## 4 Conclusions  
There is a global trend toward the use of natural flavonoids present in fruits, vegetables, oilseeds and herbs as antioxidants or functional foods.  
In the present work, two different ecotypes of _O. ficus-indica_, showing dissimilar phytochemicals profile, were studied. The total phenolic content in the extracts with solvents is almost unchanged between the fresh samples collected in June and in August. This indicates that the different maturation period, if the plant still did not produce the fruit, does not interfere with the content of secondary metabolites, such as phenols, that is almost the same. The polyphenol present in greater amount in all species is Rutin; the dried samples (Cult_AD and SL_AD) contain a greater amount than the fresh.  
Being polyphenols polar compounds, the yields % quantitative of extraction with SFE-CO2 are not high even if the extractions are very selective and the extracts nearly pure.  
Figure 8: Chromatogram resulting from the analysis of the extract by SFE-CO2 of _OFI s.l._ dried to 30% and added 20% of Diatomee Sand.  
Concerning the SFE-CO\({}_{2}\) better results were obtained with the samples of _OFI cult_. in spite of the _OFI s.l._, probably because the ecotype _OFI s.l._ was much mucilaginous, impeding to complete some extractions.  
The better SFE-CO\({}_{2}\) extraction results were obtained with samples preventively dried, as the sample CULT_90E_20O. Thus, the reduction of water, responsible for the degradation of active compounds, in presence of the Ottawa sand, produces highly selective extracts.  
The treatment with Diatomee Sand implies a reduction of both quantity and selectivity of extraction of polyphenols, perhaps because the Diatomee Sand is hygroscopic and it adsorbs water from the surrounding environment, reducing the function of co-solvent of the water naturally present in the matrix. By changing the % hydration, it is possible to modulate the selectivity of extraction.  
The polyphenols are polar compounds, therefore the pressure was increased to 250 bar to increase the polarity of CO\({}_{2}\). Nevertheless, a reduction of both the % yield quantitative and selectivity of extraction was obtained, demonstrating that the high pressures result in a loss of bioactive principles, like polyphenols. Our results evidenced as, for the first time, by improving the parameters of SFE-CO\({}_{2}\), which is an innovative, safe and cheap technique, it is possible to extract qualitatively and quantitatively the polyphenolic fraction from plant matrices hard to treat, namely the _Opuntia_ cladodes. These results increase their value as good nutraceuticals sources, instead to be considered only a by-product from prickly pear cultivation in Calabria.  
Finally, the cladodes extracts could be used as food supplements or starting materials in the pharmaceuticals industries.  
Conceptualization, B.d.C.; methodology, N.B.; validation, D.G.; formal analysis, F.R.L., V.P. and A.C.; investigation, N.B. and V.P.; writing--original draft preparation, D.I. and N.B.; writing--review and editing, D.I. and M.S.S.; supervision, M.S.S. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.  
Not applicable.  
Data Availability StatementData is contained within the article.  
The authors are grateful to the [PERSON] for the collection of the plants material and the identification of species. We also thank the Laboratory of Rheology and Food Engineering of University of Calabria, the Department of Informatics, Modeling, Electronics and Systems Engineering and the Department of Pharmacy, Health and Nutritional Sciences-University of Calabria.  
The authors declare no conflict of interest.