## 3 Result and Discussion
### Screening of Key Volatile Flavor Compounds in Bacon during LTS  
Multivariate statistical analysis of volatile flavor compounds in Bacon samples was performed using a standardized operation method for automatic calculations. As shown in Figure 1C, the Bacon samples from different stages of LTS were well separated in different regions of the OPLS-DA scatter plots, suggesting there were significant differences between the volatile flavor compounds of Bacon samples. The parameters for the orthogonal PLS-DA (OPLS-DA) model were R2X = 0.998 and R2Y = 0.963, with a model prediction index (Q2) of 0.714. After 200 permutation tests (Figure 1D), the intersection of the Q2 regression  
Figure 1: Changes in the composition of volatile flavor in Bacon during LTS. (**A**) PCA of E-nose. (**B**) Radar map of E-nose. (**C**) OPLS-DA of GC-MS. (**D**) Cross-validations of the OPLS-DA model. (**E**) VIP plot of OPLS-DA. line and the vertical axis was <0.0, indicating that the model did not exhibit overfitting and that its validation was effective. To clarify the differences in bacon samples obtained from different stages of LTS, the volatile flavor compounds with variable importance in projection (VIP) values of >1.0 and statistically significant differences (\(p<0.05\)) were defined as potential key volatile marker compounds that were considered to play a crucial role in the OPLS-DA discrimination process. A higher VIP value indicated a more significant contribution to the underlying variation. In Figure 1E, the red part of the VIP image shows the arrangement of differential metabolites with VIP > 1. In total, 11 volatile compounds (6 ketones, 3 aldehydes, 1 furan, and 1 nitride) were selected as differential volatile flavor compounds. Table 2 lists the volatile flavor compounds in bacon samples, which contribute significantly to the flavor of liquid smoked bacon. Ketones and aldehydes are the main volatile flavor compounds in meat products and are present at lower thresholds, and they produce rich flavors during meat processing [27]. As shown in Table 2, the six ketones subjected to analysis in this study accounted for a significant proportion of the characteristic volatile compounds in bacon (76.4%). This included \(2^{\prime},4^{\prime}\)-dihydroxyacetophenone, 3-methylcyclopent-2-en-1-one, 4-methyl-2H-furan-5-one, 2-furanone, 2,5-dihydro-3,5-dimethyl, 3-methyl-1,2-cyclopentanedione, and 2,3-dimethyl-2-cyclopentene-1-one. Furthermore, three aldehydes were identified as volatile flavor compounds that could differentiate between bacon samples: furufural, nonanal, and hexadecan. Other volatile compounds, including furans and nitrides, such as 2-acetyl-5-methylfuran and \(N\),\(N\)-dibutylformamide, were also identified. The content of these differential volatile flavor compounds increased significantly during LTS (\(p<0.05\)). It can be reasonably assumed that furfural and 3-methyl-1,2-cyclopentanedione have the greatest impact on flavor, given their relatively high VIP values. In previous studies, these volatile compounds were shown to be produced mainly during the oxidative denaturation of proteins and lipids. For example, ketones can be produced by lipid oxidation and amino acid metabolism [28], and aldehydes are produced in association with protein hydrolysis or amino acid degradation [29]. Moreover, it has been shown that furans are produced via the Maillard reaction [30]. It is therefore postulated that the enhancement of flavor during the LTS process may be influenced by the oxidative degradation of proteins.