## 3 Results and Discussion
### Volatile Compounds Identified in Soybean Protein Hydrolysate Samples via GC-MS
The alcohol compounds are mainly produced via the oxidation and degradation of the secondary hydroperoxides derived from polyunsaturated fatty acids, the degradation of amino acids, and the reduction from the corresponding aldehydes mediated by alcohol dehydrogenases [24]. While some straight-chain alcohols have minimal aroma, many alcohols (especially aromatic and aliphatic alcohols with relatively long chains) give aromas, such as menthol (peppermint), ethyl maltol (cooked fruit), and furaneol (strawberry) [13]. Among the detected 23 alcohol compounds, 3-methyl-1-butanol, 2-methyl-1-butanol, ethanol, and 1-octen-3-ol were the dominant compounds. Compared with SH-0, large increases in the concentrations of these alcohol compounds were found in the samples subjected to the fermentation with the LAB and yeast. Fermentation also caused small increases in the concentrations of 1-hexanol, phenol, and phenylethyl alcohol, which gave alcoholic, malty,floral, mushroom-like, and fruity aromas [13]. Alcohols with low odor thresholds are known as one type of key contributors to the flavor of fermented soybean hydrolysates, giving pleasant fruity and floral aromas. Interestingly, some new VFCs, including 2-methyl-1-propanol, 1-heptanol, 2-heptanol, and 4-methyl-1-pentanol, were produced after the fermentation. These compounds have been associated with green, nutty, and alcoholic flavors. [PERSON] et al. [14] reported that the concentrations of ethanol and phenylethyl alcohol were significantly higher in the fermented soy samples with yeasts added during fermentation, compared with those in the control group. Besides their direct contributions to product flavor (e.g., giving specific aromas), alcohols can make indirect contributions to product flavor by interacting with acids to produce esters, another important class of aroma compounds. Figure 1: Volatile flavor compounds in the soybean protein hydrolysates obtained via different processes (**A**) Venn diagram; (**B**) cluster heatmap of different categories of volatile flavor compounds based on the GC-MS data after standardization. Darker red and darker blue, respectively, indicate higher and lower concentrations of volatile flavor compounds; (**C**) changes in the relative contents of volatile flavor compounds. Aldehydes are mainly derived from lipid oxidation, degradation of precursor amino acids, and conversion of alcohols, giving pleasant aromas, such as grass, cheese, malt, and fruit flavors [25]. In this study, a total of 17 aldehydes were identified, with 2-methyl-propanal, 3-methyl-butanal, and 2-methyl-butanal being the main components. The concentrations of these aldehydes were abundant in the control samples, and fermentation with the LAB and yeast drastically decreased the concentrations of these aldehydes. These  
Figure 2: Heat map and hierarchical cluster analysis of the volatile flavor compounds in different soybean hydrolysate samples based on the GC-MS data after standardization. Darker red and darker blue respectively indicate higher and lower concentrations of volatile flavor compounds. findings were in line with previous results [26]. Aldehydes are known as inherently unstable compounds and can easily be reduced to alcohols or oxidized to acids in food systems, particularly in the presence of microorganisms [27]. This might partially account for the decreased contents of aldehydes and the increased contents of alcohols for the fermented soybean hydrolys. However, the concentrations of some aldehydes increased after the fermentation with the LAB and yeast, such as benzaldehyde (marzipan, almond, and nutty flavor), 3-methyl-2-butenal (sweet fruity, pungent, nutty with an almond and cherry nuance), hexanal (green, grassy), benzeneacetaldehyde (floral, fruity), and nonanal (beany, fatty, grassy). These aldehydes may be associated with the slight degradation of relevant amino acids and have low odor threshold values, thereby contributing greatly to the flavor of samples. [PERSON] et al.