## 3 Results and Discussion
### Color  
The obtained dried products were characterized by different brightness values from 40.01 to 81.02 (Table 6). depending on the applied drying method and the impregnating solution (Figure 1). The use of VI (\(p<0.05\)) resulted in a decrease in brightness in all samples tested compared to the control sample (F). The highest L* value was observed for untreated sweet potato (81.02), compared with 73.27 for fresh sweet potato. Other investigators have noted high L* values for fresh jujube compared to L* after drying [6]. [PERSON], [PERSON], [PERSON], [PERSON], and [PERSON] [52] observed higher L* values for apple slices after drying. VD resulted in browning of the sweet potato tested, and all samples were darker than the dried samples obtained by the FD method. Similar results were reported by [PERSON] et al. (2019). A significant effect of the drying method on the tested parameters was noted (\(p<0.05\)). \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Material** & **Method** & **Dry Matter (\%)** & **Water Activity (-)** \\ \hline F & - & 20.07 \(\pm\) 0.70 a & 0.986 \(\pm\) 0.002 f \\ \hline \multirow{4}{*}{F} & I & - & - \\  & FD & 97.05 \(\pm\) 0.15 f & 0.082 \(\pm\) 0.002 a \\  & VD & 94.93 \(\pm\) 0.04 d & 0.287 \(\pm\) 0.005 c \\ \hline \multirow{4}{*}{N} & I & 18.71 \(\pm\) 0.25 a & 0.984 \(\pm\) 0.002 f \\  & FD & 97.91 \(\pm\) 0.07 fij & 0.087 \(\pm\) 0.006 a \\  & VD & 95.66 \(\pm\) 0.14 e & 0.496 \(\pm\) 0.001 e \\ \hline \multirow{4}{*}{O} & I & 21.15 \(\pm\) 0.36 b & 0.977 \(\pm\) 0.001 f \\  & FD & 97.26 \(\pm\) 0.04 fij & 0.108 \(\pm\) 0.010 a \\  & VD & 97.66 \(\pm\) 0.06 gsh & 0.304 \(\pm\) 0.027 c \\ \hline \multirow{4}{*}{K} & I & 23.67 \(\pm\) 0.17 c & 0.989 \(\pm\) 0.011 f \\  & FD & 97.61 \(\pm\) 0.12 gsh & 0.192 \(\pm\) 0.002 b \\  & VD & 96.95 \(\pm\) 0.11 f & 0.438 \(\pm\) 0.055 e \\ \hline \multirow{4}{*}{OK} & I & 21.19 \(\pm\) 0.26 b & 0.976 \(\pm\) 0.000 f \\  & FD & 98.29 \(\pm\) 0.23 j & 0.116 \(\pm\) 0.012 a \\ \cline{1-1}  & VD & 97.39 \(\pm\) 0.11 fij & 0.022 d \\ \hline \hline \end{tabular} Note: Mean followed by different letters (a–j) in the column differs significantly (\(p<0.05\)).  
\end{table} 
Table 5: Dry matter and water activity of sweet potatoes dried by freeze drying (FD) and vacuum drying (VD) when fresh (F) and impregnated in sodium chloride (N), onion juice (O), kale juice (K), or onion–kale juice (OK). Mean followed by different letters in the column differs significantly (\(p<0.05\)). The drying process of products of plant origin is usually characterized by the degradation of pigment compounds, which results in higher values of parameters a* and b* (Table 6). This relationship holds for sweet potatoes not subjected to VI and sweet potatoes impregnated with K (due to the dark color of kale juice). VI protected the color parameters of the other impregnation solutions. Positive values were obtained for the color parameter a*, which indicated the red hue of the dried sweet potatoes. In the case of 2% sodium chloride solution and onion juice impregnating solutions, an increase in the values of the studied parameter was observed compared to the control sample, which may be related to the enzymatic browning reaction during pretreatment. Other authors have observed a similar relationship in fluidized bed drying of kale impregnated with onion juice + NaCl, and NaCl [11]. Both the impregnating solution and the drying method had a significant effect on the values of the color parameters a* and b* (\(p<0.05\)). Higher a* and b* values were recorded for the sublimation-dried samples. A significant effect of impregnating solution (\(p<0.05\)) and drying method (\(p<0.05\)) on color saturation (C*) was observed in the present study (Table 5). The C* value increased for both drying methods with the following impregnating solutions: S, O, and OK, compared to the untreated control sample. Similar observations were noted by [PERSON] et al. [28].