## 3 Results and Discussion  
### Screening of DES for Extraction Efficiency  
The evidence emerged from a previous investigation suggested that citrate salts may form DES with increased extraction efficiency [15]. Thus, the generation of a series of DES was systematically approached, by selecting two widely used HBDs, glycerol (GL) and \(\upiota\)-lactic acid (LA), and citrate salts as the HBAs (Figure 1). The salts tested were sodium citrate monobasic (SCMB), sodium citrate dibasic (SCDB) and sodium citrate tribasic (SCTB). However, as attempts to synthesize DES with either GL or LA and SCMB did not meet with success, even when HBD:HBA molar ratio (R\({}_{\text{mol}}^{\text{D/A}}\)) was 15. Thus, SCMB was not further considered. With regard to SCDB, it formed stable DES (no crystallization) with GL and LA at R\({}_{\text{mol}}^{\text{D/A}}\geq 9\); therefore, a series of GL-SCDB and LA-SCDB DES were synthesized with R\({}_{\text{mol}}^{\text{D/A}}\) varying from 9 to 15. On the other hand, SCTB formed stable DES with GL only at a R\({}_{\text{mol}}^{\text{D/A}}\) of 15. By contrast, stable DES with LA and SCTB were formed at R\({}_{\text{mol}}^{\text{D/A}}\geq 7\). Thus, LA-SCTB DES were tested within a R\({}_{\text{mol}}^{\text{D/A}}\) range of 7 to 15. In total, 14 DES were tested covering a wide pH range, from 2.86 (LA-SCDB15) to 7.50 (GL-SATB15). The extraction efficiency of the DES synthesized was compared to other green solvents, including water and 60% (_v/v_) ethanol, but also 60% (_v/v_) methanol, which is a commonly used solvent for polyphenol extraction (Figure 2). The highest Y\({}_{\text{TP}}\) was found for the extraction with 60% (_v/v_) methanol (67.86 \(\pm\) 1.70 mg GAE g\({}^{-1}\) dm), followed by 60% (_v/v_) ethanol (55.64 \(\pm\) 1.39 mg GAE g\({}^{-1}\) dm). Regarding the DES, the LA-SCDB with R\({}_{\text{mol}}^{\text{D/A}}\) = 15, termed as LA-SCDB15, gave a Y\({}_{\text{TP}}\) of 52.31 \(\pm\) 1.31 mg GAE g\({}^{-1}\) dm and it was the most efficient one (php \(<\) 0.05), as opposed to GL-SATB15, which was the least efficient (31.49 \(\pm\) 0.63 mg GAE g\({}^{-1}\) dm). Because it was observed that these extreme Y\({}_{\text{TP}}\) values coincided with the corresponding extreme pH values, concerns were raised as to what extent the pH of a DES could affect polyphenol extractability. Figure 1: Hydrogen bond donors (HBD) and hydrogen bond acceptors (HBA) tested in the current investigation. To obtain evidence for such an effect, the pH values of all DES tested were plotted against Y\({}_{\rm TP}\) (Figure 3). The linear regression gave R\({}^{2}\) = 0.67 (\(p\) = 0.0003), revealing a trend that should not be overlooked, which evidenced higher extraction efficiency for DES with lower pH. Although previous studies on polyphenol extraction with DES stressed emphatically the importance of R\({}_{\rm mol}^{\rm D/A}\) on the extraction yield [16; 21; 22; 23; 24], a correlation of yield with pH is heretofore unreported. As also reported for classical solvents, pH is critical for DES extraction efficiency. Earlier investigations with conventional volatile solvents addressed the role of pH on polyphenol extractability, demonstrating that higher total polyphenol yield from olive leaves could be achieved at pH 2 [25]. Results from following studies on onion solid wastes were in the same line, indicating pH 2 as optimal  
Figure 3: Linear regression between the pH of the DES tested and Y\({}_{\rm TP}\). All DES were tested as 70% (_w/v_) aqueous mixtures. Figure 2: Graph showing the results of screening of the deep eutectic solvents (DES) tested. Extractions were accomplished at 50 \({}^{\circ}\)C, for 150 min, under continuous stirring at 500 rpm. All DES were tested as 70% (_w/v_) aqueous mixtures. Values designated with different letters are statistically different (\(p<0.05\)). to maximize polyphenol extraction [26].