## 3 Result and Discussion
### Degradation and Oxidation of Bacon Proteins during LTS
#### 3.4.1 Protein Degradation  
As shown in Figure 2A, the content of TCA-soluble peptides significantly increased from 26.28 umol tyr/g to 30.12 umol tyr/g (\(p<0.05\)), reflecting the greater extent of protein degradation at the end of LTS. A previous study showed similar results, with protein breakdown increasing with smoking duration [31]. Myofibrillar protein (MP) and sarcoplasmic protein (SP) are important components of pork proteins, accounting for 60-70%, and 20-30%, respectively [32]. Therefore, to investigate protein degradation during LTS, this study further analyzed the changes in MP and SP during LTS (Figure 2B,D), revealing decreases from 20.21 g/100 g to 6.03 g/100 g and from 12.61 g/100 g to 5.96 g/100 g (\(p<0.05\)), respectively. This indicated that MP and SP were degraded during LTS. Similarly, SDS-DAGE results confirmed the degradation of MP and SP during LTS of bacon (Figure 2C,E). The electrophoresis bands in the 100, 75, 60, 45, 35, and 25 kDa regions show the degradation of MP (Figure 2C), potentially \(\alpha\)-actin (96 kDa), actin (43.2 kDa), \(\beta\)-tropomyosin (35 kDa), TnT (32 kDa), MLC1 (27.1 kDa), and troponin C (20 kDa) [33]. As shown in Figure 2E, at the end of LTS, bands between 25 and 75 kDa appeared weak or disappeared, indicating that SP was degraded. It is likely that phosphofructokinase (82.4 kDa), phosphoglucoisomerase (60 kDa), creatine kinase (30-40 kDa), phosphotriose isomerase (28 kDa), and other proteins were degraded. A previous study reported similar trends in Chinese Cantonese sausages, where the degree of protein hydrolysis increased  
Figure 2: Protein degradation of Hunan bacon during LTS. (**A**) TCA of bacon during LTS. (**B**) Changes in myofibrillar protein during LTS. (**C**) SDS-PAGE of myofibrillar protein during LTS. (**D**) Change in myogen protein during LTS. (**E**) SDS-PAGE of myogen protein during LTS. \({}^{\text{a--}}\) Means within rows and same breed with different superscripts differ significantly (\(p<0.05\)). with processing time [34]. It has been demonstrated that both MP and SP are susceptible to temperature and proteases. A decrease in protease activity has been shown to inhibit protein degradation [33]. Therefore, in this study, it can be speculated that the increase in temperature and protease activity promotes protein hydrolysis. Protein degradation fragments have been shown to affect flavor and have important implications for interpreting flavor formation mechanisms [35]. Similarly, [PERSON] found that in the early stage of ham processing, some of the sarcoplasmic proteins degraded into smaller molecules and then underwent a series of biochemical reactions to produce the unique taste and flavor of the ham [9]. Therefore, the present study showed that the hydrolysis of MP and SP may be closely related to the formation of volatile flavor compounds. An increase in protein degradation leads to differences in the content of free amino acids [36]; the levels of 24 free amino acids in bacon samples from different stages of LTS are presented in Table 3. Compared with control samples (0 d), the content of free amino acids increased significantly from 406.68 mg/100 g to 736.55 mg/100 g during LTS. With the exception of Cys and Gln, the content of most amino acids increased, which might have contributed to the formation of flavor compounds. It has been noted that amino acids such as Asp, Glu, Gly, Ala, Arg, and Leu are considered as precursors of meat aroma, which can react with soluble reducing sugars (e.g., glucose and fructose) to form flavors [37]. For example, Leu can react with the products of the Maillard reaction to form aldehydes, which significantly affects the volatile flavor of meat [38]. Meanwhile, the increase in free amino acid content in this study was accompanied by an increase in the content of flavor compounds, such as ketones and aldehydes, so it was hypothesized that the formation of characteristic flavor compounds of bacon during LTS is related to amino acid metabolism.