## 2 Results
### Volatile Compounds in Fresh and Dried S. crispus  
Table 2 shows the volatile compounds of fresh and dried _S. crispus_ leaves extracted using headspace solid-phase microextraction (HS-SPME). A typical chromatogram showing the volatile compounds of _S. crispus_ is shown in Figure 5. The concentration of the total volatiles in the fresh sample was 361.23 mg 100 g\({}^{-1}\) db (dry basis). The most abundant volatiles in fresh _S. crispus_ were 2-hexen-1-ol (84.11 mg 100 g\({}^{-1}\) db), 2-hexenal (51.18 mg 100 g\({}^{-1}\) db), 1-octen-3-ol (36.10 mg 100 g\({}^{-1}\) db), linalool (34.91 mg 100 g\({}^{-1}\) db), and benzaldehyde (27.61 mg 100 g\({}^{-1}\) db). The volatiles present in _S. crispus_ could be categorized under the following chemical groups: alcohol (50.2%), aldehyde (24.3%), monoterenoid (10.9%), ketone (4.1%), ester (4.0%), pyridine (3.4%), monoterpene (1.4%), and enone (0.9%). Other compounds were also found, such as cycloalkenes (0.5%), cyclohexenones (0.3%), and unidentified volatiles, represented 0.1% of the total volatiles. All of the drying methods resulted in considerable losses in volatile content (Table 2). The volatile content obtained with CD at 50 \({}^{\circ}\)C and 60 \({}^{\circ}\)C was higher than that produced by other drying methods. This finding showed that CD at 50 \({}^{\circ}\)C was suitable for the drying of _S. crispus_ in terms of retaining volatiles. However, we assumed that VMD and CPD-VMFD would retain a higher concentration of volatiles because the drying rate could be increased by the volumetric heating of microwaves. Microwave treatment could also cause a large disruption of the orderly tissue structure to improve mass and heat transfer, thereby increasing the drying rate. However, in this study, the increasing microwave power of VMD led to high losses in volatile content possibly caused by the high intensity of the drying treatment. Similar results were reported in the drying of basil [5], marjoram [6], ro  
Figure 4: Chemical structures of linalool (**1**), \(p\)-cymene (**2**), limonene (**3**), and isopulegol (**4**). \begin{table}
\begin{tabular}{l c c c c c c c c c c c c} \hline \hline  & & & \multicolumn{3}{c}{**Retention Indexes**} & \multicolumn{3}{c}{**Fresh**} & \multicolumn{3}{c}{**FD**} & \multicolumn{3}{c}{**CPD-VMPD**} & \multicolumn{3}{c}{**CD**} & \multicolumn{3}{c}{**VMD**} \\ \cline{3-13} \cline{5-13} \cline{13-13} \cline{5 Although CD had few disadvantages, such as a long drying time and being prone to volatile oxidation, convective-dried leaves are known to develop a partially dried layer on the surface following CD [6; 26]. This crust layer is important because it forms a barrier that limits losses in volatiles [27]. [PERSON] et al. (2009) reported that a temperature of 50 \({}^{\circ}\)C was advantageous to forming this layer, whereas low temperatures of 30 \({}^{\circ}\)C and 40 \({}^{\circ}\)C resulted in a high volatile loss in lemon myrtle leaves [27]. This finding agreed with our results, that is, a high amount of volatiles were retained through CD at 50 \({}^{\circ}\)C (17.4 mg 100 g\({}^{-1}\) db). At a high temperature of 60 \({}^{\circ}\)C, the concentration of volatiles was reduced by 13.8%. Our results also agreed with a previous study which reported that the VMD of rosemary leads to higher losses in volatile content (61.9 g kg\({}^{-1}\) dw) than CD (87.2 g kg\({}^{-1}\) dw) [25]. It is postulated that VMD is associated with higher losses of volatiles as the enhanced internal vapor generated in plant tissues causes an increased porosity of cellular structure compared to convective-dried leaves [28; 29]. The porous cellular structure in turn facilitates the escape of a higher amount of volatiles to the surroundings. However, it was reported that the VMD of sweet basil retains higher amounts of volatiles than CD does [5]. Therefore, no single drying method is consistently effective in ensuring high volatile retention in herbs, and losses in volatile content may vary considerably from one species of herb to another [30].