## 3 Results and Discussion
### Analysis of GC-MS
#### 3.1.2 Analysis of Heat Map of Volatile Compounds  
In order to further study the change of odor and its component in the processing of Daokou braised chicken, the relative percentage contents of 75 volatile compounds in Table 1 were standardized as variables, and then heat map was drawn and hierarchical clustering analysis was used to cluster volatile compounds (Figure 3). As shown in Figure 3, the volatile compounds could be divided into six categories. The darker the red, the higher the content, and the darker the blue, the lower the content. Class I compounds mainly appeared in raw chicken, followed by the frying stage, including aldehydes and alcohols, and mainly providing fatty odor. Class II compounds mainly appeared in the frying stage, followed by braising for 1 h, including alcohols, aldehydes, ketones and one heterocyclic compound, providing fatty and fruity fragrance. Almost all compounds in class III mainly appeared in braising for 2 h, including terpen, aromatics, enablehydes, branched-chain aldehydes and enols, probably the characteristic odor compounds of Daokou braised chicken, which provided fatty, fruity, herbal and woody fragrance [28], etc. And most of these compounds with low odor threshold, contributed the braised chicken strong and rich aroma characteristics. There were 8 volatile compounds in class IV, which appeared in the frying and braising stages, especially the frying stage, mainly included alkanes and alcohols. Compounds in class V mainly appeared in the braising stage, especially when braised for 2 h, including terpen, ketones, alcohols and heterocyclics, which mainly provided fruity, woody and nutty fragrance [29]. Class VI compounds including hydrocarbons, terpen and alcohol compounds, mainly appeared in braising for 1 h, which mainly provided fruity, herbal and spicy fragrance. The research results showed that stewing for 2 h will promote the formation of the characteristic flavor of Daokou braised chicken. Figure 2: The contents and number of volatile flavor components in Daokou braised chicken during processing. (Note: (**A**,**B**) refer to the contents and number of volatile flavor components respectively). The odor compounds of Daokou braised chicken in different processing stages changed constantly, and the types and aroma attributes of volatile compounds more abundant, thus forming the unique odor. The volatile compounds in raw chicken mainly were from class I and presented fatty odor. In the frying stage, the volatile compounds were mainly from class II and class IV, which contributed with fatty and fruity odor. After braised for 1 h, mainly volatile compounds were from class VI, and some from class V and IV, which presented fatty, fruity, herbal and spicy fragrance. Except class I, the volatile compounds in the chicken braised for 2 h were mainly from class III and V and some from class II, IV and VI. When braising for 2 h, the compounds were more diverse, so that the braised chicken had rich and harmonious odor. The above showed that the braising process was the key processing stage for the formation of characteristic odor of Daokou braised chicken. Although the content of some compounds decreased after braising for 2 h, the braising process accelerated the release of volatile compounds, and appropriate extension of braising time was conducive to the proper proportion of volatile compounds, so as to form rich and harmonious odor. The results of four techniques showed that braising promoted the release of volatile compounds, and the formation of the complex compounds, and the formation of the complex compounds, and the formation of the complex compounds, are more complex than the formation of the complex compounds. Figure 3: Analysis of heat map of volatile compounds during different processing stages. of flavor compounds, and was the key stage for the formation of characteristic flavor in Daokou braised chicken.