## 3 Results and Discussion
### Volatile Compounds in the Soybean Protein Hydrolysate Samples as Identified via GC-IMS  
The differences in VFCs among the soybean hydrolysate samples were examined further via GC-IMS, which combined the high separation efficiency of gas chromatography with the fast response and high sensitivity of ion mobility spectrometry. The drift time is a useful parameter reflecting the mass and geometric structure of the ions of a chemical and can be determined based on the normalization of the ion migration time and reactive ion peak [20]. The data are presented through 3D and 2D topographical visualization in Figure 3. As shown in Figure 3A, the 3D-topographic plots of all soybean hydrolysates showed some similarities but also some differences. The majority of the signals were detected based on a retention time from 100 s to 800 s and drift time range of 1.0-2.0. The 2D-topographic plots (Figure 3B,C) also showed the significant differences in the corresponding signal intensity among the VFCs in the three groups of soybean hydrolysates. For the investigation of the aromatic component characteristics in several groups, the difference comparison model of the internal software was used. The topographic plot of SH-0 was selected as a reference and subtracted from the plots of other samples. With the retention time changing from 100 s to 800 s; scattered red areas were found in the two groups, suggesting that these VFCs had higher concentrations than those of the reference. The concentrations of quite a few VFCs in SH-1 and SH-2 were higher, compared with those in SH-0. These results suggested that the different soybean hydrolysates had different aroma profiles, and the fermentation with the LAB and yeast generally increased the concentrations of the volatile component in the hydrolysate samples. Figure 3: \(Cont.\)  
To further evaluate the differences in VFCs among the soybean hydrolysate samples, Gallery plot was used to draw fingerprint plots (Figure 3D). Each column represents a signal peak of a volatile compound in different hydrolysate samples, whilst each row represents all the signal peaks selected in a hydrolysate sample. Approximately four areas could be categorized to reflect the characteristics of the differential VFCs among different hydrolysate groups based on the color degree of each point and statistical analysis results of the VFC peak intensities. The fingerprints showed excellent reproducibility. A total of 47 VFCs were detected in all groups, among which 37 were successfully identified based on the GC-IMS library and NIST database. Several compounds displayed two peaks (which corresponded to their monomer and dimer forms), with the dimer eluted out after the monomer. The monomer and dimer were considered one compound in this investigation. These compounds could be separated into nine types: 8 alcohols, 11 aldehydes, 2 acids, 3 esters, 10 ketones, 1 sulfide, 1 pyrazine, 1 furan, and 10 other compounds. As shown by the red boxes in Figure 3D, higher concentrations of many VFCs were observed in the hydrolysate samples compared to those in SH-0, such as 2-methyl-1-propanol, pentan-2-ol, 3-methylbutan-1-ol, oct-1-en-3-ol, 3-methylthiopropanol, ethyl acetate, isoamyl acetate, 2-pentanone, and 3-hydroxybutan-2-one. All these VFCs gave specific aromas to the hydrolysate samples. However, as exhibited in the green boxes of Figure 3D, the relative contents of some VFCs decreased after the LAB-yeast fermentation, including 2-methylpropanal, 3-methylbutanal, hexanal, nonanal, 2-hexanone, and methyl isobutyl ketone. All the VFCs with decreased contents after fermentation were mainly aldehydes and ketones, which was consistent with the finding of previous research [21] that the contents of some aldehydes (nonanal, octanal, 2-furfural, and 3-methylbutanal) and ketones (acetophenone) in fermented mung beans decreased significantly or even disappeared. Such decreases in relative contents might be associated with the reduction reaction or decomposition induced by microbial fermentation.