## 3 Results and Discussion
### Effect of the Extraction Parameters on the Responses
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline  & \multicolumn{3}{c}{**Optimal Processing Conditions**} & \multicolumn{3}{c}{**Response Optimum**} \\ \cline{2-6}  & **Time (min)** & **Power (W)** & **EOH (\%)** & **Model-Predicted Values** & **Experimental Values** \\ \hline \multicolumn{6}{c}{_Individual conditions for each response variable_} \\ Extraction yield (extract) & 34.4 & 483.0 & 34.1 & 61 \(\pm\) 1\% (_in/w_) & - \\ B-type (epi)catch dimer & 11.2 & 393.0 & 94.8 & 0.64 \(\pm\) 0.01 mg/g dw & - \\ Epicatechin & 24.6 & 222.6 & 41.6 & 1.06 \(\pm\) 0.02 mg/g dw & - \\ Quercetin-3-O-glucose & 39.2 & 191.4 & 59.2 & 0.164 \(\pm\) 0.002 mg/g dw & - \\ Quercetin-3-O-rhammoside & 45.0 & 257.6 & 53.2 & 0.148 \(\pm\) 0.001 mg/g dw & - \\ Total flavonoids & 24.9 & 5.0 & 34.1 & 1.82 \(\pm\) 0.03 mg/g dw & - \\ \hline \multicolumn{6}{c}{_Global conditions considering all response variables_} \\ Extraction yield (extract) & & & & 46 \(\pm\) 1\% (_in/w_) & 46 \(\pm\) 2\% (_in/w_) \\ B-type (epi)catch dimer & & & 0.426 \(\pm\) 0.008 mg/g dw & 0.432 \(\pm\) 0.006 mg/g dw \\ Epicatechin & 14.8 & 94.4 & 68.4 & 0.78 \(\pm\) 0.02 mg/g dw & 0.78 \(\pm\) 0.04 mg/g dw \\ Quercetin-3-O-glucose & & & 0.148 \(\pm\) 0.002 mg/g dw & 0.150 \(\pm\) 0.003 mg/g dw \\ Quercetin-3-O-rhammoside & & & 0.145 \(\pm\) 0.001 mg/g dw & 0.1468 \(\pm\) 0.0002 mg/g dw \\ Total flavonoids & & & 1.49 \(\pm\) 0.03 mg/g dw & 1.51 \(\pm\) 0.04 mg/g dw \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Optimal processing conditions that maximize the extraction of flavonoids from kiwi peel and model-predicted and experimental response optimum. Figure 2: _Cont._  
The extraction solvent was the variables that most affected the extraction yield and the highest extract weights were obtained when sonicating the kiwi peel sample with lower ethanol concentrations (Figure 2). A higher ultrasonic power and longer extraction times also favored the recovery rate. However, it should be noted that the increase in the extract weight does not necessarily resulted in a greater extraction of flavonoids. As seen in Figures 2 and 3, although the more intense conditions of time and ultrasonic power have led to a greater extract weight, these have negatively affected the total flavonoids content. Therefore, compounds other than flavonoids were being extracted (possibly fibers and other carbohydrates, given the intrinsic nature of the kiwi peel) and represented the major fraction of the obtained extract. The optimal points of these two responses are also shown in the cube plots in Supplementary Materials Figure S2A,B, respectively, which shows how the three variables combine to affect the response. A flavonoid-rich extract can thus be obtained more selectively through the control of these two process variables (\(t\) and \(P\)). Interestingly, the ethanol percentage that led to a greater amount of from kiwi peel extract also favored the recovery of flavonoids (Figures 2 and 3 and Supplementary Materials Figure S2). Figure 3: 2D response graphs for the effects of the independent variables on the total content of flavonoids (\(Y_{6}\)) and extraction yield (\(Y_{1}\)) obtained from kiwi peel. In each graph, the excluded variables were fixed at their optimal value. Figure 2: Response surface graphs for the combined effects of the independent variables on the total content of flavonoids (\(Y_{6}\)) and extraction yield (\(Y_{1}\)) obtained from kiwi peel. In each graph, the excluded variable was fixed at its optimal value.