## 4 Conclusions  
This study explored the recovery of a by-product of the Grappa industry, SGP, using UAE. This approach falls under the context of a circular economy, in which by-products from the wine industry are transformed into precious resources, contributing to environmental sustainability. UAE was shown to improve the extraction of antioxidant bioactive compounds, achieving superior results in terms of the TPC more quickly than when using conventional methods, such as a WB and the combined WUS method. Grapeseed oil fortified with 30% EGPs experienced significantly delayed oxidation compared to that seen when adding the synthetic antioxidant BHT at the highest level allowed by law (200 ppm). Therefore, using these extracts as natural antioxidants represents a valid alternative to the use of synthetic antioxidants such as BHT, reducing the environmental impact and promoting greater sustainability in the food industry. Future studies should focus on an in-depth metabolomics characterization of the optimized extracts to clarify their composition and identify the bioactive compounds responsible for their intriguing antioxidant activity. In addition, the fatty acid profile of the fortified oil during storage must be monitored in future to ensure its long-term stability. The following supporting information can be downloaded at: [[https://www.mdpi.com/article/10.3390/app142210184/s1](https://www.mdpi.com/article/10.3390/app142210184/s1)]([https://www.mdpi.com/article/10.3390/app142210184/s1](https://www.mdpi.com/article/10.3390/app142210184/s1)), Table S1. Total phenolic content (mg GAE/g) of grape pomace after distillation along the extraction time. Conceptualization, M.C. and A.L. ; methodology, M.C. and E.C. ; software, M.C. and E.C. ; validation, M.C., E.C. and A.L. ; formal analysis, M.C. and E.C. ; investigation, M.C. and E.C. ; resources, M.C. and A.L. ; data curation, M.C., E.C. and A.L. ; writing--original draft preparation, M.C., E.C. and A.L. ; writing--review and editing, M.C., E.C., D.M. and A.L. ; supervision, A.L. ; funding acquisition, A.L. All authors have read and agreed to the published version of the manuscript. This research was supported by the University of Padova under prot. DOR 2032990. Not applicable. The authors declare that the data supporting the findings of this study are available within the paper. Should any raw data files be needed in another format, they can be made available by the corresponding author upon reasonable request. The authors thank [PERSON], the Distilerie Bonollo (Conselve, Italy) and the \"Associazione Nazionale Industriale Distillatori di Alcoli e Acquaviti\" for their technical support. The authors declare no conflicts of interest. AAI, Antioxidant Activity Index; AOA, antioxidant activity; BBD, Box-Behnken Design; BHT, Butylhydroxytoluene; EGPs, extracts from grape pomace; FRAP, ferric reducing antioxidant potential; GAE, Gallic Acid Equivalent; GP, grape pomace; IT, Induction Time; RP, red pomace; RSM, Response Surface Methodology; SGP, spent grape pomace; TE, Trolox Equivalent; TPC, total phenolic content; TPTZ, 2,3,5-Triphenyltetrazolzolium; Trolox, Tetramethylchromane-2-carboxylic acid; UAE, ultrasound-assisted extraction; WB, water bath; WP, white pomace; WUS, water bath + ultrasound.