## 3 Results and Discussion
### Volatile Components  
Volatile flavor substances are crucial for flavor formation in fermented soybean products and are significantly influenced by microbial diversity, as well as their metabolic effect during the fermentation process. The contents of volatile compounds in the douchi samples are shown in Figure 4. A total of one-hundred volatile compounds were detected, including twelve alcohols, five phenols, thirteen aldehydes, seven acids, seven ketones, twenty-two esters, eight pyrazines, and twenty-six others (Table S2). The proportions of various volatile compounds varied when the content of each volatile component was normalized. As is shown in Figure 4a, esters and alcohols were predominant. Furthermore, Figure 4b depicts the Venn diagram of the number of volatile compounds in each douchi. Fifteen volatile compounds were common among all seven samples, and six, two, five, three, three, twelve, and seven volatile compounds were unique from DC-1 to DC-7. In addition, DC-7 (57) had the highest number of volatile compounds, followed by DC-6 (52) and DC-4 (50). DC-2 (39) and DC-5 (39) had the lowest numbers of volatile compounds (Figure 5). Figure 4: The composition of the volatile components in the douchi samples. (**a**) The relative abundances of volatile components; (**b**) Venn analysis. Figure 3: The contents of amino acids in douchi samples. (**a**) The amino acids of different flavors; (**b**) the relative abundances of amino acids. From the heatmap visualization of the flavor substances in the douchis (Figure 5), DC-2, DC-5, and DC-1 were grouped together, while DC-4, DC-7, and DC-3 were grouped together, and DC-6 was classified in a separate category. Ethyl benzoate, ethyl phenylacetate, ethyl lactate, and ethyl hexanoate had the highest contents in DC-6. Furthermore, some olefin compounds, like myrcene, alpha-terpinene, and gamma-terpinene, were detected only in DC-6. 1-Octen-3-ol, phenylethyl alcohol, and phenethyl acetate, were high in DC-2 and DC-5, while pyrazine compounds were found to be more abundant in DC-3 and DC-4. Figure 5: Heatmap of the volatile substances in different douchis. Esters were one of the main compounds in the douchis, which is homologous to the results in the previous coverage of fermented products [33; 36]. A total of 22 esters were detected (Table S2). They were mainly produced by the esterification of free fatty acids with organic acids and alcohols in douchi as precursors [37]. Among the esters, the contents of ethyl acetate, ethyl benzoate, ethyl phenylacetate, and diethyl phthalate were the most dominant, being formed in the post-fermentation process, bringing sweet, floral, fruity, and other good flavors [38]. Alcohols were one of the main compounds in the douchis, forming during fermentation, and their synthesis is linked to the amino-acid metabolism and sugar metabolism of yeast [39]. Ethanol, 1-octen-3-ol, phenylethyl alcohol, and 3-methyl-1-butanol were observed to be the highest-abundance alcohol components, and these are often described as having mushroom-like, fruity, and honey-like flavors [40]. Ethanol was the dominant alcohol in all the douchi products; it may be relevant to adding baijiu during the mixing and the metabolism of microorganisms [24]. 1-Octen-3-ol was found to be a common flavor compound in fermented soybean products in previous studies [41], but in this study, it was only found in DC-2 and DC-5. 2,3-Butanediol is one the characteristic flavor substances in fermented products and displays a sweepty fruity aroma. These contributed greatly to the formation of the douchi flavor. In this study, eight pyrazines have been detected, such as 2-methylpyrazine, 2,6-dimethylpyrazine, 2,5-dimethyl pyrazine, 2-ethyl-6-methylpyrazine, trimethyl-pyrazine, 3-ethyl-2,5-dimethylpyrazine, and tetramethylpyrazine. Pyrazines are released from the non-enzymatic browning in cooked soybeans or microbial metabolism, which occurs during the degradation of protein into amino acids when the fermentation time begins [3; 38; 42].