## 3 Results and Discussion
### Volatile Compounds Analysis
The percentages of ethyl 2-methylbutyrate, Z-3-Hexenyl acetate, propyl hexanoate, ethyl octanoate, E-2-nonenal, linalool, hexyl 2-butenoate, naphthalene, hexyl octanoate, anthehole and dodecanol were lower among all the varieties over two years, as shown by the red color. Ethyl 2-methylbutyrate could be produced from isoleucine by isoleucine catabolism, which shares the same precursor substance as 2-methyl-1-butanol. [PERSON] et al. [33] treated Granny Smith apple skin with deuterium-labeled isoleucine and found ethyl 2-methylbutyrate was mostly formed. Since the 2-methyl-1-butanol was high yield, it may lack isoleucine for ethyl 2-methylbutyrate generation, thus affecting the concentration. Moreover, [PERSON] et al. [34] pointed out that ethyl 2-methylbutyrate was a postharvest-induced volatile, and that the concentration would be increased in apples ripened off the tree, and after a long time in cold storage. However, since all the apple samples were treated when reaching the same stage of maturity as measured by the starch-iodine index, and stored without a long time in cold storage, the postharvest-induced generation effect may be negligible. The precursor substance for Z-3-hexenyl acetate was Z-3-hexenal, which was produced from linolenic acid (C18:3) by the LOX pathway. However, Z-3-hexenal could not only form Z-3-hexenyl acetate through ADH and alcohol acyltransferase (AAT), but could also generate E-2-hexenal via E-2-enol isomerase [8]. In other words, Z-3-hexenyl acetate and E-2-hexenal share the same precursor substance; Z-3-hexenal. [PERSON] et al. [35] demonstrated that the lack of linolenic acid may cause the absence of Z-3-hexenal, E-2-hexenal and E-2-hexen-1-ol. Nevertheless, the concentrations for E-2-hexenal and E-2-hexen-1-ol were relatively high for all varieties over two years, which may indicate the content of linolenic acid is high. Accordingly, Z-3-hexenyl acetate concentrations were contrasted with E-2-hexenal. It is worth noting that LOX is highly positively related to generation of these \"green note\" compounds (hexanal, E-2-hexenal and E-2-hexen-1-ol) [5]. The activity of LOX is increased during apple ripening in various apple varieties [8; 35]. Naphthalene had been reported in apple [29], apple cider [36], mango [37], pear [38] and melon [22]. Coincidentally, [PERSON] et al. [29] had identified naphthalene in apples, where the apples were harvested from the same orchard as this study. There may be several reasons that cause the presence of naphthalene. Naphthalene-derived compounds, like 1-naphthaleneacetic acid, are usually used as plant growth regulators to increase yield in fruit, such as apple, pears and grapes [39]. There may be residues from the use of related regulators in field management. Some authors indicated naphthalene may be generated from the thermal degradation of \(\beta\)-carotene [40; 41]. Moreover, some authors demonstrated that mild thermal treatment like simultaneous distillation and extraction caused the formation of naphthalene in cashew apple fruit. When the extraction temperature was below 37 \({}^{\circ}\)C, no naphthalene was detected [42]. In this study, although the extraction temperature was 35 \({}^{\circ}\)C, the extraction time was longer. Whether this affects the formation of naphthalene is unclear. The lower concentration of naphthalene may be because it was a residue from field management, \(\beta\)-carotene degradation products, or formed in the extraction process. Because of this uncertainty naphthalene was not excluded in the following analysis. As for the comparison between the two years among the 39 volatiles, some of the volatiles indicated similar trends. For example, the concentration percentages for pentyl acetate, D-limonene, hexyl acetate, 1-heptanol, hexyl 2-butenoate and anethole in the first year were lower than the second year for most apple varieties. Especially for D-limonene, 1-heptanol and anethole, where the concentration percentages were increased with time for all varieties.