## 2 Results  
### Drying Kinetics  
The drying kinetics of _S. crispus_ leaves dried using CD, VMD, and CPD-VMFD were best described by the modified Page model (Equation (1)) based on the highest \(R^{2}\) and the lowest root-mean-square error (RMSE).  
\[MR=a\cdot\exp\,(-k\cdot t^{n}), \tag{1}\]  
where \(a\) is the model constant, \(k\) is the drying constant, and \(n\) is the dimensionless empirical constant.  
Figure 1 shows the relationship between the moisture ratio (MR) and the time consumed for the leaves to dry under various drying methods and conditions. CD at 40 \({}^{\circ}\)C and 50 \({}^{\circ}\)C required 180 and 150 min to complete drying, respectively. By contrast, CD at 60 \({}^{\circ}\)C required 120 min. VMD at 6, 9, and 12 W/g required shorter drying durations of 28, 21, and 14 min, respectively. The lengthy drying process of CD was shortened to 105 min by introducing VMFD as observed in CPD-VMFD.  
Figure 2 illustrates the relationship between the drying rate and the MR. The drying rates between CD and VMD largely differed. VMFD increased the drying rate to 0.016 min\({}^{-1}\) from a low drying rate of 0.003 min\({}^{-1}\) during CPD at 90 min. When the moisture content of the leaves was high during the initial drying period, the drying rate increased with time regardless of the drying methods. In CD, surface water evaporated easily to the surrounding air through external diffusion. The removal of surface water produced a moisture gradient in the material, and the removal of moisture at this stage depended on internal diffusion that occurred at a slow rate. In VMD and VMFD, the high drying rate observed at the beginning could be explained by the high amount of available water molecules, resulting in high microwave energy absorption. However, the amount of moisture that reduced as drying progressed corresponded to the low absorption of microwave energy, resulting in the decreasing drying rate.  
The maximum drying rates observed for VMD at 6, 9, and 12 W/g were 0.086, 0.115, and 0.173 min\({}^{-1}\), respectively. In CD at 40 \({}^{\circ}\)C, 50 \({}^{\circ}\)C, and 60 \({}^{\circ}\)C, the maximum drying rates were 0.015, 0.022, and 0.031 min\({}^{-1}\), respectively. In VMD, the drying duration of _S. crispus_ leaves was reduced from 81% to 92%. When VMFD at 9 W/g was introduced to CPD at 50 \({}^{\circ}\)C, the drying rate increased  
Figure 1: The relationship between moisture ratio and time for the drying of _Strobilanthes crispus_ with convective drying (CD) at 40, 50, and 60 \({}^{\circ}\)C, vacuum microwave drying (VMD) at 6, 9, and 12 W/g, and convective pre-drying followed by vacuum microwave finish drying (CPD-VMFD) at 50 \({}^{\circ}\)C and 9 W/g.  
Figure 2: The relationship between drying rate and moisture ratio for the drying of _S. crispus_ with CD at 40, 50, and 60 \({}^{\circ}\)C, VMD at 6, 9, and 12 W/g, and CPD-VMFD at 50 \({}^{\circ}\)C and 9 W/g.  
from 0.003 min\({}^{-1}\) to 0.044 min\({}^{-1}\). This condition could be attributed to the volumetric heating of VMD in which high internal moisture could be effectively removed at a fast rate, thereby shortening the drying time from 150 min to 105 min.  
Increasing the parameter level, that is, microwave power and hot air temperature, led to an increase in the drying rates. As the microwave power increased from 6 W/g to 9 W/g, the average drying rate also increased from 0.035 min\({}^{-1}\) to 0.047 min\({}^{-1}\). On the contrary, when the microwave power increased from 9 W/g to 12 W/g, the average drying rate increased from 0.047 min\({}^{-1}\) to 0.070 min\({}^{-1}\). When hot air temperature increased by 10 \({}^{\circ}\)C, a marginal improvement was noted. The average drying rate also increased from 0.008 min\({}^{-1}\) to 0.009 min\({}^{-1}\) by increasing the hot air temperature from 40 \({}^{\circ}\)C to 50 \({}^{\circ}\)C. Similarly, the average drying rate increased from 0.009 min\({}^{-1}\) to 0.010 min\({}^{-1}\) when temperature increased from 50 \({}^{\circ}\)C to 60 \({}^{\circ}\)C.