# HS-SPME Combined with GC-MS/O to Analyze the Flavor of Strong Aroma Baijiu _Daqu_
## 3 Results
### Analysis of Daqu at Different Maturity Stages by HS-SPME Combined with GC-MS
\begin{table}
\begin{tabular}{c c c c c c c c c c c} \hline \hline \multirow{2}{*}{**No.**} & \multirow{2}{*}{**Compound**} & \multirow{2}{*}{**CAS**} & \multirow{2}{*}{**RI**} & \multirow{2}{*}{**Identification a**} & \multicolumn{6}{c}{**Area Percentage \% b**} \\ \cline{5-10}  & & & & & \multicolumn{3}{c}{**S-0D**} & \multicolumn{1}{c}{**S-4D**} & \multicolumn{1}{c}{**S-6D**} & \multicolumn{1}{c}{**S-20D**} & \multicolumn{1}{c}{**S-3M**} \\ \hline \multicolumn{1}{c}{Furans} & & & & & & & & & & \\ \cline{5-10}
115 & \multicolumn{2}{c}{5-Methyl-2-} & \multirow{2}{*}{3857-25-8} & \multirow{2}{*}{1726} & \multirow{2}{*}{MS,RI} & \multirow{2}{*}{—} & \multirow{2}{*}{—} & \multirow{2}{*}{—} & \multirow{2}{*}{—} & \multirow{2}{*}{0.100} & \multirow{2}{*}{0.108} \\  & \multicolumn{2}{c}{} & & & & & & & & \\ \cline{5-10}
116 & \multicolumn{2}{c}{3-Methyl-2(SH)-} & \multirow{2}{*}{2212-36-7} & \multirow{2}{*}{1732} & \multirow{2}{*}{S,MS,RI} & \multirow{2}{*}{—} & \multirow{2}{*}{—} & \multirow{2}{*}{—} & \multirow{2}{*}{—} & \multirow{2}{*}{0.170} & \multirow{2}{*}{0.200} \\  & \multicolumn{2}{c}{} & & & & & & & & \\ \cline{5-10}
117 & \multicolumn{2}{c}{3-Phenylfuran} & \multirow{2}{*}{13679-41-9} & \multirow{2}{*}{1861} & \multirow{2}{*}{MS,RI} & \multirow{2}{*}{—} & \multirow{2}{*}{—} & \multirow{2}{*}{0.028} & \multirow{2}{*}{0.090} & \multirow{2}{*}{0.066} & \multirow{2}{*}{0.031} \\  & \multicolumn{2}{c}{} & & & & & & & & \\ \cline{5-10}
118 & \multicolumn{2}{c}{Dibenzofuran} & \multirow{2}{*}{132-64-9} & \multirow{2}{*}{2277} & \multirow{2}{*}{S,MS,RI} & \multirow{2}{*}{3.286} & \multirow{2}{*}{0.531} & \multirow{2}{*}{0.794} & \multirow{2}{*}{1.425} & \multirow{2}{*}{0.661} & \multirow{2}{*}{0.710} \\  & \multicolumn{2}{c}{} & & & & & & & & \\ \cline{5-10}  
\\ \hline \hline \end{tabular}  
*: NIST 11 was used to qualitative analysis, with matching degree \(\geq 80\); RI: retention index; S: standard substance was used to qualitative analysis; b: The area normalization method was used to express the relative content of the compounds. MS: compounds identified by MS spectra.  
\end{table} 
Table 1: _Cont._  
Figure 3: Types and changes of volatile components in _Dau_ at different maturity stages (**A**); PCA score plots of different maturity stages _Dau_ (**B**). Esters, which account for 60% of all flavor substances in baijiu, play an essential role in baijiu [22]. It can be seen from Table 1 and Figure 3A that the ester compounds in _Daqu_ (S-4D) after early fermentation increased significantly, and the relative peak area reached 43.381%. With the fermentation, the relative peak areas of esters in S-10D, S-22D and S-3M were more than 50%, the kinds of esters increased, and the content of long-chain fatty acid esters such as ethyl pentadecanoate, ethyl octadecenoate, and ethyl hexadecanoate increased significantly. The relative content of nitrogen-containing substances also increased significantly with the extension of fermentation time. After storage, the range of nitrogen-containing substances in _Daqu_ reached 7.228%. Nitrogen-containing substances significantly increased after a slow rising period (about S-10D) compared with ester compounds. Among them, the content of tetramethylpyrazine changed the most, which was the most nitrogen-containing compound in _Daqu_ after storage. This is because nitrogen-containing substances were mainly produced by the Maillard reaction. With the fermentation, the fermentation temperature of medium high temperature _Daqu_ could reach 50-60 \({}^{\circ}\)C. In the high temperature transformation stage of no less than one week, amino acids and proteins in wheat undergo the Maillard reaction to produce a variety of pyrazines (one of the nitrogenous compounds) [23]. The compounds mainly show a roasting and nut-like aroma, which might contribute to the flavor of the finished _Daqu_. Nitrogen-containing substances in _Daqu_ might be brought into baijiu. The number of aldehydes and ketones decreased first and then increased.