## 2 Results and Discussion  
### Volatile Profile of Marinated Pork Hocks Characterized by GC-MS/O  
#### 2.1.1 Volatile Composition of Marinated Pork Hocks  
The odour descriptions, thresholds and relative concentrations of the volatile compounds identified in the marinated pork hocks are presented in Table 1. A total of 62 volatile compoundswere detected in the marinated pork hocks, and there were 35, 37, 33 and 25 volatile compounds in DHM, DXC, HHT and TFH, respectively. These compounds can be further categorized into 10 major groups, as follows: 13 aldehydes, nine alcohols, five ketones, three esters, 14 hydrocarbons, two ethers, two phenols, seven furans, three \(N\)-containing compounds and four \(S\)-containing compounds. Total estimated concentrations of volatile compounds in DHM, DXC, HHT and TFH were 1224.2 ug\(\cdot\)kg\({}^{-1}\), 1073.4 ug\(\cdot\)kg\({}^{-1}\), 1791.8 ug\(\cdot\)kg\({}^{-1}\) and 822.7 ug\(\cdot\)kg\({}^{-1}\), respectively. TFH contained the least number and lowest levels of volatile constituents. Conversely, the greatest number and highest levels of volatile compositions were detected in DXC and HHT, respectively. The majority of aldehydes significantly contributed to the flavour profiles of various food matrices due to their low odour thresholds [11; 20]. Aldehydes are mainly generated from two pathways, i.e., lipid oxidation and Strecker degradation of amino acids [3; 5]. In this study, the 2,4-decadienal isomer with fatty and deep-fried odour had the lowest threshold of 0.07 ug\(\cdot\)kg\({}^{-1}\) in DHM. However, this compound was not observed in the other three pork hock samples. [PERSON] et al. [21] reported that the 2,4-decadienal isomer was considered as an oxidation product of linoleic acid, which was the main polyunsaturated fatty acid found in cooked cured pork ham. Nonanal with a citrus and fatty odour was identified and had the highest concentration in all samples. This aldehyde was generated from lipid oxidative degradation [6]. Additionally, the remaining four aldehyde compounds, including hexanal, octanal and benzaldehyde, were also detected in all samples. Among these compounds, benzaldehyde (with a bitter almond smell) has the larger odour threshold, which demonstrates the lesser contributions to the aroma profiles of the marinated pork hocks. Note that the aroma profile of DXC was characterized by the presence of 2-methylbutanal and 3-methylbutanal with a nutty odour, which were not detected in the other three samples. [PERSON] et al. [11] reported that 2-methylbutanal was known to be a Strecker reaction product of isoleucine in steamed Chinese mitten crab. [PERSON] et al. [22] found that 3-methylbutanal was formed in salty boiled duck in water, and the formation may be associated with leucine. [PERSON] and [PERSON] [23] reported that alcohols were one of the key flavour compounds in steamed Rangia clam, which could be associated with the decomposition of hydroperoxides of fatty acids or the reduction of aldehydes. Regarding the relative concentrations of alcohols detected in the marinated pork hocks, 1,8-cineole, linalool and terpinen-4-ol were detected in all samples. As shown in Table 1, the relative concentration of 1,8-cineole with the minimum threshold was significantly higher than all other listed alcohol compounds, which indicated its greatest contribution to the complete flavour profile of marinated pork samples. In addition, linalool and 1-octen-3-ol were known to be the most important aroma-active alcohols and have been found in the essential oil [24] and fish products [12], respectively. The \(\beta\)-oxidation of linoleic acid has been considered as the main pathway to form 1-octen-3-ol in dry cured loin [25]. The odour threshold of 1-octen-3-ol (2 ug\(\cdot\)kg\({}^{-1}\)) was three times lower than that of linalool (6 ug\(\cdot\)kg\({}^{-1}\)); therefore, a higher OAV was achieved by 1-octen-3-ol in DHM, HHT and TFH compared with linalool.