## 3 Results and Discussion
### Identification of Key Volatile and Non-Volatile Compounds in Raw Meat during Refrigeration
#### 3.3.1 Changes in Key Volatile Compounds  
As shown in Table 1 and Figure 3a, 38 volatile compounds were detected in raw meat, mainly including aldehydes, alcohols, acids, esters, ketones, heterocyclic, and alkanes. Hexanal (526.32-2375.77 ng/g) had the most abundant content, followed by 1-octen-3-ol (398.33-916.86 ng/g), 2,3-octanedione (127.39-858.26 ng/g), and nonanal (192.54-1095.56 ng/g). Based on OAVs and the contribution rates, 1-octen-3-ol, (E, E)-2,4-decadienal, and nonanal were recognized as key volatile compounds in raw meat during refrigeration with different chilling rates. In terms of contribution rate, the contribution rate of 1-octene-3-ol at 72 h post-slaughter was significantly lower (\(p<0.05\)) than that at other post-slaughter times at chilling rates of 1.44 \({}^{\circ}\)C/h, but the contribution rate of aldehydes was significantly higher (\(p<0.05\)). When the chilling rate was 32.4 \({}^{\circ}\)C/h, the contribution rate of aldehydes at 24 h post-mortem was significantly higher (\(p<0.05\)) than that at other post-mortem hours, while 1-octen-3-ol was significantly lower (\(p<0.05\)). (E, E)-2,4-decadienal was the highest contributing aldehyde, which enhanced the aroma of raw meat [51]. The results also showed (E, E)-2, 4-decadienal was the predominant volatile compound at 72 h post-slaughter when the chilling rate was 1.44 \({}^{\circ}\)C/h and 22.2 \({}^{\circ}\)C/h, while (E, E)-2, 4-decadienal was the largest contributor of the volatile compound at 24 h when the rate was 32.4 \({}^{\circ}\)C/h. This might be due to the delayed phase of the decrease in ATP concentration associated with a rapid chilling rate (32.4 \({}^{\circ}\)C/h). ATP remained almost constant during chilling at \(-\)20 \({}^{\circ}\)C or lower [52], at which temperature the breakdown of ribonucleotides, protein hydrolysis, and glycolysis would be inhibited. Conversely, some oil-derived volatiles, including (E, E)-2, 4-decadienal, were increased through the breakdown of ribonucleotides, protein hydrolysis, and glycolysis [53; 54; 55]. Therefore, the contribution rate of (E, E)-2,4-decadienal in raw meat at 24 h post-slaughter was higher at 32.4 \({}^{\circ}\)C/h than 1.44 \({}^{\circ}\)C/h.  
The contents of 1-octen-3-ol, (E, E)-2,4-decadienal, and nonanal in raw meat first increased and then decreased during refrigeration, and their OAVs had the same trend. Similar results were obtained from studies on Jingyuan meat [35]. 1-octen-3-ol was mainly derived from linoleic acid when amino acids and ribose interact during lipid oxidation [56], and aldehydes also could be formed through lipid oxidation [57]. Therefore, the contents of 1-octen-3-ol, (E, E)-2,4-decadienal and nonanal increased in the early post-slaughter period, which might be due to lipid oxidation of raw meat. Generally, the fatty flavor was richer under the rapid chilling rate, especially at 24 h postmortem. In addition, more aromatic compounds were observed in lamb meat at 72 h postmortem with a chilling rate of 1.44 \({}^{\circ}\)C/h and at 24 h with a chilling rate of 32.4 \({}^{\circ}\)C/h.