## 3 Results and Discussion
### Identification and Determination of Volatile Compounds in Forty Apple Cultivars
#### 3.2.1 Esters  
Esters are the dominant aromatic compounds in apples that form and contribute to the characteristic fresh and fruity apple flavour [29; 30]. In this study, esters constituted the largest proportion of volatile compounds and 47 types were identified by SPME/GC-MS in the peels of 40 apple cultivars. HC had the highest ester content (21,457.95 \(\pm\) 2230.10 \(\upmu\)g/kg FW, 77.15% of total volatiles), followed by RXH (21,403.00 \(\pm\) 2350.36 \(\upmu\)g/kg FW, 79.23% of total volatiles), Qinguan (QG, 19,396.06 \(\pm\) 2010.57 \(\upmu\)g/kg FW, 70.39% of total volatiles) and JZ (19,352.58 \(\pm\) 1523.65 \(\upmu\)g/kg FW, 70.39% of total volatiles) (Table 4 and Table S4). By comparison, HS (474.23 \(\pm\) 35.21 \(\upmu\)g/kg FW), GS (504.68 \(\pm\) 38.94 \(\upmu\)g/kg FW) and COP (608.05 \(\pm\) 52.84 \(\upmu\)g/kg FW) had a lower ester content, accounting for 12.84%-23.23% of the total volatiles (Table 4 and Table S4). These results confirmed a previous observation that the volatile compound profile is highly cultivar-dependent, owing to the variation in esters, which is under strong genetic control [24]. In this study, the major ester compounds (average content \(>\) 100 \(\upmu\)g/kg FW) were butyl acetate (E7), 2-methylbutyl acetate (E8), butyl butyrate (E14), butyl 2-methylbutyrate (E15),  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Cultivars** & **Esters** & **Aldehydes** & **Alcohols** & **Others** \\ \hline RG & 1691.11 \(\pm\) 210.25 & 741.34 \(\pm\) 80.43 & 94.28 \(\pm\) 10.42 & 392.54 \(\pm\) 32.51 \\ GD & 3240.04 \(\pm\) 295.30 & 973.72 \(\pm\)  
hexyl acetate (E17), hexyl propanoate (E23), butyl caproate (E26), hexyl butyrate (E27), hexyl 2-methylbutyrate (E28), hexyl hexanoate (E40) and butyl caprylate (E41) (Table 2), which was in agreement with the previous research [31; 32]. Moreover, the most abundant esters (average content \(>\) 700 \(\mathrm{\SIUnitSymbolMicro g}\)/kg FW) as determined by GC-MS were hexyl butyrate (E27), hexyl 2-methylbutyrate (E28) and hexyl hexanoate (E40). By comparison, ethyl propanoate (E2), propyl propionate (E5), propyl butyrate (E9), heptyl acetate (E25), ethyl octanoate (E29), 3-methylbut-2-enyl hexanoate (E39) and butylolactone (E43) were present in relatively low amounts (average content \(<\) 3 \(\mathrm{\SIUnitSymbolMicro g}\)/kg FW) in peels of each apple cultivar (Table 2). Hexyl acetate, hexyl hexanoate, and hexyl 2-methylbutyrate, which are the most important esters, have a great influence on apple aroma because of their abundance [7; 8]. Consistent with previous reports [33; 34], hexyl 2-methylbutyrate (E28) was the most abundant in most of the cultivars analysed in this study, such as HC (10,087.55 \(\pm\) 1534.58 \(\mathrm{\SIUnitSymbolMicro g}\)/kg FW), JZ (8980.62 \(\pm\) 850.45 \(\mathrm{\SIUnitSymbolMicro g}\)/kg FW) and OI (7589.21 \(\pm\) 865.32 \(\mathrm{\SIUnitSymbolMicro g}\)/kg FW) (Table S3). Hexyl acetate has a sweet and fruity odour, with floral notes [35]. The contents of hexyl acetate (E17) in RXH (1399.58 \(\pm\) 145.63 \(\mathrm{\SIUnitSymbolMicro g}\)/kg FW) and EV (1380.88 \(\pm\) 15.86 \(\mathrm{\SIUnitSymbolMicro g}\)/kg FW) were higher than in the other apple cultivars. Conversely, the cultivars with the lowest content of hexyl acetate (E17) were Qinyun (QYN, 10.61 \(\pm\) 1.85 \(\mathrm{\SIUnitSymbolMicro g}\)/kg FW) and Ruixue (RX, 11.88 \(\pm\) 1.02 \(\mathrm{\SIUnitSymbolMicro g}\)/kg FW) (Table S3). Hexyl hexanoate is another main ester in apples [30]. HC, QG, and RXH had higher levels of hexyl hexanoate (E40), at concentrations of 5688.01 \(\pm\) 415.97 \(\mathrm{\SIUnitSymbolMicro g}\)/kg FW, 5494.48 \(\pm\) 475.20 \(\mathrm{\SIUnitSymbolMicro g}\)/kg FW, and 5403.15 \(\pm\) 586.30 \(\mathrm{\SIUnitSymbolMicro g}\)/kg FW, respectively, whereas Yuhuazaofu (YH), COP, and GS had lower levels, at 83.05 \(\pm\) 7.64 \(\mathrm{\SIUnitSymbolMicro g}\)/kg FW, 88.31 \(\pm\) 9.20 \(\mathrm{\SIUnitSymbolMicro g}\)/kg FW and 139.78 \(\pm\) 12.96 \(\mathrm{\SIUnitSymbolMicro g}\)/kg FW, respectively (Table S3).