## 3 Results and Discussion  
### Volatile Compounds Identified in Soybean Protein Hydrolysate Samples via GC-MS  
Typically, odor is one of the most important factors in accepting a product from the consumers perspective. The relationship between volatile compounds and odor descriptions has been well established. Thus, the identification of volatile compounds can predict the odor profile of each soybean protein hydrolysate obtained and determine the differences in volatile profiles among these hydrolysate samples. In the present study, a total of 127 VFCs were identified via SPME-GC-MS, with 86 in SH-0, 108 in SH-1, and 104 in SH-2, respectively (Figure 1A). The Venn diagrams indicated that the presence of fermentation with the LAB (_T. halophilus_ 10286) and yeast (_Z. rouxii_ 1378) greatly increased the number of species of volatile flavor compounds (compared with SH-0). Also, the number of species of volatile flavor compounds was affected by the method of adding the LAB and yeast for fermentation, simultaneous inoculation of the LAB, and yeast or sequential addition-incubation of the LAB and yeast. It has been reported that the flavor of fermented foods may be changed due to the generation of new substances or a change in the concentrations of VFCs during fermentation [22]. The distinct flavor of fermented bean products is determined mainly by some species of volatile compounds, such as alcohols, acids, esters, ketones, and aldehydes. In this study, 11 types of volatile compounds were generated during fermentation, including alcohols (23), aldehydes (17), acids (10), esters (11), ketones (21), phenols (3), sulfurs (6), pyrazines (19), furan and pyran (8), pyridine and pyrrole (3), and others (6). Figure 1B shows the changes in the concentrations of the 11 types of compounds in the different soybean hydrolysates. Cluster analysis results indicated that alcohols, acids, esters, phenols, sulfur-containing compounds, pyridine, and pyrrole, as well as furan and pyran, clustered into group I. Meanwhile, aldehydes, ketones, pyrazines, and others were categorized into group II. Compared with those in SH-0, the concentrations of the identified compounds of group I in the samples fermented with the LAB and yeast increased significantly, whereas the concentration of group II compounds exhibited a notable decrease compared to that in the control samples (\(p<0.05\)). Based on the relative contents of VFCs (Figure 1C), it was found that aldehydes, ketones, and pyrazines were predominant in the control groups. In SH-0, ketones were the most abundant species (accounting for 42.6%), followed by aldehydes (38.40%). Nevertheless, in SH-1/-2, alcohols had the highest proportion, followed by aldehydes. These results confirmed that the fermentation with the LAB and yeast greatly altered the proportions of the VFCs in the soybean hydrolysate samples. Hierarchical clustering was used to help detect the patterns of the GC-MS data to elucidate the relationships between the various VFCs and the preparation methods of soybean hydrolysate samples [12]. The red and blue areas of Figure 2 represent the volatile flavor compounds at higher and lower concentrations, respectively. Two obvious clusters are shown in Figure 2. The first cluster was SH-0, which had higher proportions of aldehydes (e.g., 3-methyl-butanal, 2-methyl-butanal, and heptanal), and ketones (e.g., 2-butanone, 4-methyl-2-pentanone, and 3-methyl-2-pentanone). The other cluster included SH-1 and SH-2, which had higher proportions of alcohols (benzyl alcohol, 2-heptanol, and 1-pentanol), acids (acetic acid, 3-methyl-butanoic acid, and 2-methyl-propanoic acid), and esters (phenethyl acetate, ethyl acetate and isoamyl acetate). These results were consistent with the finding of previous research [23] that fermentation with LAB and yeast could significantly improve the formation of VFCs. All the identified compounds are listed in Table S1. In general, the analyzed soybean hydrolysate samples differed in the number of species and concentrations of alcohol compounds.