## 3 Results and Discussion
### Identification of Volatile Compounds in Six Citrus EOs
The top five volatiles in HY EOs were \(D\)-limonene (77.25-83.10%), \(\gamma\)-terpinene (6.74-8.48%), germacrene D (1.14-3.83%), \(\beta\)-myrrene (1.24-1.73%) and \(\alpha\)-pinene (0.33-1.29%), while in GF EOs they were D-limonene (88.21-94.01%), \(\beta\)-myrrene (1.51-2.20%), carophyllene (0.22-1.58%), \(\alpha\)-pinene (0.42-0.84%) and \(\alpha\)-copaene (0.19-0.98%). Most of these have also been identified as characteristic flavor compounds of citrus in previous studies [28; 29; 30]; however, the concentration of germacrene D in HY was significantly higher than that in most citrus [13]. Compared to GF EOs, multiple terpen were unique in HY EOs; i.e., \(\alpha\)-terpinene, \(p\)-cpame, \(\delta\)-elemene, \(\beta\)-ylangene, \(\gamma\)-elemene, \(\beta\)-sesquiphellandrene and germacrene B. On the other hand, \(\alpha\)-muurolene, _cis_-muurola-4(15),5-diene, isocadinene, \(\gamma\)-muurolene, \(\beta\)-selinee, calamenene and \(\alpha\)-calacorene were unique terpenids that only existed in GF EOs. Aldehydes play an important role in citrus aroma due to their low threshold values [31]. In both HY and GF EOs, decanal was the predominant aldehyde, with citronellal, undecanal, nonanal and dodecanal being the other common aldehydes. However, octanal, neral and geranial were representative aldehydes with a high level in GF EOs but only detected in trace amounts in HY EOs. \(\beta\)-sinensal was the second most abundant aldehyde in HY EOs but almost absent in GFEOs. Previous studies [32] have demonstrated that fatty acid degradation aldehydes (e.g., nonanal and decanal) impart a fatty and green scent, while others (e.g., citral and citronellal) exhibit fresh, pleasant and citrus notes. It was suggested that the differences in aldehydes composition may account for the aroma difference between HY and GF. Alcohols are also one of the primary contributors to citrus aromas. Elemol was found to be the only alcohol compound present in all six EOs, and it was found in close proximity in HY EOs and GF EOs. However, most alcohols were present in higher levels in GF EOs compared to HY EOs, including octanol, linalool, nonanol, \(\alpha\)-terpineol, _cis_-carveol, nerol, citronellol, geraniol and decanol, which may be another reason for the aroma difference between HY and GF. One exception to this trend was germacrene D-4-ol, which was the most concentrated alcohol in HYEOs and found to be significantly higher than in GF EOs. Moreover, isospathulenol, \(\alpha\)-cadinol and intermedeol only existed in HY EOs, whereas (\(E\))-2-hexen-1-ol, _trans_-nerolidol, shyobunol and (\(2E\),\(6E\))-farnesol were only present in GF EOs. Esters also constitute an important part of citrus aroma, which are primarily formed through the esterification of acids and alcohols. There were three aliphatic esters (i.e., hexyl acetate, octyl acetate and bornyl acetate) and four monoterpene esters (i.e., _cis_-carveyl acetate, \(\alpha\)-terpinyl acetate, citronellyl acetate and neryl acetate) detected in six EOs. Of these, citronellyl acetate, neryl acetate and _cis_-carvyl acetate were significantly richer in HY EOs than in GF EOs, hexyl acetate and bornyl acetate were only present in trace amounts in HY SCC extracts and \(\alpha\)-terpinyl acetate was only present in GF EOs. Carvone and nootkatone were the two ketones detected in six citrus EOs. Carvone was probably generated by the oxidation of limonene during oxidative storage and thermal treatment [33]. Surprisingly, we found that the content of nootkatone in HY EO was nearly one hundred times higher than that in GF EO, despite previous studies predominantly