## 4 Conclusions  
The chromatographic analysis allowed identifying four flavonoids in kiwi peel extracts, namely B-type (epi)catechin dimer, epicatechin, quercetin-3-O-glucose, and quercetin-3-O-thamnoside. In order to valorize this by-product as a source of bioactive compounds, a five-level CCRD design coupled to RSM was successfully implemented. The UAE was significantly affected by the independent variables time, ultrasonic power and ethanol concentration, and the theoretical models, fitted to the experimental data by means of least squares calculation using a second-order polynomial equation, were validated based on different statistical criteria. Under the optimized overall UAE condition, it was possible to obtain 46% extract weight and 1.51 mg/g dw of flavonoids. After experimental validation of the predictive model, the bioactivity of the kiwi peel extract obtained under the optimal UAE conditions was evaluated in vitro, showing a somehow promising bioactive potential. Thus, this study contributed to the valorization of kiwi peel through its recycling into a bioactive extract with potential for application in the food industry as a natural preservative (given the antioxidant and antimicrobial properties). The reuse of kiwi peel could also contribute to a circular bioeconomy and resource-use efficiency. **Supplementary Materials:** The following are available online at [[https://www.mdpi.com/article/10.3390/app11146416/s1](https://www.mdpi.com/article/10.3390/app11146416/s1)]([https://www.mdpi.com/article/10.3390/app11146416/s1](https://www.mdpi.com/article/10.3390/app11146416/s1)), Figure S1: HPLC-chromatographic profile of phenolic compounds in kiwi peel extract and chemical structure of (-)-epicatechin and quercetin-3-O-glucose. The compounds \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline  & **Kiwi Peel Extract** & \multicolumn{3}{c}{**E211**} & \multicolumn{3}{c}{**E224**} \\ \hline Antibacterial activity & MIC & MBC & MIC & MBC & MIC & MBC \\ \hline _Staphylococcus aureus_ & 1 & 2 & 4 & 4 & 1 & 1 \\ _Bacillus cereus_ & 2 & 4 & 0.5 & 0.5 & 2 & 4 \\ _Listeria monocytogenes_ & 2 & 4 & 1 & 2 & 0.5 & 1 \\ _Escherichia coli_ & 1 & 2 & 1 & 2 & 0.5 & 1 \\ _Salmonella Typhimurium_ & 2 & 4 & 1 & 2 & 1 & 1 \\ _Enterobacter cloace_ & 2 & 4 & 2 & 4 & 0.5 & 0.5 \\ \hline Antifungal activity & MIC & MFC & MIC & MFC & MIC & MFC \\ \hline _Aspergillus ochracus_ & 0.5 & 1 & 1 & 2 & 1 & 1 \\ _Aspergillus niger_ & 1 & 2 & 1 & 2 & 1 & 1 \\ _Aspergillus versicolor_ & 0.5 & 1 & 2 & 2 & 1 & 1 \\ _Penicillium functilosum_ & 0.5 & 1 & 1 & 2 & 0.5 & 0.5 \\ _Penicillium anurantiogriseum_ & 1 & 2 & 2 & 4 & 1 & 1 \\ _Trichoderma viride_ & 0.5 & 0.5 & 1 & 2 & 0.5 & 0.5 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Antimicrobial activity of the kiwi peel extract obtained under optimized UAE conditions. identification is presented in Table 1. Figure S2: Cube plots illustrating the optimal values as a function of the three independent variables for extraction yield (A), total content of flavonoids (B), and all responses simultaneously (C). The values at each vertex of the first two cubes are response values. The model-predicted values represented in each cube plot are shown in Table 3; Table S1: Natural and coded values of the independent variables used in the five-level central composite rotatable design (CCRD) used to optimize the extraction of flavonoids from kiwi peel. ; Table S2. Parametric values estimated with the polynomial Equation (1) and statistical information of the model fitting procedure. Parametric superscripted 1, 2, and 3 stand for the variables time, ultrasonic power, and ethanol concentration, respectively. Conceptualization, L.B., I.C.F.R.F., and C.C. ; methodology, M.G., C.C., J.P., M.I.D., R.C.C. and D.S. ; software, J.P.; validation, M.S., A.L.C., I.C.F.R.F. and L.B. ; formal analysis, M.G., C.C., J.P., M.I.D., R.C.C. and D.S. ; writing--original draft preparation, M.G., C.C., J.P. and M.I.D. ; writing--review and editing, J.P., R.C.C., D.S., M.S., D.T., A.L.C., I.C.F.R.F. and L.B.