## 3 Results and Discussion
### Principal Component Analysis of Volatile Compounds  
Principal component analysis (PCA), an unsupervised clustering method, is often used to provide a partial visualisation of data in a reduced-dimension plot [43; 44]. PCA was used extract important information from the 78 volatile compounds detected in the 40 apple cultivars. As shown in Figure 3, the first two principal components accounted for 63.92% of the variation in the data, with PC1 and PC2 explaining 38.24% and 25.68% of the total variance, respectively. Scatter plots of the 40 apple cultivars are shown in Figure 3A, and the corresponding loadings establishing the relative importance of the variables are shown in Figure 3B. The 40 cultivars were divided into five groups based on the relationships between cultivars (scores) and their volatile compounds (loadings). The first group included five cultivars (RXH, JNG, CM, HY, HC), which contained high relative contents of butyl acetate (E7), hexyl acetate (E17), butyl caproate (E26), butyl heptanoate (E35), and estragole (O10). The second group contained eight cultivars (MI, FI, RL, RD, MYK, JZ, RGL, PNM) characterised by high relative contents of 2-methylbutyl acetate (E8), amyl propionate (E13), 2-methylbutyl 2-methylbutyrate (E19), hexyl 2-methylbutyrate (E28), 2-methylbutyl hexanoate (E30), and 2-methylbutanoic acid (O4). The third group was composed of three cultivars (HF, HG, JNT), which contained high levels of propyl butyrate (E9), 2-methyl-1-butanol (B3), and 1-hexanol (B5). The fourth group included five cultivars (GS, HS, RX, COP, and ID) with low relative contents of esters and high relative contents of aldehydes such as 2-hexenal (A4), (E,E)-2.4-heptadienal (A9). The fifth group contained the other 19 cultivars, and showed no consistency in the composition of volatile compounds.  
Among these cultivars, JNG in group 1 was characterised by high levels of butyl acetate (E7), hexyl acetate (E17), and butyl caproate (E26), in agreement with previous studies [25]. However, hexyl acetate (E17) was the major ester compound and was present in high levels in cultivar GD, which did not cluster into Group 1, possibly due to the influence of the content of other esters, such hexyl butyrate (E27) and hexyl hexanoate (E40). Cultivar FJ, one of the most widely cultivated apples in China, clustered into group 2 based on high relative content of 2-methylbutyl acetate (E8), amyl propionate (E13), 2-methylbutyl 2-methylbutyrate (E19), and hexyl 2-methylbutyrate (E28). 2-methylbutyl acetate is the main compound in the aroma profile of Fuji apples [45]. Granny Smith apples have low volatile emission compared with other apple varieties [46]. In this study, GS had low total content of volatile compounds, but the high relative content of 2-hexenal (A4) clustered it into group 4. As expected, group 5 contained the highest number of apple cultivars, and these cultivars had different types and contents of volatile compounds. These differences in the volatiles in cultivars contributed to diversity among apple varieties. According to PCA analysis results in this study, the most abundant esters in apple peels (hexyl butyrate, hexyl 2-methylbutyrate and hexyl hexanoate), together with hexanal, (E)-2-hexanal, 1-hexanol, estragole and \(\alpha\)-farnesene could been proposed for apple cultivar classification in the future.