## 3 Results and Discussion
### GC-MS Analysis of Volatile Profile of Dried Citrus Peel as Effected by Drying Methods
#### 3.1.9. Correlation and PCA Analysis  
For overall analysis of all detected volatiles by GC-MS in dried citrus peel from different drying methods, correlation and PCA analysis was employed in Table 1 with results shown in Figures 3, 4 and S1, respectively. In Figure 3, the Pearson's correlation coefficient between samples showed that the triplicate samples of the same citrus peel with the same drying method were highly correlated, indicating the good repeatability of the samples within the group. Sample correlation analysis also showed that the differences between FAD and FSD, GAD and GSD were the smallest, followed by the differences between GSD and FAD, GSD and FSD, GFD and FFD, GSD and GFD, and the differences between GFD and FAD, GFD and FSD were the largest. In PCA analysis, the first three principal components covered 88.54% of total variance (principal component-1 = 46.30%, principal component-2 = 30.70%, principal component-3 = 11.54%). Figure 4 shows the first two principal component score/loading plots (covering 77.00% of total variance) while the plots of principal component-1 vs. principal component-3 and principal component-2 vs. principal component-3 are shown in Figure S1. It can be directly observed from Figures 4a and S1a,c that both drying methods and cultivars showed great influences on the volatile composition of dried citrus peel, and SD peel was more similar with AD peel in GC-MS analysis of volatiles.  
Figure 3: Correlation between dried citrus peel samples of different groups (FAD, FSD, FFD, GAD, GSD, GFD) as characterized by GC-MS analysis.  
In Figures 4b and S1b, all three \(N\)-containing compounds, four sesquiterpenes ((-)-\(\beta\)-caryophyllene, (Z, E)-\(\alpha\)-famesene, \(\alpha\)-selinene, \(\alpha\)-farmesene), and thymol were of positive higher loadings at principal component-1, while five aldehydes (dodecanal, \(\beta\)-sinensal, furfural, decanal, octanal) and four sesquiterpenes (germacrene B, \(\gamma\)-elemene, trans-\(\beta\)-famesene, \(\delta\)-elemene) were of negative higher loadings at principal component-1, suggesting these compounds represented most important information of principal component-1. Based on the interpretation of principal component-1, it can be seen from Figures 4a and S1a that \(N\)-containing compounds, sesquiterpenes, and aldehydes were important compounds to clearly discriminate dried Chachi and Ponkan peel, with both possessing its individual characteristic sesquiterpenes and aldehydes, and much higher content of \(N\)-containing compounds in dried Chachi peel.  
For principal component-2, as shown in Figures 4b and S1d, two monoterenes (D-limonene and \(\beta\)-myrcene) and two alcohols (linalool and elemol) were of positive higher loadings, while four straight-chain fatty acid methyl esters (methyl palmitoteate, methyl palmitate, methyl linoleic and methyl linolenate) and two phenols (p-thymol, carvacrol) were of negative higher loadings, suggesting these compounds represented most important information of principal component-2. Based on this interpretation, it can be seen from Figures 4a and S1c that monoterenes (including the main compound D-limonene), straight-chain fatty acid methyl esters and phenols were important compounds to clearly discriminate FD peel and AD/SD peel, with FD favorable to preserve phenols and esters while unfavorable to preserve monoterenes when being compared with AD or SD.