## 2 Results and Discussion
### Discrimination of Marinated Pork Hocks by GC-MS/O  
To visualize a total picture of the distributions of 24 odour-active compounds (OAV \(>1\)) in all samples, PCA was applied (Figure 1). The two principal axes accounted for 84.69% of the entire variations of the four pork hocks; the two PCA components, PC1 and PC2, explained 48.56% and 36.13% of the variation, respectively. As shown in Figure 1A, it can be found that the sample dots of the marinated pork hocks were well separated. PC1 clearly distinguished DHM and HHT. DHM was in the positive side of PC1, while HHT appeared in the negative side, indicating that there were obvious differences of flavour features. Both DXC and TFH were located in the lower left quadrant of PC and close to each other, which meant that they have similar flavour. Hence, the four different marinated pork hocks were divided into three groups, i.e., group I: DHM, group II: HHT, and group III: DXC-TFH.  
Figure 1: _Cont._  
Moreover, the major odour-active compounds contributing to DHM were hexanal (green, grass), 2,4-decadien isomer (fatty, deep-fried), 2-acetylthiazole (caramel, sweaty), ethyl hexanoate (fruity, sweet), 2-nonenal isomer (fatty, cucumber), 2-octenal isomer (fatty, green), estragole (aniseed-like) and octanal (orange peel, fatty). As seen from Table 2, the OAV values of these aldehyde compounds in DHM were significantly higher than those in the other samples (\(p<0.05\)). As had been reported on all types of foodstuff [6; 32; 33], the eight compounds above have been widely studied with respect to their sources and contributions to food aroma [4; 34; 35]. HHT was in the first quadrant and highly associated with four hydrocarbons (\(p\)-cymene, 3-carene, D-limonene and \(\beta\)-phellandrene) and two furans (2-ethylfuran and 2-pentylfuran). Among them, the hydrocarbon compounds could originate from the animal feeds [24] and Chinese traditional spices [3]. 2-ethylfuran, 2-pentylfuran and eugenol could have been found in the cooked meat products [28; 36] and the traditional salted vegetables [27]. Only three volatile components were related to DXC and TFH, including ethyl acetate, 2-methylbutanal and 3-methylbutanal. As discussed previously, the formation of 2-methylbutanal and 3-methylbutanal can be attributed to the amino acid Strecker reaction [6; 20].  
In addition to PCA analysis, PLS-DA was applied for the discrimination of all marinated pork hocks. Figure 1B illustrated the PLS-DA score of the marinated samples. HHT was located on the negative side of axis 1, DHM was found on the positive side of axis 2, and TFH and DXC were clustered on the negative side of axis 2. Hence, there were three separate groups (HHT, DHM and DXC-TFH) for marinated pork hocks. It could also be concluded that the flavour of DXC and TFH was similar, and they were different from HHT and DHM.  
Figure 1: (**A**) PCA for odour-active compounds of the four marinated pork hocks. The blue dots represent the samples from marinated pork hocks, and the red dots represent odour-active compounds. (**B**) PLS-DA score plot from different marinated samples (R\({}^{2}\)X = 0.978, R\({}^{2}\)Y = 0.997, Q\({}^{2}\) = 0.994). The red dots represent DHM, the yellow dots represent HHT, the blue dots represent TFH, and the green dots represent DXC.