## 3 Results and Discussion
### Analysis of GC-IMS
#### 3.2.2 Fingerprints Analysis  
For a clearer comparison of the volatile compounds during the different processing stages, signals of the volatile compounds were selected to develop a comprehensive spectral fingerprint using a gallery plot plug-in ([PERSON] et al., 2021) (Figure 5). In Figure 5, rows represent the signal peaks from the samples and columns represent the signal peak of the same volatile compounds in different samples. As shown in Figure 5, obvious differences among the samples were clearly observed. The parts with obvious differences in fingerprints were detected and marked as \"a\", \"b\", \"c\" and \"d\" respectively. These distinct regions indicated that the flavor of the samples had markedly changed during processing. From Table 2 we could see the names of compounds in regions a, b, c and d.  
Figure 4: GC-IMS two-dimensional qualitative topographic plot (**A**) and topographic plot of the four processing stages (**B**). The contents of substances in area \"a\" were lowest in raw chicken, gradually increasing after frying and the stewing stage, and highest in the stewing stage. These mainly included ethyl-3-methylbutanoate, methyl-2-methylbutanoate, ethyl butanoate, ethyl pentanoate, Z-3-hexenol, butanoic acid, etc. The content of substances in area \"b\" were detected in the highest contents in raw chicken, i.e., mainly 2-methylpropionic acid, hydroxyacetone, pentan-2 3-dione, 2-pentanone, isoamyl alcohol, 2-methylpyrazine, hexanal, P-xylene, etc. The peroxidation of fatty acid may promote the production of hexanal. These substances disappeared after thermal processing and could be regarded as characteristic volatile compounds in raw chickens. Substances in region \"c\" were more abundant in raw chicken and fried chicken samples but lower after the stewing stage, such as 1-butanol, 3-methyl-3-butenol, 2-hexenol, 2-methyl pyrazine, propyl acetate, benzaldehyde, pentanol, heptanal, etc. The volatile compounds in part \"d\" mainly appeared during braising, especially after braising for 1 h, including ethyl-3-methylbutanoate, phenylacetaldehyde, 2-penthyl \begin{table}
\begin{tabular}{c c c c} \hline \hline
**a** & **b** & **c** & **d** \\ \hline Ethyl-3-methyl butanoate & 2-Methylpropionic acid & 1-Butanol & Hydroxyacetone \\ Methyl-2-methyl butanoate & Ethanol & 3-Methyl-3-butenol & Ethyl-3-methylbutanoate \\ Ethyl butanoate & Hydroxyacetone monomer & N-nitrosodimethylamine & Phenylacetaldehyde \\ Ethyl pentanoate & Pentan-2 3-dione & 2-Hexenol & 2-Pentyl-furan \\ Z-3-hexenol & 2-Pentanone & 2-Methyl pyrazine & Ethyl propionate \\ Butanoic acid & Isoamyl alcohol & Propyl acetate & Alpha-pinene \\ Pentanoic acid & 2-Methylpyrazine & Benzaldehyde & 2-Methocyphenol \\ Hexanal & Pentanol & Acetophenone \\ P-xylene & Heptanal & Isovaleric acid \\ Hexanol & & 2-Octanone \\
2-Furanmethanol-furfuryl & & & 2 3-Butanediol \\ alcohol & & & 2-Hexanone \\ Ethyl pentanoate & & & 3-Methylpentanoic acid \\  & & & nonanal \\  & & 2-Phenylethanol \\  & & Phenol-4-methyl \\  & & 1-Octanol \\  & & 1 2-Furanly-ethanoneone \\  & & 2-Nonanone \\ Alpha-pinene & & \\ \hline \hline \end{tabular} Note: The numbers represent the names of compounds that could not be accurately identified. The names of compounds are listed from left to right in regions a, b, c and d.  
\end{table} 
Table 2: The names of compound in Figure 5 in regions “a, b, c and d”. Figure 5: Fingerprint spectra (C) of volatile compounds in Daokou braised chicken at different processing stages. (Note: The numbers represent compounds which could not be accurately identified). furan, ethyl propionate, A-pinene, 2-methoxyphenol, acetophenone, isovaleric acid, 2-octanone,2,3-butanediol,2-hexanone,3-methylpentanoic acid, nonanal, 2-phenylethylanol, phenol-4-methyl, 1-octanol, 1,2-furanyl-ethanoneone, and 2-nonanone, while the contents of these compounds in raw chicken and fried chicken were extremely low.