# Nutrition, Flavor, and Microbial Communities of Two Traditional Bacterial Douchi from Gansu, China
## 3 Results and Analysis
### Analysis of Volatile Flavor Compounds in the Two Types of Douchi
#### 3.2.2 Identification of Key Flavor Compounds Using ROAV  
In identifying the compound that contributes the most to the flavor of the douchi, when the concentration of the compound is fixed, the sensory threshold is inversely proportional to the degree of perceptual sensitivity; namely, the lower the compound threshold, the easier it is to perceive. When the sensory threshold of a compound is fixed, the concentration is directly proportional to the degree of perceptual sensitivity; namely, the higher the compound concentration, the easier it is to perceive [26]. The results of an analysis of the odor threshold of volatile compounds and ROAV calculations are shown in Table 5. In Longnan douchi, the relative content of ethyl isobutyrate was 4.36%, and the flavor threshold value was 0.1 ug/kg, which contributed the most to the overall flavor of Longnan douchi. The results show that there were five main flavor compounds (ROAV \(\geq\) 1) in Longnan douchi, namely, ethyl isobutyrate, ethyl butyrate, isoamyl acetate, ethyl hexanoate, and 1-octen-3-ol, and there were three compounds that played a modifying role in the overall flavor of Longnan douchi (0.1 \(\leq\) ROAV \(<\) 1), namely, \begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**No.**} & \multirow{2}{*}{**Category**} & \multirow{2}{*}{**Compound Name**} & \multicolumn{2}{c}{**Relative Content (\%)**} \\ \cline{3-4}  & & & **LN Douchi** & **QY Douchi** \\ \hline \multicolumn{4}{c}{**Others**} \\ \hline
73 & & Acetonitrile & - & 0.16 \(\pm\) 0.09 \\
74 & & Dimethyl disulfide & - & 1.19 \(\pm\) 0.15 \\
75 & & (2E,6E)-4,5-Dimethyl-2,6-octadiene & 0.11 \(\pm\) 0.02 & - \\
76 & & Dimethyl trisulfide & 0.14 \(\pm\) 0.01 & - \\
77 & & Ligustrazine & 13.40 \(\pm\) 0.35 a & 2.44 \(\pm\) 0.36 b \\ \hline \hline \end{tabular} Note: “-” not perceived; letters indicate [PERSON]’s pairwise differences among different samples (\(p<0.05\)).  
\end{table} 
Table 4: _Cont._ethyl heptanoate, 3-octanone, and 2,4,5-trimethylthiazolople. In Qingyang douchi, 1-octen-3-ol, with a relative content of 1.38% and a flavor threshold of 1 \(\upmu\)g/kg, contributed the most to the overall flavor of the Qingyang douchi. There were four main flavor compounds (ROAV \(\geq\) 1) in Qingyang douchi, namely, 1-octen-3-ol, 2,5-dimethylpyrazine, 2-pentylfuran, and 2,4,5-trimethylthiazolople. There were nine substances (0.1 \(\leq\) ROAV \(<\) 1) that contributed to the modification of the overall flavor of Qingyang douchi, namely, 1-penten-3-ol, 2-butanone, 2-pentanone, 2-methylpyrazine, 2,6-dimethylpyrazine, 2,3-dimethylpyrazine, 2-ethylpyrazine, 2-ethyl-5-methylpyrazine, and 2,3,5-trimethylpyrazine. The compounds playing a flavor role in Longnan douchi were detected less in Qingyang douchi, and compounds playing a flavor role in Qingyang douchi played a smaller role in Longnan douchi. According to the ROAVs of volatile flavor compounds, the key flavor compounds in Longnan douchi mainly consisted of esters, whereas the key flavor compounds in Qingyang douchi mainly consisted of pyrazines. Among them, 1-octen-3-ol was the main flavor compound shared by the two types of douchi; it is a typical bean flavor compound with a mushroom flavor [33]. Ethyl isobutyrate has a strawberry-like odor [28], ethyl butyrate has a pineapple flavor and a floral aroma [29], isoamyl acetate has a banana flavor [30], ethyl hexanoate has an apple-like flavor [31], 2,5-dimethylpyrazine has a baking flavor [34], 2-pentylfuran has a legume and grassy flavor [35], and 2,4,5-trimethylthiazolople has an earthy flavor [36]. These flavor compounds are usually produced by the metabolism of microorganisms in douchi during