## 3 Results and Discussion  
### Comparative Performance of Different DESs for Antioxidant Extraction  
The experimental AC values of 36 extracts of four herbs (Cha, LB, M, and N) and two agricultural residues (BH and ChoP) obtained by six green DESs (ChClCl:CitA, Gu:CitA, Gu:U-30, Gu:U-50, B:CitA, and B:U) and determined using three analytical methods (CUPRAC, ABTS, and DPPH) are depicted in Figure 1.  
As can be seen, the values of ACs for studied extracts determined by the modified analytical procedures differ significantly from each other (Figure 1, Duncan test). This variability can be explained by the influences of (1) the chosen DES type, its viscosity and \begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Independent Variable**} & \multicolumn{4}{c}{**Levels with the Codes**} \\ \cline{2-4}  & \(-\mathbf{1}\) & \(\mathbf{0}\) & \(\mathbf{1}\) \\ \hline Choline chloride/Urea & 1:2 (0.5) & 1:1 (1) & 3:2 (1.5) \\ ratio (ChCLU) (mol/mol) & & & \\ Water content (WC) (\%) & 20 & 30 & 40 \\ Extraction time (t) (min) & 5 & 10 & 15 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Independent variables for the optimization of DES–UAE of antioxidants from lemon balm. polarity affecting the extraction efficiency of antioxidants, (2) analytical parameters of the applied analytical assays, and (3) genetic, agronomic, and environmental factors, which would affect the level of antioxidants in natural plant materials and residues from them. DES polarity and viscosity mainly depend on their constituents, the molar ratios of HBAs to HBDs, and the water content in the DES [30]. These studies suggest that amide HBDs (urea) were more suitable for extracting antioxidant compounds present in the analyzed plant materials than acid HBDs (citric acid). Among the tested green solvents, the DES based on betaine (B) and urea (U) (1:1) had the highest performance in extracting antioxidants capable of forming a color Cu(I)-chelate after the reduction of Cu(II) to Cu(I). Therefore, the studied extracts (except BH) prepared based on B:U (1:1) revealed the highest CUPRAC values ranging between 56.7 umol TE/g for Cha and 429.9 umol TE/g for LB (Figure 1a). Similarly, B:U (1:1) was the best DES to recover the antioxidants from LB, M, and ChoP, which could scavenge ABTS**+** and DPPH**+** radicals (ABTS = 136.8-146.5 umol TE/g and DPPH = 145.2-170.3 umol TE/g, Figure 1b.c). Interestingly, the highest ABTS results (40.5-136.5 umol TE/g) for Cha, N, and BH extracts were obtained using glucose (Gu) and urea (U) as DES after adding 30% of water (Gu:U-30), whereas these samples had the highest DPPH values (101.7-111.5 umol TE/g) using also the same DES mixed with a higher water concentration of 50% (Gu:U-50). These results suggest that the addition of a higher WC (50%) reduced the viscosity of a DES based on Gu:U and improved the mass transfer. Thus, the extraction efficiency of antioxidants with the ability to scavenge DPPH**+** radicals was enhanced. However, adding excessive amounts of water probably suppressed the interactions between the DES constituents (Gu and U) and reduced the performance of this green solvent in the extraction of antioxidants capable of quenching ABTS**+** cation radical. Unexpectedly, more water can break the hydrogen bond network between HBD and HBA, resulting in a ruptured Gu:U structure [31]. Apart from this, high WC in Gu:U solvent may enhance the preferential Gu hydration, thereby reducing the extraction efficacy. Surprisingly, the ABTS**+** cation radical is the least sensitive to antioxidant compounds present in the investigated plant extracts prepared by all proposed types of DESs (ABTS = 0.0-146.5 umol TE/g < DPPH = 11.9-170.3 umol TE/g < CUPRAC = 0.0-429.9 umol TE/g). Evidently, among the studied herbs, the richest source of antioxidants (regardless of the applied analytical tests) was LB, while the ChoP as agricultural waste revealed more potent antioxidant properties than BH (Figure 1).