## 3 Results and Discussion
### Volatile Compounds in the Soybean Protein Hydrolysate Samples as Identified via GC-IMS
These results indicated that the LAB-yeast fermentation might be conductive to increasing the concentrations of VFCs in soybean protein hydrolysates. Figure 3: Topographic plots of soybean hydrolysate samples. Each colored dot on the spectrum represents a volatile compound, with red color indicating a high intensity and blue color indicating a low intensity. (**A**) 3D-topographic features of the volatile compounds (within the blue background, the red vertical lines on the left represent the reactive ion peaks); (**B**) 2D-topographic features of the volatile compounds; (**C**) 2D-topographic plot with a difference comparison model. Red and blue dots indicated that the concentrations of the compounds were higher or lower than the reference, respectively. The red vertical line at the abscissa 1.0 represents the RIP peak (Reaction Ion Peak, normalized); (**D**) Gallery plot fingerprints of the volatile compounds in soybean hydrolysate samples. Each dot represents a volatile flavor compound, with the intensity of its color indicating its concentration. The identified compounds based on the RI and drift time are shown in Table S2. Among the detected substances, aldehydes and ketones were the main compounds in control groups, whilst the primary components in the groups involving the LAB-yeast fermentation were alcohols, aldehydes, esters, and ketones. As depicted in Table S2, the relative contents of the majority of alcohols increased after the fermentation with the LAB and yeast, such as 2-methyl-1-propanol, pentan-2-ol, 3-methylbutan-1-ol, and pentan-1-ol, thereby making high contributions to the aromas of samples. It was reported that the alcohols in soy sauce are generated primarily from sugars and amino acids during the fermentation under aerobic conditions, while some alcohols are produced through the reduction of the corresponding aldehydes mediated by yeasts [38]. In this study, the raw materials used for preparing the soybean protein hydrolysate samples were the same as those for making soy sauce. Moreover, soy protein hydrolyses could promote the growth of brewer's yeast and drive fermentation, inducing faster reducing sugar consumption and higher ethanol generation [1]. This phenomenon might be associated with the relatively lower molecular weight and the more hydrophilic and electropositive amino acid residues brought in by the protein hydrolysate. Notably, the relative contents of oct-1-en-3-ol in SH-1 and SH-2 were higher than that of SH-0, which was in line with the result of GC-MS. Furthermore, the relative contents of 1-butanol in the treatment groups were lower compared with those in their corresponding control groups. These results might be related to the growth and metabolism of LAB and yeast and the possible transformation of acidic substances into esters [39]. This was further confirmed by the increase in butyl ester substances, such as butyl acetate and isoamyl acetate. The relative contents of some aldehydes increased after the LAB-yeast fermentation, such as benzaldehyde and 3-methylthiopropanol. The generation of the majority of aldehydes possibly resulted from to the oxidation of unsaturated fatty acids. Previous studies reported that benzaldehyde and 3-methylbutanal could be produced due to the degradation of phenylalanine and leucine during the Maillard reaction or Strecker reaction [40]. However, the contents of 2-methylpropanal, 3-methylbutanal, (E)-2-methyl-2-butenal, (E, E)-2,4-hexadienal, nonanal, and hexanal decreased after fermentation. Similar results were investigated by [PERSON] et al. [26], and the authors explained that the phenomenon was mainly related to the hydrolysis of proteins and the metabolite, which were the precursors of many volatile flavor substances. After fermentation, the aldehydes were further oxidized to acids or reduced to alcohols, which diminished the effect of the aldehydes. Acid compounds were the major cause of acidic odors. Only two acids were detected via GC-IMS, which resulted in fewer species than those identified via GC-MS.