## 3 Results and Discussion
### Effect of the Extraction Parameters on the Responses  
The 3D response surface graphs constructed to illustrate the effect of the independent variables involved in the UAE of flavonoids from kiwi peel are presented in Figure 1. In each graph, the excluded variable was positioned at its individual optimal value shown in Table 3. The B-type (epi)catechin dimer showed an opposite extraction tendency compared to that of the other flavonoids. As observed in Figure 1, the increase in the value of the three variables up to intermediate levels decreased the extraction of this proanthocyanidin, but the consequent increase induced a new improvement in its recovery rate. The extraction trend thus followed quadratic effects, which were more pronounced for EtOH \(>P>t\). In this particular case, the central projections of the surfaces at the base of each 3D graph illustrate the variable ranges where the extraction of B-type (epi)catechin dimer was lower. Regarding epicatechin, its extraction was promoted by the increase in processing time and ethanol concentration up to around 25 min and 42% EtOH, respectively, but the consequent increased caused a reduction in the recovery yield, probably due to its degradation. Actually, the red colored response surface area of the 3D graph of the combined effects of these two variables illustrates very well the optimal response (Figure 1). Contrariwise, the ultrasonic power did not significantly affect (\(p>0.05\)) the extraction of this antioxidant flavonoid (Supplementary Material Table S2). Based on these observations and the reverse response surfaces of B-type (epi)catechin dimer and epicatechin (Figure 1), it may be suggested that the UAE process may have promoted the breakdown of this B-type proanthocyanidin into its flavanol subunits. Figure 1: Response surface graphs for the combined effects of the independent variables on the content (mg/g) of individual flavonoids (\(Y_{2}\)–\(Y_{5}\)) recovered from kiwi peel. In each graph, the excluded variable was fixed at its optimal value. For the two quercetin glycosides, the 20 experimental runs of the CCRD design yielded comparable mean responses, as shown by the intersection values in Equations (5)-(6) and Supplementary Material Table S2. The longer the processing time, the greater the linear recovery of these quercetin glycosides, but this variable was significant (\(p<0.05\)) only for quercetin-3-O-rhammoside. In turn, the ultrasonic power merely impacted the extraction of quercetin-3-O-glucose, whose yield increased with the application of up to \(\sim\)190 W and then decreased for higher powers. Both compounds were significantly affected by the extraction solvent (\(p<0.05\)); while the recovery of quercetin-3-O-glucose mainly followed a linear effect, quadratic effects marked the trend of quercetin-3-O-rhammoside extraction. Ethanol/water ranging from 50 to 60% appeared to be preferable mixtures. In the particular case of quercetin-3-O-glucose, it is also interesting to note the strong positive interaction between ultrasonic power and ethanol concentration. Figure 2 illustrates the response surface graphs for the effects of the independent variables on the total content of flavonoids and extraction yield obtained from kiwi peel. The excluded variable in each 3D graph was fixed at its optimal value. In general, the extraction of the total flavonoids content follows a trend comparable to that discussed above for epicatechin, with additional interactive effects between the solvent and the other two variables. So, the lower the ethanol concentration and ultrasonic power (EtOH \(\times\)\(P\)), the greater the recovery of flavonoids, while lower ethanol concentrations combined with longer processing times (EtOH \(\times\)\(t\)) seem to improve the UAE process efficiency.