## 2 Results and Discussion
### Sources of Key Aroma-Active Compounds in Yongchuan Douchi  
Fatty acid is one important class of key aroma-active compounds. Four fatty acids including acetic acid, 3-methylbutanoic acid, 3-methylvaleric acid and 4-methylvaleric acid were characterized as key aroma-active compounds contributing sour, rancid aromas to Yongchuan douchi. Although these acids had relatively high concentrations (acetic acid, 535.58 \(\upmu\)g/g; 3-methylbutanoic acid, 14.21 \(\upmu\)g/g; 3-methylvaleric acid, 2.04 \(\upmu\)g/g; 4-methylvaleric acid, 3.55 \(\upmu\)g/g) and OAVs \(>\) 1 causing rancid flavor in douchi, the flavor was pleasant and mild without an overpowering pungent aroma because of the delicate interactions of some volatiles with fruity notes [25]. As for the likely origins of fatty acids, the acetic acids are the metabolic product of the carbohydrate fermentation, 3-methylbutanoic acids are derived from leucine and valine degradation and acids with six or more carbons were from the lipolysis [25; 26]. 2,3-Butanedione contributed butter, milky flavor, and had an OAV of 114, which was considered an important odorant in this study. Research showed the compound might originate from the lipolytic free fatty acids through enzymatic catabolism or antioxidants by the microflora in the sample [27]. 2-Methoxyphenol (OAV = 1317) with a savory, smoky aroma was thought to come from lignin-related phenolic carboxylic acid by thermal degradation [15]. Eugenol (OAV = 5) gave a woody, spicy odor, and the source was not clear. 3-(Methylthio)propionaldehyde (FD = 256, OAV = 229) and dimethyl trisulfide (FD = 32, OAV = 8818) were two sulfur-containing compounds found in the Yongchuan douchi sample. Sulfur-containing compounds were discovered in many fermented products and gave a great contribution to the whole aroma of samples, although they were found in relatively small amounts compared to others [28; 29; 30]. Dimethyl trisulfide may have derived from the methionine abundantly present in soybean by enzymatic or nonenzymatic breakdown [27]. In addition, heterocyclic compounds, including one pyrazine (acetylpyrazine, nutty note), one pyrone (maltol, caramel note) and one furanone (\(\gamma\)-nonanolactone, milky, coconut-like note), were characterized as predominant odorants in this study. The preparation of douchi includes several processes that can generate pyrazine compounds, which mainly involves cooking the soybeans and the drying and ageing processes. Alkyl pyrazines have been discovered generating naturally by the condensation of aminoketones as a result of the Maillard reaction and the Strecker degradation during the ageing process. Acetylpyrazine contributed a roasted, burnt aroma which has been identified in coffees [31; 32] and wines [33], but if its origin is same as alkyl pyrazines remains to be explored. The maltol and \(\gamma\)-nonanolactone were two important substances for the flavor of many foods, and could be products of the Maillard reaction. Benzeneacetaldehyde, phenylethyl alcohol and phenylacetic acid are regarded as the volatiles providing the aroma of flora and honey in flowers and foods. Benzeneacetaldehyde and 3-(Methylthio)propionaldehyde belong to Strecker aldehydes, most of which are generated by the decarboxylation and deamination reaction of corresponding amino acids during the fermentation of soybeans, and some of them are generated by the Millard reaction during the heating process [34]. Many studies have been carried out to elucidate the pathway for the synthesis of phenylethyl alcohol in bacteria and yeast. It was discovered that the yeast (Saccharomyces cerevisiae) can produce phenylethyl alcohol fromphenylalanine via benzeneacetaldehyde and phenylpyruvate [35]. In addition to yeast, phenylethyl alcohol production can be through the benzeneacetaldehyde/phenethylamine route, as well as the pathway of transcinnamic acid and phenyl lactate. Both decanal (FD = 16, OAV = 2) and methyl palmit (FD = 128, OAV = 4) have an aroma that is waxy and fatty, but the note of decanal is greener.