## 3 Results and Discussion
### Differences of Volatiles in Citrus EOs with Different Extraction Methods  
There were 54, 66, 71, 60, 58 and 70 volatile compounds detected in HY_CP, HY_MADH, HY_SCC, GF_CP, GF_MADH and GF_SCC, respectively. The result showed that both the citrus varieties and extraction methods would affect the composition and content of volatile compounds in citrus EOs. As shown in Figure 2A, the SCC extracts contained extremely low levels of volatile compounds, which was in agreement with the results of the sensory and e-nose analysis. However, compared to CP and MADH, SCC extracts contained some unique volatiles, which were mainly sesquiterpenes such as \(\gamma\)-cadinene, calamenene, 1,4-cadinadiene and \(\alpha\)-cadinene.  
Compared to CP, the EOs extracted by MADH and SCC had an expanded number of volatile compounds, as shown in Figure 2B. This was attributed to the fact that the two methods involved a heating process in which some heat sensitive compounds were degraded as well as the hydrolysis of readily hydrolysate compounds, resulting in the generation of a range of new compounds. Most notably, there was a significant increase in the number and content of alcohols. Most alcohol compounds were significantly more abundant in MADH EOs than in CP EOs. The similar phen  
Figure 2: Comparison of the concentration and number of volatile compounds among six EOs. Bar chart for the concentration of volatile compounds in six EOs (**A**); bar chart for the number of volatile compounds in six EOs (**B**). EOs, essential oils; HY, Huyou; GF, grapefruit; CP, cold pressing; MADH, microwave-assisted hydrodistillation; SCC, spinning cone column.  
sterilized orange juice compared to untreated juice [36]. In addition, six alcohol compounds were only present in MADH and SCC extracts, i.e., (_E_)-2-hexen-1-ol, hexanol, terpinen-4-ol, nonanol, decanol and T-muurolol. Among these, terpinen-4-ol was generally identified as an off-flavor marker in heat-treated orange juice, which was produced by the degradation of linalool [32; 37].  
The content of terpenes was also strongly affected by the extraction method, particularly terpenes, most of which were significantly lower in MADH EOs than in CP EOs, such as \(\beta\)-cubebene, caryophyllene, _cis_-\(\beta\)-farnesene and germacrene D. Terpenes were prone to oxidation and degradation reactions when exposed to oxygen and heat, converting them into terpene alcohols or oxides [38]. Thus, we observed an increase in limonene oxide and terpenic alcohols in MADH EOs at the same time. However, such molecular change produces unpleasant odors like medicinal, woody and waxy that negatively affect the sensory quality [39].  
The effect of the extraction method on aldehyde content was different in the two citrus. In HY EOs, most aldehydes were lower in MADH EO than in CP EO, such as hexanal, nonanal, citronellal, decanal and undecanal. Previous studies have confirmed that thermal treatments easily lead to the loss of aldehydes [30], but some direct chain aldehydes such as (_E_)-2-hexenal may increase due to the degradation of polyunsaturated fatty acids [36]. However, in GF EOs, we observed that the contents of most aldehyde compounds were increased in MADH EO compared to CP EO.  
Carvone was also significantly abundant in MADH EOs and it was also an off-flavor compound, resulting from the oxidative production of \(D\)-limonene during heating processing [40]. Additionally, some nonvolatile compounds such as coumarins (i.e., 7-hydroxycoumarin, bergaptene, osthole, isoauraptene, meranzin and auraptenol) only existed in CP EOs. In conclusion, the appropriate extraction method can be selected according to the individual needs in practice.