## 4 Conclusions  
The application of the Box-Behnken design and response surface methodology to the ultrasound- and microwave-assisted extraction of antioxidant compounds from chicory and fennel by-products were effective tools to study the impacts of the process variables and to optimize the extraction conditions. The efficiency of the sustainable techniques depended on the matrix. Compared to a conventional extraction performed with a 70% ethanol aqueous solution as extraction solvent, UAE and MAE performed with water as solvent allowed the obtainment of chicory extracts richer in phenolic compounds (+48% and +34%, respectively), significantly decreasing the extraction times (4-fold and 90-fold reduction, respectively.) Regarding the fennel, the application of UAE and MAE determined the extraction of a slightly lower amount of phenolics (\(-11.7\%\) and \(-10.5\%\), respectively) but halving the extraction time (UAE) or reducing it 60-fold (MAE). Based on these findings, microwave-assisted extraction seems to be the most sustainable technique. The solid-water ratio was the variable with the highest effect in models applied to microwave-assisted extraction and it was one of the main variables affecting ultrasound-assisted extraction, contributing sometimes positively and sometimes negatively to the extraction of antioxidant compounds. The applications of the models allowed the prediction of TPC and AA values close to the experimental results, thus proving the adequacy of modelsthemselves. Extraction techniques strongly affected the number, amount and type of phenolic compounds extracted, and were beyond consideration relating to sustainability aspects; this information can guide the choice of extraction technique based on the type of phenolic profile that is to be obtained. The following supporting information can be downloaded at: [[https://www.mdpi.com/article/10.3390/app13074191/s1](https://www.mdpi.com/article/10.3390/app13074191/s1)]([https://www.mdpi.com/article/10.3390/app13074191/s1](https://www.mdpi.com/article/10.3390/app13074191/s1)), Figure S1: 3D response surface plots for antioxidant activity (mmol Trolox/g dm) of chicory extracts obtained through UAE: effect of solid-water ratio (A), temperature (B), extraction time (C), power (D), and their interactions; Figure S2: 3D response surface plots for TPC (mg gallic acid/g dm) of fennel extracts obtained through UAE: effect of solid-water ratio (A), temperature (B), extraction time (C), power (D), and their interactions; Figure S3: 3D response surface plots for antioxidant activity (mmol Trolox/g dm) of fennel extracts obtained through UAE: effect of solid-water ratio (A), temperature (B), extraction time (C), power (D), and their interactions; Figure S4: 3D response surface plots for antioxidant activity (mmol Trolox/g dm) of chicory extracts obtained through MAE: effect of solid-water ratio (A), extraction time (B), power (C), and their interactions; Figure S5: 3D response surface plots for TPC (mg gallic acid/g dm) of fennel extracts obtained through MAE: effect of solid-water ratio (A), extraction time (B), power (C), and their interactions; Figure S6: 3D response surface plots for antioxidant activity (mmol Trolox/g dm) of fennel extracts obtained through MAE: effect of solid-water ratio (A), extraction time (B), power (C), and their interactions. Conceptualization, A.B. ; methodology, A.B. and A.F. ; software, A.B. ; validation, A.B. ; formal analysis, A.B. ; investigation, A.B. and A.F. ; resources, A.B. ; data curation, A.B., R.R. and A.F. ; writing--original draft preparation, A.B. ; writing--review and editing, A.B. ; visualization, A.B., R.R. and F.G.; supervision, A.B. and R.R. ; project administration, A.B. ; funding acquisition, A.B. All authors have read and agreed to the published version of the manuscript. This work was carried out in the frame of the TOFACS project. The project receives funding from the Italian Minister of University and Scientific Research under the Italian PNR 2015-2020. Not applicable.