## 2 Results and Discussion
### Analysis of Volatile Contents
modifiers_. Leaves dried using CPD-VMFD (179.02 mg 100 g\({}^{-1}\) db) achieved higher retention of volatiles compared with fresh leaves (176.81 mg 100 g\({}^{-1}\) db), but the differences in values were marginal (Table 3). Thermal treatment of leaves during CPD-VMFD may have inactivated polyphenol oxidase (PPO), thereby reducing enzymatic degradation of volatiles [27]. In addition, short thermal treatment, achieved by the introduction of VMFD assured that volatiles were not exposed to high temperature for an excessively long period. Consistent with the literature, CD and VMD led to considerable losses of volatiles, evidenced by the drying of thyme [17], basil [13], and rosmary [35]. In this study, CD retained a higher amount of volatiles than did VMD. A similar result was reported in the drying of rosmary [35]. In another study, VMD was shown to retain higher total volatiles because of the fast drying process at a mild temperature [18]. However, in this study, average temperature throughout VMD, irrespective of microwave power, was 46 \({}^{\circ}\)C. Slightly higher temperatures of CD at 50 and 60 \({}^{\circ}\)C and CPD-VMFD at 50 \({}^{\circ}\)C were advantageous in forming a partially dried surface layer on leaves that function as a barrier, thereby limiting the diffusion of volatiles and minimizing their loss [36]. Furthermore, the formation of a crust layer is characteristic of products dried using CD. Parameters in CD and VMD also affect the total concentration of volatiles. A moderate temperature of 50 \({}^{\circ}\)C and microwave power of 9 W/g were preferable in yielding higher total volatiles. At these conditions, leaves were subjected to moderate temperature at a relatively short time. It was suggested that VMD led to higher amount of volatiles loss as microwave treatment causes a more porous structure in plant tissue because of the high internal vapor formed during the drying process. The higher porosity of tissue structure of vacuum microwave-dried leaves, compared to convective-dried leaves, could be a possible reason that a higher amount of volatiles could escape to the surrounding medium [37; 38]. Analysis of volatile compounds of dried samples showed that CPD-VMFD and CD 50 \({}^{\circ}\)C yielded the highest volatile concentrations. Therefore, both methods were used to determine the influence on phytosterols concentration. Table 4 shows the identified phytosterols in _P. nodiflora_ and their respective concentrations. The total concentrations of phytosterols in fresh leaves were 1237.39 mg 100 g\({}^{-1}\) db. \(\beta\)-Sitosterol was the major phytosterol in fresh and dried samples, followed by stigmasterol and campesterol. CPD-VMFD was able to retain a higher total yield of phytosterols (711.07 mg 100 g\({}^{-1}\) db) compared with CD 50 \({}^{\circ}\)C samples (668.66 mg 100 g\({}^{-1}\) db). Due to the chemical structures of phytosterols, such as the unsaturation at the side chain or ring, these compounds may undergo oxidation when exposed to air and heat treatment at high temperatures. The oxidation process of phytosterols is reported to increased when phytosterols are subjected to heat and the stability of phytosterols is also influenced by the temperature and time of heat treatment [39]. In this study, microwave treatment during VMFD benefited the retainment of higher phytosterol content. Lower temperature and reduced oxygen condition during VMFD may have minimized the thermo-oxidation of phytosterols, thereby increasing the stability of phytosterols. This result is in agreement with the results obtained by [PERSON] et al. [40], [PERSON], [PERSON], and [PERSON] [41], which all reported that phytosterol concentration decreased with increased drying temperature and prolonged drying time.