## 3 Results and Discussion
### Color
Based on the analysis of the change in the \(\Delta\)E parameter, it was found that the change in color of all VI-treated materials differed significantly from that of fresh sweet potatoes. Similar observations have been noted in previous studies [52]. The change in value of the \(\Delta\)E parameter was smaller in the FD process compared with VD, which indicates better color retention during FD. Similar results were obtained in tests of drying methods on jujube color, in which the smallest color change for dried products was obtained with the FD method [6]. In samples pretreated with onion juice (O) and 2% sodium chloride solution (N), the range of changes in \(\Delta\)E values between the pre-drying and post-drying levels was smaller than that of the control sample (F). This finding indicates a protective effect of the impregnating solutions used on kale color during drying. Both the impregnating solution (\(p<0.05\)) and drying method (\(p<0.05\)) had a significant effect on the total color difference index. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Material** & **Method** & **L*** & **a*** & **b*** & **C*** & \(\Delta\)E** & **BI** \\ \hline F & - & 73.27 \(\pm\) 0.37 h & 28.61 \(\pm\) 0.29 l & 54.19 \(\pm\) 0.32 l & 61.28 \(\pm\) 0.26 l & - & 0.11 \(\pm\) 0.03 f \\ \hline \multirow{2}{*}{F} & FD & 81.02 \(\pm\) 0.41 k & 18.43 \(\pm\) 0.16 f & 38.28 \(\pm\) 0.28 c & 42.48 \(\pm\) 0.21 d & 20.42 \(\pm\) 0.21 f & 0.04 \(\pm\) 0.00 a \\  & VD & 58.95 \(\pm\) 0.05 e & 19.28 \(\pm\) 0.08 g & 37.31 \(\pm\) 0.23 b & 41.99 \(\pm\) 0.20 c & 24.03 \(\pm\) 0.17 h & 0.10 \(\pm\) 0.00 e \\ \hline \multirow{3}{*}{N} & I & 66.60 \(\pm\) 0.13 s & 30.46 \(\pm\) 0.06 m & 56.44 \(\pm\) 0.08 k & 64.13 \(\pm\) 0.06 m & 7.28 \(\pm\) 0.12 a & 0.17 \(\pm\) 0.00 s \\  & FD & 74.45 \(\pm\) 0.28 i & 21.03 \(\pm\) 0.30 g & 40.32 \(\pm\) 0.09 e & 45.47 \(\pm\) 0.16 h & 15.86 \(\pm\) 0.17 d & 0.06 \(\pm\) 0.00 b \\  & VD & 59.21 \(\pm\) 0.09 e & 26.81 \(\pm\) 0.02 i & 38.15 \(\pm\) 0.29 e & 46.63 \(\pm\) 0.25 f & 21.41 \(\pm\) 0.22 g & 0.12 \(\pm\) 0.00 f \\ \hline \multirow{3}{*}{O} & I & 65.23 \(\pm\) 0.13 f & 33,40 \(\pm\) 0.18 h & 57.40 \(\pm\) 0.12 i & 66.41 \(\pm\) 0.14 n & 9.90 \(\pm\) 0.16 c & 0.19 \(\pm\) 0.00 h \\  & FD & 75.35 \(\pm\) 0.31 l & 23.49 \(\pm\) 0.15 h & 47.55 \(\pm\) 0.11 h & 53.04 \(\pm\) 0.13 j & 8.63 \(\pm\) 0.15 b & 0.08 \(\pm\) 0.00 c \\  & VD & 57.29 \(\pm\) 0.28 d & 28.14 \(\pm\) 0.11 k & 45.00 \(\pm\) 0.07 g & 53.07 \(\pm\) 0.09 l & 18.45 \(\pm\) 0.25 f & 0.17 \(\pm\) 0.00 s \\ \hline \multirow{3}{*}{K} & I & 53.46 \(\pm\) 0.78 c & 9.84 \(\pm\) 0.48 c & 48.27 \(\pm\) 0.27 l & 49.26 \(\pm\) 0.23 l & 27.93 \(\pm\) 0.61 i & 0.21 \(\pm\) 0.01 i \\  & FD & 59.05 \(\pm\) 0.43 e & 1.83 \(\pm\) 0.34 a & 38.80 \(\pm\) 0.33 d & 38.84 \(\pm\) 0.33 b & 34.00 \(\pm\) 0.40 j & 0.08 \(\pm\) 0.00 c \\  & VD & 40.10 \(\pm\) 0.60 a & 5.01 \(\pm\) 0.21 b & 35.15 \(\pm\) 0.29 a & 35.51 \(\pm\) 0.30 a & 44.94 \(\pm\) 0.59 l & 0.25 \(\pm\) 0.01 j \\ \hline \multirow{3}{*}{OK} & I & 59.39 \(\pm\) 0.28 e & 19.23 \(\pm\) 0.1 f & 52.39 \(\pm\) 0.41 l & 55.83 \(\pm\) 0.41 k & 16.82 \(\pm\) 0.29 e & 0.20 \(\pm\) 0.00 h \\  & FD & 65.42 \(\pm\) 0.27 f & 12.66 \(\pm\) 0.16 d & 44.35 \(\pm\) 0.36 f & 46.12 \(\pm\) 0.33 g & 20.31 \(\pm\) 0.18 g & 0.09 \(\pm\) 0.00 d \\ \cline{1-1}  & VD & 42.82 \(\pm\) 0.38 b & 14.08 \(\pm\) 0.34 e & 40.53 \(\pm\) 0.24 e & 52.91 \(\pm\) 0.19 e & 36.40 \(\pm\) 0.37 k & 0.33 \(\pm\) 0.01 k \\ \hline \hline \end{tabular} Note: Mean followed by different letters (a–n) in the column differs significantly (\(p<0.05\)).  
\end{table} 
Table 6: Color changes: L*—lightness, a*—greenness, b*—yellowness, C*—saturation, \(\Delta\)E—total color of sweet potato 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 (\(p<0.05\)).