## 3 Results and Discussion
### Optimization-Effect of Process Variables
Although the total polyphenol content of PP may exhibit variations attributed to varietal, cultivation and climatic factors, in a survey on 60 different potato cultivars, no value higher than 12.59 mg GAE g\({}^{-1}\) dm was found for the total polyphenol content [20]. This fact strongly evidenced that the level of Y\({}_{\text{TP}}\) achieved in this study was indicative of a high-performance extraction system. The direct effect of pH (X\({}_{1}\)) was non-significant, but the influence exerted by pH was manifested by its quadratic term X\({}_{1}\)\({}^{2}\). The optimum pH 5 determined by the model was the upper limit of the experimental design, suggesting that extraction was not favored in a more acidic environment. The role of pH has been implicated in polyphenol-cyclodextrinin interactions because it affects the dissociation of either carboxyl functions or phenolic hydroxyls. Experiments with caffeic acid (CA) and HP-\(\beta\)-CD indicated that constants of complex formation were sensitive to pH and decreased with pH increases [21]. This behavior was ascribed to the hydrophobicity of CA, which depends on its ionization state. Since the main inclusion interactions are hydrophobic, developed between the HP-\(\beta\)-CD cavity and the guest molecule, acidic pH would contribute to suppressing CA ionization, maintaining it in a non-ionized and thus, more hydrophobic form, hence, the greater stability constant. Results from rosmarinic acid/\(\beta\)-CD interactions were in concurrence [22]. However, the carboxyl group of the quinic moiety on CGA, which is the principal polyphenol in PP [5], has a pK\({}_{\text{a}}\) of 3.33, and at pH 5, this group would be deprotonated [23]. Therefore, it would be expected that polyphenol extraction from PP with HP-\(\beta\)-CD would be favored at lower pH. This finding might indicate that other interactions between CGA and HP-\(\beta\)-CD and/or interactions with other PP constituents affected the overall extraction yield. The effect of R\({}_{\text{L/S}}\) and S\({}_{\text{S}}\) was significant (Figure 2, inset table), as demonstrated for several extraction processes [24; 25; 26; 27]. Both variables have been closely associated with diffusion phenomena that govern solid-liquid extraction, and their proper regulation might be critical in attaining high extraction yield. Regarding R\({}_{\text{L/S}}\), it defines the concentration gradient that is the driving force of diffusion, which in turn, favors mass transfer [28; 29; 30]. Thus, suitable setting of R\({}_{\text{L/S}}\) would entail negligible resistance to mass transfer and effective entrainment of solute (polyphenols) into the liquid medium [30]. In this study, the optimum R\({}_{\text{L/S}}\) estimated was 80 mL g\({}^{-1}\), which matches the optimum 82-84 mL g\({}^{-1}\) found for polyphenol extraction from PP and eggplant peels using water/ethanol mixtures [16; 31]. Likewise, careful regulation of S\({}_{\text{S}}\) results in effective dispersion of the solute that diffuses from the internal part of the solid particles, into the liquid volume. This would minimize external resistance and increase mass transfer [32]. To achieve higher diffusivity and increased mass transfer, S\({}_{\text{S}}\) must be properly set, since it has been shown that diffusivity is highly correlated with S\({}_{\text{S}}\)[33]. The optimum S\({}_{\text{S}}\) of 800 rpm determined in this study was equal to those estimated for polyphenol extraction with deep eutectic solvent from saffron processing wastes [26] and _Moringa oleifera_ leaves [27], and comparable to 900 rpm found for polyphenol extraction from onion solid wastes [34]. However, optimum S\({}_{\text{S}}\) of as low as 200 rpm has also been reported [35].