## 3 Results
### Design of Experiments (DoE) and Process Optimization
Additionally, it can be seen from the plot that if the _MeOH_% were fixed and only \(q_{\text{CO2}}\) were increased, there would be a certain threshold from where a negative effect would start to be seen on the final response. This is because, as emphasized before, the solvent would probably not have enough residence time in the extraction vessel to penetrate the matrix structures or interact with the molecules of interest, lowering the extraction yields. In order to put this theory to test, yet another extraction (Run 12, Table 6) was performed using a very high \(q_{\text{CO2}}\) (25 g/min), while maintaining the other conditions constant. The results of this extraction along with the results of the two axial points and the central point of the optimization study (runs 07, 08, and 09, respectively; Table 4), are replicated in Table 6 for ease of comparison. At the very high flow rate of 25 g/min, both dependent variables decreased. Thus, the limit for \(q_{\text{CO2}}\) can be set at around 20 g/min and the adjusted optimum conditions of the DoE then become: 80 \({}^{\circ}\)C, 350 bar, MeOH at 20%, and 18 g/min, which corresponds exactly to run 04 of the DoE (Table 4). Despite the good recovery of caffeic acid (CFA) observed, it is clear that the central reason for the relatively low overall recovery was the complete absence of chlorogenic acid (CGA) in the extracts. In potato peel samples, this compound accounted for 64% of the TPC and hence the low TPR observed across the experiments. It is not easy to explain exactly why this happens, but we discuss some possible explanations for this in the following paragraphs. Although the theories debated are specifically related to the results of this work, the arguments made could also provide good insights on how to tackle and improve the extraction of other phenolic compounds by SFE. One possible reason for the null recovery of CGA may be linked to the molecular weight and polarity of this molecule. While CFA, successfully extracted in this work, has a molecular weight of 180 g/mol, CGA is almost twice as heavy (354 g/mol) [40]. Usually, to aid the dissolution of heavier molecules in supercritical fluids, the cosolvent concentration can be increased [38], but as a result, the fluid phase becomes highly polar. However, CGA is an ester of CFA, and consequently, less polar than its simpler counterpart. Due to the opposing nature of these properties at play during SFE, the extraction of CGA from potato peels with the aid of very polar solvents may be difficult to achieve. Very high cosolvent concentrations (25% and above) could potentially increase recoveries, as confirmed by the model generated in this work, and also by the work of [PERSON] et al. [36], who used 40% of ethanol as cosolvent in the SFE of lemon peels. However, it is also important to keep in mind that by definition, cosolvents should be efficient at very small concentrations [38]. Their addition in large amounts is also not interesting from an economic standpoint, since this would result in more energy and solvent requirements, apart from the need for longer post-operations of concentration, isolation, and purification. It is important to acknowledge that one counterargument to molecular weight being a limiting property could be the very successful extraction of other molecules heavier than phenolics by SFE, such as carotenoids [41], which weigh around 500-600 g/mol. However, such molecules are non-polar, as is the supercritical solvent, and this characteristic strongly aids their migration to the fluid phase and explains the very high yields observed. Another possible explanation for the poor CGA extraction performance might be that, when submitted to high \(T\) and \(P\) during extraction, the compound can conjugate to other