## 3 Results and Discussion  
### Volatile Compounds Analysis  
According to the retention time, retention index and qualitative masses (\(m/z\)), 39 volatile compounds were identified, including 22 esters, 8 alcohols, 4 aldehydes, 1 ketone, 1 terpenoid and 3 others (estragole, naphthalene and anethole) in all apple varieties over two years. All the volatiles had been reported in apple or apple juice in previous studies [1; 4; 7; 9; 26; 29]. The volatiles were then quantified by external standards, calculated with standard curves. The total ion current (TIC) chromatogram graph for internal and external standards is shown in Figure 1. Detailed apple volatile concentrations for 1 st year and 2 nd year were in the Supplementary Materials Tables S1 and S2. In order to compare and interpret the volatile compounds visually within 35 apple varieties, the quantified data were converted to percentages for both years (Figure 2). The percentages were counted by dividing the concentration of each compound by the total concentration for each variety, which are revealed by different colors. Butyl acetate, hexanal, 2-methyl-1-butanol, E-2-hexanal, 1-hexanol, E-2-hexan-1-ol, hexyl 2-methylbutanoate and 1-heptanol had relatively higher percentages in all varieties for both years, which are shown with the percentages near 70%. In other words, the contents for these compounds are dominant in apples. These compounds are all identified in the literatures [9; 10] among different apple varieties. Figure 1: Total ion current (TIC) chromatogram graph for 1 internal standard and 39 external standards. Figure 2: Heatmap displaying the percentages of 39 volatile compounds among 35 apple varieties in the first year (**a**) and the second year (**b**). The colors from red to blue represent the percentages from low to high for each variety. The apple cultivar abbreviation codes are MG-Miguo, WP-White Pippin, GG-Grimes Golsen, SGD-Stark Spur Golden Delicous, CD-Cloden, CDN-Cardinal, CR-Calville Rouge, XH-Xinhua, JD-Jonagold, JT-Jonathan, HB-Hadi Bolaite, CP-Clapp, JTW-Jonathan (New), MS-Mutsu Spur, HAH-Hahong, WH-Wenhong, HQ-Hongqiaowang, NG-Ningguan, JL-Judeline, HH-Huahong,  
HF-Hanfu, CH-Changhong, PN-Pinova, DN-Dounan, SR-Starkrimson, GD-Gloden Delicious, QJ-Qiujin, QG-Qinguan, JG-Jiguan, RAW-Ralls (weeping), RA-Ralls, GT-Gongteng, QH-Qihu 7, HUF-Huafu and CF-Changfu 2. It was noted that the concentrations of butyl acetate, 2-methyl-1-butanol and 1-hexanol were all at high levels, which is consistent with the literature. However, 2-methyl-1-butanol was not an original compound in apple, but is generated by processing, especially when mashing the apples. Although the apples were not processed, they were cut into small pieces during the extraction procedure, which may have affected formation. 2-methyl-1-butanol may form by transamination and decarboxylation of leucine and isoleucine, respectively [30]. Although hexanal, E-2-hexenal and E-2-hexen-1-ol reached higher concentrations in this study, this was not in agreement with the literature [9]. This is probably because these compounds were highly dependent on variety, as proven by [PERSON] and [PERSON] [31]. For instance, ascorbic acid content in apples had been reported as one of the factors that may have effects on their concentrations [32]. Moreover, all of these three compounds were generated from fatty acid metabolism. The precursor substance for hexanal and 1-hexanol is linoleic acid (C18:2). Hexanal is formed initially by the lipoxygenase (LOX) pathway, and then hexanol is produced by alcohol dehydrogenase (ADH) [8]. At the same time, hexanal could also yield E-2-hexenal from linoleic acid. Furthermore, E-2-hexen-1-ol could be produced from E-2-hexenal via ADH. Because the concentrations for 1-hexanol, hexanal, E-2-hexenal and E-2-hexen-1-ol were all at high levels in this study, it implies that the precursor contents for linoleic acid and linolenic acid may also be at high levels in these varieties.