# The Effect of Hydrolysis on the Antioxidant Activity of Olive Mill Waste
## 5 Conclusions  
In this study, methanol, acid and alkaline hydrolysis extraction was applied on olive oil waste collected from the Maltese Islands. From the results obtained, it can clearly be seen that significant changes occur in the phenolic content of olive mill waste obtained from olives at different maturity stages. Alkaline and acid hydrolysis had a major influence on phenolic content and antioxidant activity. In fact, alkaline-hydrolysis-derived extracts showed the highest antioxidant activity with respect to TPC, _DPPH Radical Savenging_ activity and metal ion reducing activity, which includes FRAP and CUPRAC assays, whereas acid hydrolysis had the highest _ABTS_ scavenging activity. From the results obtained, acid hydrolysis showed losses of phenolic content and antioxidant activity due to heating during the hydrolysis procedure. Thus, this might be the reason why acid hydrolysis had lower concentrations than those of alkaline hydrolysis. The correlation analysis carried out on the different phenolic classes revealed that most of the extracts showed a positive correlation between TPC, TFC and TdOPC. Furthermore, these phenolic classes tended to be highly correlated with metal ion reducing activity and _Radical Savenging_ activity.  
**Supplementary Materials:** The following supporting information can be downloaded at: [[https://www.mdpi.com/article/10.3390/app122312187/sl](https://www.mdpi.com/article/10.3390/app122312187/sl)]([https://www.mdpi.com/article/10.3390/app122312187/sl](https://www.mdpi.com/article/10.3390/app122312187/sl)). Figure S1: a graph showing the percentage of _DPPH_ inhibition for all extracts in alkaline hydrolysis, Figure S2: a graph showing the percentage of _DPPH_ inhibition for all extracts in acid hydrolysis, Figure S3: a graph showing the percentage of _DPPH_ inhibition for all extracts in maceration extraction, Figure S4: a graph showing the percentage of _ABTS_ inhibition for all extracts in alkaline hydrolysis, Figure S5: a graph showing the percentage of _ABTS_ inhibition for all extracts in acid hydrolysis, Figure S6: a graph showing the percentage of _ABTS_ inhibition for all extracts in maceration extraction, Figure S7: a graph showing the percentage of superoxide inhibition for all extracts in alkaline hydrolysed extracts, Figure S8: a graph showing the percentage of superoxide inhibition for all extracts in acid hydrolysed extracts, Figure S9: a graph showing the percentage of superoxide inhibition for all extracts in macerated extracts, Table S1: extraction yield obtained.  
**Author Contributions:** Conceptualization, K.A. and F.L.; methodology, K.A. and F.L.; validation, K.A.; formal analysis, K.A. and F.L.; investigation, K.A.; resources, F.L.; data curation, K.A., F.L. and M.H.O.; writing--original draft preparation, K.A. and F.L.; writing--review and editing, K.A., F.L. and M.H.O.; visualization, K.A.; supervision, F.L.; project administration, F.L.; funding acquisition, F.L. All authors have read and agreed to the published version of the manuscript.  
**Funding:** This research was funded by the Malta Council for Science & Technology and the Scientific Technological Research Council of Turkey (Tubitak) through the MCST-Tubitak 2021 Joint Call for R&I Projects under grant agreement No MCST-TUBITAK-2021-04 for OPobicell project.  
**Institutional Review Board Statement:** Not applicable.  
**Informed Consent Statement:** Not applicable.  
**Data Availability Statement:** Not applicable.  
**Acknowledgments:** [PERSON] for providing the olive waste from different cultivars.  
**Conflicts of Interest:** The authors declare no conflict of interest.