## 3 Results and Discussion
### Optimisation of Extraction Performance
Furthermore, for the extractions without pretreatment, R\({}_{\text{L/S}}\) (X\({}_{2}\)) had no direct impact on Y\({}_{\text{TP}}\), but its cross effect (X\({}_{1}\)X\({}_{2}\)) with \(C_{\text{DES}}\) was negative. For the ultrasound-pretreated extractions, quadratic effects of both R\({}_{\text{L/S}}\) (X\({}_{2}\)) and S\({}_{\text{S}}\) (X\({}_{2}\)) were positive and significant. The impact of the ultrasonication pretreatment could be more characteristically depicted by the changes found in the optimization settings of all three variables. Pretreatment resulted in shifting the theoretically required DES amount from 85% to 75% (_w/v_), R\({}_{\text{L/S}}\) from 20 to 100 mL g\({}^{-1}\), and S\({}_{\text{S}}\) from 800 to 200 rpm (Table 5). This outcome suggested a profound influence of ultrasonication on the pattern through which the process variables can affect polyphenol extraction. On the other hand, it is to be emphasized that the ultrasonication pretreatment exerted virtually no effect on the extraction performance, since the optimal predicted values for Y\({}_{\text{TP}}\), for the extraction without and with pretreatment, had no statistically significant difference, being 80.93 \(\pm\) 6.68 and 82.87 \(\pm\) 4.28 mg GAE g\({}^{-1}\) dm, respectively (Table 5). A critical assessment of these data would point out that samples received no ultrasonication pretreatment required higher DES concentration and a relatively high speed of stirring to yield extracts with increased Y\({}_{\text{TP}}\). The optimum \(C_{\text{DES}}\) determined was 85% (_w/v_), very close to the 80% (_w/v_) found for _M. oleifera_ polyphenol extraction with a DES composed of glycerol and sodium acetate (6:1) [13]. Likewise, the optimum S\({}_{\text{S}}\) (800 rpm) was comparable to 900 rpm determined for polyphenol extraction from onion solid wastes, using a DES of glycerol/sodium propionate (8:1) [25]. By contrast, the optimum R\({}_{\text{L/S}}\) (20 mL g\({}^{-1}\)) was rather low compared with those reported for polyphenol extraction with DES, ranging from 29 mL g\({}^{-1}\)[27] to 100 mL g\({}^{-1}\)[25]. This picture was fundamentally changed when samples were ultrasonicated prior to stirred-tank extraction. The optimum \(C_{\text{DES}}\) dropped to 75% (_w/v_), while ultrasonication-pretreated samples required 5 times higher R\({}_{\text{L/S}}\) but 4 times lower S\({}_{\text{S}}\) to provide maximum Y\({}_{\text{TP}}\) (Table 5). Ultrasonication is known to contribute in intensification of extraction efficacy, owed to several phenomena that accompany irradiation with ultrasounds, including propagation of ultrasound pressure waves through  
Figure 4: Three-dimensional diagrams given on a comparative arrangement, to illustrate the effect of ultrasonication pretreatment on the variations in the response (Y\({}_{\text{TP}}\)), as a function of different variable levels. the solvent and resulting cavitation effects [28; 29]. The increased performance usually observed in the ultrasound-assisted extractions is generally attributed to mechanical, cavitation, and thermal effects, which can provoke cell wall disruption and reduction of particle size, resulting eventually in enhanced mass transfer across cell membranes. On such a theoretical background, it could be postulated that the reduced \(C_{\text{DES}}\) required for the ultrasonicated-pretreated samples to reach optimum \(Y_{\text{TP}}\), could probably be ascribed to higher polyphenol diffusivity and solubilization, as a result of cell wall/membrane disruption. Such an event could facilitate leaching and dissolution of polyphenols into the liquid phase, an assumption corroborated by the drastic decrease in the optimum \(S_{\text{S}}\) required, from 800 to 200 rpm. It would appear that lower \(S_{\text{S}}\) (200 rpm) is sufficient to provide the turbulence necessary for effective diffusion, whereas higher \(S_{\text{S}}\) (800 rpm) might provoke adsorption effects, thereby hindering higher extraction yields.