## 2 Results
### Antioxidant Activity
All of the drying methods led to considerable losses of phenolic compounds, including FD that involved no heat treatment. The low TPC of the heat-treated samples could be caused by the thermal and oxidative degradation of phenolic compounds. In VMD, TPC considerably decreased when microwave power was 12 W/g. In CD, a higher phenolic concentration was retained at 40 \({}^{\circ}\)C and 60 \({}^{\circ}\)C than at 50 \({}^{\circ}\)C. The highest TPC (1086.71 mg/100 g dw) was detected with CD at 60 \({}^{\circ}\)C among the drying methods. Overall, the TPC and antioxidant values did not correlate well. Therefore, this suggests that it is possible that other antioxidant compounds other than phenolic compounds, such as essential oil volatile components, were present in _S. crispus_, which contributed to the overall antioxidant activity. Antioxidant compounds derived from plants such as phenolic compounds and terpene derivatives are widely reported to exhibit antioxidative properties. In our study, the most abundant phytosterols identified in _S. crispus_ were \(\beta\)-sitosterol and stigmasterol, both of which were reported to have an antioxidant effect [18]. Phytosterols are triterpenes and are characterized by the tetracyclic cyclopenta (\(\alpha\)) phenanthrene structure [19]. The chemical structures of both compounds are shown in Figure 3.  
The volatile composition of _S. crispus_ was also investigated in this study; 2-hexen-1-ol 2-hexenal 1-octen-3-ol linalool, and benzaldehyde constituted the major compounds of _S. crispus_ volatile content. However, the antioxidant activity of _S. crispus_ is more likely to be contributed by terpenoids present in _S. crispus_, such as linalool, _p_-cymene, limonene, and isopulegol. Figure 4 shows the chemical structures of these compounds. Linalool, which is the most dominant terpenoid in _S. crispus_, is a monoterpene alcohol reported to have an antioxidant effect [20]. A vast number of terpenoids are reported as potential antioxidant molecules due to their ability to interact with free radicals. The most effective antioxidant compounds are the ones that disrupt the free-radical chain reaction. These antioxidants usually consist of aromatic or phenolic rings with the ability to donate H\({}^{+}\) to free radicals produced during oxidation, becoming radicals themselves. The resonance delocalization of electrons in the aromatic ring will act to stabilize these radical intermediates [21]. Therefore, the H\({}^{+}\)-donating abilities are predicted to contribute to the structure-activity relationship [22]. With the presence of various constituents in the extract of _S. crispus_, such as terpenoids and phytosterols, the overall antioxidant activity may have been contributed by the synergistic interactions of these compounds. Synergism between antioxidants involves several mechanisms. Synergism occurs when there is a combination of two or more antioxidants with different antioxidant mechanisms, or when there is a combination  
Figure 3: Chemical structures of \(\beta\)-sitosterol (**1**) and stigmasterol (**2**). of two or more different free-radical scavengers, in which an antioxidant is found to be regenerated by other antioxidants. More specifically, an antioxidant is characterized as a sacrificial antioxidant which is oxidized to exert protection on another antioxidant. Furthermore, regeneration of a primary antioxidant, with higher reduction potential by a secondary antioxidant (co-antioxidant), with less reducing power, is able to contribute to a higher net interactive antioxidant effect compared to the total individual antioxidant effects [23]. Synergistic interactions between antioxidant compounds derived from hops extract were reported in a previous study [24]. Drying intensity slightly influenced the antioxidant activity and TPC. However, CD at a low temperature of 40 \({}^{\circ}\)C produced a sample with the highest antioxidant activity among the convective-dried samples (50 \({}^{\circ}\)C and 60 \({}^{\circ}\)C).