## 2 Results
### Color Analysis  
Color is often perceived as an index of freshness and is associated with a nutritional value. Therefore, color is an important cue used by consumers to evaluate the quality of a certain product. Among factors contributing to the changes in the color profile of dried herbs are hydrolysis, caramelization, and nonenzymatic and enzymatic reactions. Chlorophylls are unstable and may undergo transformation or degradation, forming olive-brownish and greenish derivatives or colorless substances. Figure 10 shows the image of dehydrated _S. crispus_ dried using various drying methods. The visual images were confirmed by the results obtained from instrumental measurement of color (Table 6). In our study, the freeze-dried samples had the lightest (highest \(L^{*}\) value) and greenest (lowest \(a^{*}\) value) color. The convective-dried samples at 40 \({}^{\circ}\)C and the VMD-treated samples (6, 9, and 12 W/g) also exhibited lightness and greenness values comparable with those of the freeze-dried samples. The freeze-dried samples effectively preserved the color of herbs because heat was not involved in dehydration, in addition to a reduced oxygen condition. This finding was possible because the most widely occurring process in plants is the conversion of chlorophyll \(a\) and \(b\) to pheophytins (olive green) during heat treatment. Drying inherently led to the mechanical disruption of the cytoplasmic membrane, thereby allowing membrane permeability to increase, causing cellular acids to be released and degrading chlorophylls to pheophytins [40]. VMD conditions (reduced oxygen, low boiling temperature with reduced pressure, and short drying durations) effectively preserved the color of herbs. High hot air temperatures (50 \({}^{\circ}\)C and 60 \({}^{\circ}\)C) in the convective-dried and convective pre-dried samples produced dark leaves because the subject to high drying temperature for a long duration might have intensified chlorophyll degradation.  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Drying Method** & **Water Activity, a\({}_{\text{w}}\)** \\ \hline Fresh & 0.9879 \(\pm\) 0.000 a \\ Freeze-drying & 0.0245 \(\pm\) 0.000 b \\ CD 40 \({}^{\circ}\)C & 0.1577 \(\pm\) 0.010 c \\ CD 50 \({}^{\circ}\)C & 0.1071 \(\pm\) 0.004 d,f \\ CD 60 \({}^{\circ}\)C & 0.0968 \(\pm\) 0.004 d,e \\ VMD 6 Wg\({}^{-1}\) & 0.1539 \(\pm\) 0.002 c \\ VMD 9 Wg\({}^{-1}\) & 0.0821 \(\pm\) 0.005 e \\ VMD 12 Wg\({}^{-1}\) & 0.1239 \(\pm\) 0.003 f,g \\ CPD-VMFD, 50 \({}^{\circ}\)C, 9 Wg\({}^{-1}\) & 0.1272 \(\pm\) 0.004 g \\ \hline \hline \end{tabular} CD—convective drying, VMD—vacuum microwave drying, CPD—convective pre-drying, VMFD—vacuum microwave finish drying. Values with the same letter in the same column were not significantly different (\(p<0.05\)), according to [PERSON]’s test.  
\end{table}
Table 5: Water activity of dried _S. crispus_ dehydrated using different methods.  
In \(b^{\star}\), a high \(b^{\star}\) (yellow) was obtained for the freeze-dried samples, indicating that carotenoids were preserved well during FD, followed by VMD at 12, 6, and 9 W/g, and all of these processes exposed the samples to a reduced oxygen condition. Carotenoids are susceptible to oxidation. As such, this result was expected. Carotenoids were preserved to a greater extent with CPD-VMFD at low hot air temperature (40 \({}^{\circ}\)C) and exposure to oxygen than with CD at 50 \({}^{\circ}\)C and 60 \({}^{\circ}\)C. For the fresh samples, the period between harvest and analysis could have allowed an extensive chlorophyll degradation to occur because of the high water content in the samples (a\({}_{\rm w}\) of 0.9879). Hence, fresh leaves appeared dark, and their green and yellow intensity decreased.