## 3 Results and Discussion
### Odor-Active Compounds Detected Using GC-O
All of these organic acids have been reported as volatiles and odor-active compounds of GSS in the published literature [4; 9; 10; 14], and they were formed as microorganism metabolic products. For example, the metabolism of lactic acid bacteria led to the production of acetic acid, propionic acid, butanoic acid, etc. [19]. The precursors of 2-methylpropionic acid, 3-methylbutanoic acid and phenylacetic acid were valine, leucine and phenylalanine, respectively; these acids could be produced as yeast metabolic products by transamination and decarboxylation [20]. Nine pyrazines (16; 17; 18; 20; 22; 25; 26; 27 and 34) were also detected as flavor compounds in six CSSs; among them, neither 3-methyl-2-isobutylpyrazine (27) nor 2-acetylpyrazine (34) had been identified in GSS as volatiles and odor-active compounds. 3-methyl-2-isobutylpyrazine was only detected in the C2 sample, and 2-acetylpyrazine was found in six samples. These pyrazine compounds could be formed by three pathways. Firstly, they might originate from the raw materials of SS, including roasted wheat and wheat bran, which contained pyrazine compounds, such as 2-methylpyrazine, 2,6-dimethylpyrazine, 2-ethyl-5-methylpyrazine, etc. [21; 22]. Secondly, they were formed by a Maillard reaction during processing; their precursors were \(\alpha\)-amino acids, carbohydrates, and \(\alpha\)-dicarbonyl compounds. Soybean was an important material for producing SS; it contained oil and 18 free \(\alpha\)-amino acids [23]. Soybean was roasted under heating before being used; oil in soybean could yield \(\alpha\)-dicarbonyl compounds upon oxidation [24]; the wheat contained carbohydrates. These substances were conducive to the Maillard reaction. Thirdly, some pyrazines were among the microbial metabolic products; for example, under the same fermentation conditions, some pyrazines, including 2,5-dimethylpyrazine, 2,6-dimethylpyrazine, 2-ethyl-5-methylpyrazine, 2,3,5-trimethylpyrazine and 2-ethyl-3,5-dimethylpyrazine, were identified as volatile constituents of the solid-state fermentation product of bacteria, but they were not detected in the solid-state fermentation product of yeast [25]. Tetramethylpyrazine could be synthesized by _Bacillus subtilis_ through the multi-step bioconversion of glucose to aceton as a precursor [26]. Seven aldehydes (3, 12; 14; 21; 32; 36 and 54) were identified as odor-active compounds. 2(3)-methylbutanal and benzeneacetaldehyde belonged to Strecker aldehyde. Not only could they be formed through the Strecker degradation of isoleucine (or leucine) and phenylalanine, but also were derived from the corresponding amino acid catabolism by the Ehrlich pathway [20]. Hexanal, octanal, nonanal and (E,Z)-2,6-nonadieneral were lipid-derived compounds. Soybean seeds contained more than 20% soybean oil, which contained monounsaturated and polyunsaturated fatty acids, such as oleic acid, linoleic acid, arachidonic acid, etc. [27]. These four aliphatic aldehydes could be derived from unsaturated fatty acid (UFA) through an oxidation reaction. Vanillin could be produced by microorganisms, such as bacteria, fungi, yeast or engineered microbial cells; its precursor was fertile acid, present in the cell wall of wheat (6.6 g/kg), which was one of the materials of SS, or lignin, which exists in soybeans and wheat. The bioconversion of ferulic acid into vanillin occurs in both aerobic and anaerobic conditions [28]. Seven ketones (7, 9; 15; 44; 47; 49 and 50) were identified as odor-active compounds; all of them have been found in GSS. There were two main pathways for the formation of 2,3-butanedione and 2,3-pentanedione. The first was that they were generated during the Maillard reaction. 2,3-butanedione was formed through the sugar degradation pathway, and its precursor was glucose. 2,3-pentanedione was produced by the sugar degradation pathway and through the further interaction of sugar degradation products with amino acids, and its precursors were glucose and L-alanine [29]. The second pathway was yeast fermentation.