## 3 Results
### Extraction of Phenolics by SFE vs. Other Techniques
The main solvents used in low-pressure techniques (MAE, UAE, SWE, etc.) are aqueous solutions of ethanol or methanol, with the optimized concentration varying across different methods. Extractions are also usually quite fast, with most of them lasting only a few minutes in the case of MAE, UAE, and SWE. This is in agreement with [PERSON] [43], who confirms that MAE and UAE usually require shorter extraction times than high-pressure extractions, such as pressurized-liquid extraction (PLE) and SFE. However, the temperatures used in these methodologies are very high (80-220 \({}^{\circ}\)C), which can lead to the degradation of phenolic molecules. [PERSON] [43] also mentions that SFE requires much lower working temperatures, but that MAE still yields the highest TPCs, followed by PLE, UAE, and SFE. Another advantage of the aforementioned techniques relative to SFE is that samples do not have to undergo previous drying and milling, since the extraction is carried out in contact with liquid solvent. In the case of SFE, however, drying the sample is usually required because the high amounts of water present in vegetable matrices are believed to compete with the solute and to interact with the solvent, decreasing the yield of the process [38]. One noticeable trend in the SFE of phenolics from other vegetable peels is that, across almost all the matrices tested, the optimized conditions for their extraction were found at the lowest levels of the extraction parameters. Optimal temperatures usually ranged from 35 to 50 \({}^{\circ}\)C; pressures, from 100 to 250 bar; cosolvent concentration, from 7 to 10%; and extraction times were 20-30 min. The only exceptions to this trend were the extraction of apple peels, which was optimal at 80 \({}^{\circ}\)C and with MeOH at 25% for 200 min, and of potato peels (this work), also performed at 80 \({}^{\circ}\)C and with MeOH at 20%, for 60 min. The former was also the only other work to report the yields of individual phenolics, with a 0.16 mg CGA per g of extract recovered in the final extracts. Despite some similarities, each of the above methodologies relies on slightly different mechanisms of action, and effective molecule extraction happens according to the nature of the distinct solvent-matrix interactions involved. The high efficiency of UAE, for example, is associated with the phenomenon of ultrasound-induced cavitation, where the constant collapsing of bubbles disrupts the vegetable cells and facilitate compound migration to the liquid solvent. This is similar to what happens in MAE, where microwaves overheat the water structures and cause cell burst. In the case of SWE, this is obtained via the extremely high water temperatures involved, which greatly increase compound solubility. A similar characteristic across all these techniques, however, is that the samples, prior to extraction, are put in contact with a liquid solvent, which serves as the fluid phase that will later dissolve the molecules extracted. From this perspective, a clear difference in SFE, relative to the aforementioned techniques, is that the fluid phase is a solvent in its supercritical state, which grants the fluid gas-like properties of diffusion, viscosity and surface tension, and liquid characteristics of density and solvation power. This combination of features renders SFE a highly efficient methodology for the extraction of many phytochemicals. However, comparatively, the extraction of phenolics from vegetable food matrices by SFE tends to deliver lower yields. On this, it is important to highlight that while there is evidence to suggest that the conditions used in SFE processes (especially \(T\) and \(P\)) are strong enough to also promote some disturbance of the vegetable cell structures, this effect does not seem nearly as pronounced as in the other techniques mentioned above [45]. This also indicates that successful extraction is still possible for molecules that are easily dispersed on the surface of the solid particles, and which can be transferred to the fluid phase via fluid convection.