrolysates had a significant acidic smell, likely due to a large amount of acetic acid detected in these samples. Both the number of species and the concentrations of acid compounds increased greatly after the fermentation with the LAB and yeast, compared with those in the control group. Similar results were found in the fermentation of beans by _Lactobacillus plantarum_[21]. Among the identified 10 acid compounds, 2-methyl-propanoic acid (cheese), butanoic acid (cheese, fruity), and 3-methyl-butanoic acid (fruity) had significantly increased contents after the fermentation with the LAB and yeast, thereby contributing greatly to the sour flavor of the samples. Compared with the control samples, the concentrations of propanoic acid and octanoic acid also increased significantly, which might be associated with the high concentrations of alcohols available for further oxidation to acids [14]. Carbohydrate metabolism or lipid degradation in fermentation systems could lead to the production of acids. Esters are usually generated due to the non-enzymatic esterification of alcohols and organic acids or enzymatic catalysis mediated by microorganisms. The esters produced by fermentation possess low flavor thresholds, thereby contributing greatly to fermented product flavor (mostly fruity and floral flavors) [28]. As shown in Table S1, esters were the most prominent VECs, in terms of the increases in the number of species and the concentrations of compounds. The total concentrations of esters in SH-1 (54.29 \(\mathrm{\SIUnitSymbolMicro}\)/L) and SH-2 (75.85 \(\mathrm{\SIUnitSymbolMicro}\)/L) were 22 and 31 times higher than that of SH-0 (2.45 \(\mathrm{\SIUnitSymbolMicro}\)/L), respectively. The formation of esters was mainly through the enzymatic esterification of fatty acids and alcohols mediated by microorganisms during fermentation [28]. In this study, 11 esters were detected, with ethyl acetate accounting for a high proportion, which was associated with the relatively high concentration of ethanol in the fermented samples. These results were in line with previous research [14] on the role of aroma-producing yeasts in the fermentation of soy sauce. Isoamyl acetate with a low odor threshold value was generated after the fermentation, giving a banana-like fruity flavor to the samples. Interestingly, isobutyl acetate, ethyl lactate, phenethyl acetate, and \(\gamma\)-nonanolactone were also detected in the soybean hydrolysates after the fermentation. Similar results were reported by [PERSON] et al. [29]. The authors found that _W. versatilis_ exhibited a high metabolic capacity and could facilitate the formation of esters via alcohol acetyltransferasease and ester hydrolase. Ketones are generated mainly through amino acid degradation or the beta-oxidation of unsaturated fatty acids and provide tallow and burnt aromas or a pleasant fruity, floral, and fragrance aroma (for ketones with relatively long carbon chains) [30]. Some ketones can be formed via the Maillard reaction and microbial metabolism [31]. In this study, a total of 21 ketones were detected in the soybean hydrolysate samples. The primary ketones were 2-butanone, acetoin, 2-heptanone, and isophorone, which gave fruity, cool wood, and cheese flavors. Furthermore, the concentrations of some ketones, such as methyl cyclopentenolone (sweet), acetophenone (flower and almond), and 2,6,6-trimethyl-2-cyclohexene-1,4-dione (tea and nutty), increased significantly after the fermentation with the LAB and yeast. Interestingly, \(\beta\)-damascenone was also detected in the fermented soybean hydrolysates. This compound has a low threshold and thus was anticipated to have an impact on the final flavor of the fermented hydrolysates.