# Quality Control of Fried Pepper Oils Based on GC-MS Fingerprints and Chemometrics
## 3 Results and Discussion
### Orthogonal Partial Least Squares Discriminant Analysis (OPLS-DA)  
OPLS-DA A is a supervised discriminant method widely used for quality evaluation. In the classification, the model becomes simple and easy to explain, and its discrimination effect and principal component, the visualization effect of the score chart, are prominent [20]. Figures 5 and 6 show the OPLS-DA score scatter plots for ZBO and ZAO samples. Nine samples were clearly distinguished according to their different characteristics and most ZBOV and ZAOC samples were in the negative quadrant of t [1], while most ZBOC and ZAOV samples were in the positive quadrant of t [1]. The classification results are homologous to those of the PCA results in Figure 4. All results indicated that this OPLS-DA model has good stability and the ability to distinguish two kinds of pepper oil.  
Figure 4: PCA clustering results of ZAOOs and ZBOs. (**a**) Scores plot of ZBOs; (**b**) loading plot for common characteristic in ZBOs; (**c**) scores plot of ZAOs; (**d**) loading plot for common characteristic in ZAOs.  
Figure 5: OPTLS-DAD model of ZAO samples. (a) Scatter plot of OPTLS-DDA of ZAOVs and ZAOCs. (b) 200 permutation tests for OPTLS-DDA model. (c) VIP value of different compounds in ZAOVs and ZAOCs. (d) S-plot of different compounds in ZAOVs and ZAOCs (sq01: \(\&\)Cadnirener; sq02: Bicydo[3.1.0]-bex2-ene, 2-methyl)-5-(1-methyl)ethyl; xq03: D-Limonene; xq04: \(\&\)Phellandrene; sq05: \(\&\)B-Elemene; xq06: \(\&\)Y-Terpinene; sq07: Nerololol; xq08: Germacrenne D; xq09: Linaolo; sq10: Sabinene; sq11: Humulene; sq12: Caryophyllene; sq13: Terpinolene; sq14: Linaolyl acetate; sq15: Myrocne; sq16: Heroadence). The red dots indicate odors with VIP values exceeding 1, and the green dots indicate odors with VIP values not exceeding 1.  
The variable important for the project (VIP) values were analyzed to determine the aroma components contributing greatly to fried pepper oils. VIP is the weight value of the OPLS-DA model variable that can measure the influence intensity and interpretation of the difference in accumulation for each component for the classification and discrimination of each sample group. The contribution rate increases proportionally with the VIP value. The greater the VIP value, the more significant the difference in variables among different varieties of fried pepper oils. Generally, VIP > 1 is the standard screening basis of differential components [34]. This study used VIP > 1 and \(p\) < 0.05 as the criteria. As Figure 5c illustrated, xq4 (germacrene D), xq5 (linalool), xq6 (sabinene), xq8 (linal acetate), and xq10 (\(\beta\)-myrcene) were selected by the VIP method as characteristic indicators for distinguishing between ZBOV samples and ZBOC samples. Additionally, xq9 (linalool), xq10 (sabinene), and xq14 (linal acetate) were markers for distinguishing between ZAOV samples and ZAOC samples.  
S-plot was used to confirm the differences between different fried pepper oils and identify potential markers for distinguishing different samples, with the points at the ends of the \"S\" indicating the variables with the largest contributions to the model. In contrast, variables that contributed less were clustered around the origin. As shown in Figures 5d and 6d, the red dots indicate odors with VIP values exceeding 1. Therefore, xq9 (linalool), xq10 (sabinene), and xq14 (linaly acetate) were markers for distinguishing between ZAOV samples and ZAOC samples. The characteristic indicators for distinguishing ZBOV samples from ZBOC samples were xq4 (germacrene D), xq5 (linalool), xq6 (sabinene), xq8 (linaly acetate), and xq10 (\(\beta\)-myrcene).