## 3 Results
### Design of Experiments (DoE) and Process Optimization
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Run** & **MeOH (\%)** & **Q\({}_{\text{CO2}}\) (g/min)** & **CFA (\%)** & **TPR (\%)** \\ \hline
**07** & 15 & 5.9 & 11.2 & 7.7 \\
**09** & 15 & 13 & 35.3 & 21.8 \\
**08** & 15 & 20.1 & 62.2 & 33 \\
**12** & 15 & 25 & 32.1 & 21.9 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Run 12 (very high CO2 flow rate) compared to runs with lower solvent flow rates. matrix components (e.g., carbohydrates) or to cell wall structures [42], or even undergo hydrolysis to caffe and quinic acids with the amount of free water still present in the samples, even with this being considerably low. For instance, [PERSON] and [PERSON] [16], report an increase in the CFA contents of potato peel extracts from SWE owing to the fact that the subcritical water provides access to ionic reaction conditions that enhance acid- and base-catalyzed reactions, which can cause hydrolysis of CGA to CFA. In our work, we also observed an increase in CFA and the occurrence of other CGA isomers, such as ICA, as we increase the levels of the extraction parameters. We cannot be sure, however, if this was due to a more efficient extraction of these individual compounds, or if they are products of either CGA hydrolysis or of isomerization reactions. A possible solution for improving the extraction of phenolics from waste matrices by SFE in general could be the combination and integration of different techniques, solvents, and extraction conditions [43; 44; 45]. Alternatively, another approach could be the pretreatment of samples with weak alkalis or enzymes [13]. However, these steps are likely to incur on extra production costs and this also needs to be considered in the design of pilot-scale extractions. Looking at the very few works from the literature specifically reporting on the extraction of CGA by SFE, [PERSON] et al. [46], when simulating and modelling the extraction and fractionation of caffeine from coffee beans by SFE and water as a cosolvent, mentioned that CGA could only be extracted in the water phase. [PERSON] et al. [47], tested different cosolvents in the SFE of asparagus and reported that there was no recovery of CGA when using methanol or ethanol. Only when the authors used a mixture of water:ethanol (1:1) did they observe some recovery of CGA. Other works also confirm the absence of CGA in their supercritical extracts obtained with pure solvents as cosolvents, such as in those of _Echinacea purpurea_[48]. Additionally, the vast majority of the works from the literature that do report some success in extracting CGA via SFE employed plant leaves or flowers as matrices, such as _Hibiscus sabdarififa_[49], _Lonicera japonica_[50], and _Chrysanthemum indicum_[51], which most likely present a simpler structure than the starch-rich peels used in this study, facilitating CGA extraction. The only two exceptions to this involved the use of aqueous solvent solutions as cosolvents: The work of [PERSON] et al. [52], who successfully recovered 52% of the initial CGA content of samples of sun flower seed kernels with a SFE performed at 180 bar, 40 \({}^{\circ}\)C, 105 min, 5% ethanol as cosolvent, and 1.6 mL/min (=1.3 g/min) of CO\({}_{2}\) flow rate; and the SFE of apple peel [37], in which the authors used 4% of water in their ethanolic solution employed as a cosolvent at a 25% cosolvent-to-solvent concentration [37]. Even so, the recovery of CGA was comparatively low. Thus being, a final experiment was carried out to test our general theory that the SFE recovery of CGA might be null due to the technique not involving a liquid phase that facilitates cell disruption and heavy-molecule dissolution in the fluid phase. A solution of water + methanol (1:1) was used as a co-solvent in an SFE run in replacement of the pure methanol used across the experiments in this work. The other optimized conditions were maintained constant (20% cosolvent-to-solvent ratio, 80 \({}^{\circ}\)C, 350 bar, and 15 g/min of CO\({}_{2}\)).