## 2 Results and Discussion
### Antioxidant Properties  
The results of this study further showed that the 1,1-diphenyl-2-picrylhydrazyl (DPPH) clearing capacity of the citrus oleoresin extracted by SFE was 9.69\(\sim\)40.15 \(\upmu\)M Trolox/g, while the 2,2\({}^{\prime}\)-Azinobis(3-ethylbenzothizolin-6-sulfonic Acid) (ABTS) clearing capacity was 19.07\(\sim\)73.80 \(\upmu\)M Trolox/g (Figure 2). The Pokan oleoresin had the best antioxidant capacity among the three varieties. In addition, in terms of the essential oils extracted with steam distillation, the scavenging DPPH capacity of the Pokan oils was 0.12\(\sim\)0.19 \(\upmu\)M Trolox/mL, while the ABTS clearing capacity was 0.31\(\sim\)0.37 \(\upmu\)M Trolox/mL (data not shown). Essential oils have poorer antioxidant capacity than citrus oleoresin because the terpenoid compounds in essential oils accelerate oxidant reactions. The independent or complex combinations of antioxidants from oil are able to slow down the evolution of peroxide compounds effectively under Rancimat testing conditions [39]. In this study, citrus oleoresin extracted via SFE was dissolved in sunflower oil at 1 mg/mL, and the results showed that the sunflower oil group prolonged the induction time for 2.2 h during the evolution of peroxide compounds, such that citrus oleoresin extracted via SFE was able to extend the induction time to 2.42\(\sim\)2.47 h (Figure 3), with the obtained antioxidative indexes for the Ponkan, Tankan, and Murcott extracts being 1.21, 1.21, and 1.23, respectively (Figure 2). However, the antioxidative indexes of the essential oils extracted via steam distillation were \(<\)0.8 (data not shown). The experimental results indicated that all of the citrus-extracted oleoresins exerted fat-oxidative inhibition, although there were no differences in the three citrus varieties. Flavonoids are powerful antioxidants against free radicals because they act as radical scavengers [34; 35]. This activity is attributed to their hydrogen-donating ability. Three structural groups are important for the evaluation of their antioxidant capacity: (I) the ortho-dihydroxy structure in the B-ring, which confers greater stability to radicals, possibly through hydrogen bonding, and which participates in electron dislocation; (II) the 2,3-double bond, which, in conjugation with a 4-oxo function, is responsible for electron dislocation from the C-ring; and (III) the 3-hydroxyl group in the C-ring [34]. According to the structures of tangeritin and nobilentin, in comparison to flavonoids, PMF has fewer hydroxyl groups, revealing that the 2,3-double bond conjugated with a 4-oxo function, which is responsible for electron dislocation from the C-ring, could be responsible for free radical clearness. Figure 2: ABTS and DPPH scavenging abilities and antioxidative indexes of citrus oleoresin. \({}^{\mathrm{a}}\) Data presented are in mean \(\pm\) SD (\(n=3\)), with the different letters indicating significant differences (\(p<0.05\)). However, the DPPH scavenging effects do appear to be under the influence of the structure [40]. The results of this study revealed that citrus extract fared better in terms of scavenging for ABTS. The results also indicated the citrus oleoresins were rich in limonoids (11.2%-40.6%) and PMF (8.6%-25.9%). Limonoids are highly oxygenated triterpenoids with fewer hydroxyl groups than flavonoids, such that their free radical scavenging ability is poor [41]. The results of this study revealed that antioxidant ability is positively associated with the PMF content in citrus oleoresin. Other studies have shown that the protection of antioxidants can prevent the degradation of phytosterol; however, while antioxidants are important, the interactions between the different compounds and their compositions are also important [42].