## 3 Results and Discussion
### Analysis of GC-MS
#### 3.1.1 Volatile Compounds in Different Processing Stages
Ketones may arise from the oxidation of fat. The relative content of ketones increased with processing, and increased significantly in the stewing stage (\(p<0.05\)) This indicates that the thermal processes had a positive effect on the formation of ketone compounds. The relative contents of 2,3-Octanedione and 3-Hydroxy-2-butanone were higher than those of other ketones, which may contribute to harmonizing the flavor of Daokou praised chicken. The proportion of aldehydes was higher in each stage of processing, especially nonanal, hexanal, octanal and heptanal, which played important roles in the characteristic flavor because of their low odor thresholds [24]. Although the content of enal and dienal were lower, they were characteristic flavor compounds owing to their extremely low thresholds when chicken fat was heated [25]. The Strecker degradation of leucine may promote  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**NO. ** & **Compounds** & \multicolumn{4}{c}{**Relative Percentage Content (\%)**} \\  & & **L1** & **L2** & **L3** & **L4** \\ \hline
44 & Ethylbenzene & 0.06 \(\pm\) 0.01 c & 0.12 \(\pm\) 0.01 c & 0.36 \(\pm\) 0.04 a & 0.24 \(\pm\) 0.03 b [FOOTNOTE:c]Footnote c: \(\equiv\) 0.05, 2.61 \(\pm\) 0.01 c & 0.36 \(\pm\) 0.04 a & 0.24 \(\pm\) 0.03 b [FOOTNOTE:c]Footnote c: \(\equiv\) 0.05, 2.61 \(\pm\) 0.01 c & 0.36 \(\pm\) 0.04 a & 0.24 \(\pm\) 0.03 b [FOOTNOTE:b]Footnote c: \(\equiv\) 0.05, 2.61 \(\pm\) 0.01 c & 0.36 \(\pm\) 0.04 a & 0.24 \(\pm\) 0.03 b [FOOTNOTE:b]Footnote b: \(\equiv\) 0.05, 2.61 \(\pm\) 0.01 c & 0.36 \(\pm\) 0.04 a & 0.24 \(\pm\) 0.03 b [FOOTNOTE:b]Footnote b: \(\equiv\) 0.  
the production of 3-Methyldil-butanal [26]. Additionally, 3-Methyl-butanal was detected in Dezhou braised chicken [5; 6]. Previous research indicated that aldehydes are mainly derived from lipid oxidation. Aldehydes changed significantly during the processing (\(p<0.05\)). The content of aldehydes was the highest in raw meat, lowest after braising for 1 h and then increased significantly after braising for 2 h. Heptanal, octanal and nonanal decreased significantly (\(p<0.05\)) during the frying stage. Aldehydes mostly have a fatty odor, and their proportion directly affected the odor of the Daokou braised chicken. The main volatile compounds of raw chicken were aldehydes with animal fat odor if no heat processing was applied. Increasingly volatile components were produced under the heat of frying and braising, while the proportion of aldehydes decreased after frying and braising for 1 h. Similarly, aldehydes continued to be produced with lipid oxidation and the degradation of certain amino acids, the proportion of which increased (while remaining significantly lower than in raw chicken), giving Daokou braised chicken a unique and harmonious odor. Hydrocarbons have a high odor threshold and are mostly odorless or with a faint odor [27]; as such, they have little direct effect upon the flavor of braised chicken. Aliphatic hydrocarbons may be produced by the oxidative degradation of lipids, and aromatic hydrocarbons may be produced by the oxidation of free amino acids with aromatic groups. However, the relative content of hydrocarbons in the samples changed significantly during processing (\(p<0.05\)), and increased significantly during frying and braising, from 0.55% of raw chicken to 8.47% after braising for 1 h, indicating that hot processing is beneficial to the formation of hydrocarbons in braised chicken. Octahydro-4,7-methano-1H-indene, 3-Ethyl-2-methyl-1,3-hexadiene and (E)-2,2-dimethyl-4-decene were detected in the samples; these compounds are precursors of ketones and aldehydes, and likely have effects on flavor. All terpen except 1,8-Cineole were generated from spices added during the braising process, indicating that the braising stage plays a crucial role in the formation of terpen [8].