# Extraction of Antioxidants from Grape and Apple Pomace: Solvent Selection and Process Kinetics
## 3 Results and Discussion  
### Kinetic Study of Extraction Processes  
The first step was to perform solid-liquid extractions of the selected residues with different solvents of industrial interest: ethanol, acetone, ethylene glycol, propylene glycol, water, and water:solvent mixtures, as well as a known deep eutectic solvent (choline chloride-ethylene glycol -ChCl:EG- with a molar ratio of 1:4). After the extraction process, the TPC and antioxidant capacity of the samples were measured. The results from the AP extraction are shown in Figures 1-6 while the extraction results from the GP extraction are displayed in Figures 7-10. In these figures, experimental data is shown as points while the fitting of the empirical second-order kinetic model is displayed as lines. When referring to results for the AP extraction using ethanol 70% (_v/v_) extraction at several temperatures (Figure 1), final TPC values increase with temperature from 10.57 \(\pm\) 1.21 mg GAE/gds at the final extraction point (60 min) at 25 \({}^{\circ}\)C to 13.13 \(\pm\) 1.43 mg GAE/g gds at 70 \({}^{\circ}\)C, a value that is very similar to the one obtained at 90 \({}^{\circ}\)C. The effect of the temperature is also perceived in the initial extraction rate, thus a fast washing of the residue in the first minute leads to 1.8 mg GAE/gds at 25 \({}^{\circ}\)C, 3.0 at 50 \({}^{\circ}\)C, 4.3 at 70 \({}^{\circ}\)C, and 5.7 mg GAE/gds at 90 \({}^{\circ}\)C. Considering the analytical method here performed, sampling at particular time values followed by a subsequent spectrophotometric analysis, the dissolution of readily available material on or near the surface of waste particles was so fast that it cannot be followed, thus leading to an apparent value at zero time, as reflected by \(\mathrm{q_{0}}\) in the kinetic model or, more visibly, by a perceived TPC value at zero time in Figure 1 (and for this parameter and the antioxidant activity in all other figures). In this case of AP extraction with hydroalcoholic solutions at several temperatures, the increment in TPC at zero time indicates a sharp increase in phenolic solubility with temperature. In fact, this increment is slightly exponential--TPC = 1.18-exp(0.0177-T[\({}^{\circ}\)C]); R2 = 0.995, a phenomenon that is also observed for (+)-catechin solubility in water and water-ethanol mixtures in similar temperature intervals [17]. After this fast solubilization, a progressive extraction occurs, reaching values that are maximal at 70 \({}^{\circ}\)C. The subsequent reduction in final TPC values observed at higher temperature could be ascribed to pore collapse by dehydration or a similar phenomenon that hinders mass transfer of phenolics out of the porous structure, as an increase in temperature always favors the desorption of the compounds out of the pore inner surface. Alternatively, phenolics degradation due to a relative high temperature usually happens, and affects TPC measurement, as indicated by [PERSON] et al. [18]. According to these authors, in the case of grape marc or pomace, polyphenol oxidase can reduce TPC values during the first hour of extraction at temperatures of 80 \({}^{\circ}\)C or higher. This deactivation is evident for raw grape marc but does not happen in the case of filtered extracts, suggesting that the phenolic composition is key to the behavior of phenolics under oxidizing conditions at mild-high temperatures. A solvent comparison is shown in Figure 2 where the effect of water in the solvent mixture was studied. Water extraction presents a final yield for TPC in the case of AP, 11.56 \(\pm\) 0.61 mg GAE/gds, which is similar to those obtained with a hydroalcoholic mixture containing 70% ethanol and 30% \(v/v\) water. In the case of acetone and acetone 80%/water 20% AP extraction, the results are also similar (10.42 \(\pm\) 1.08 and 10.90 \(\pm\) 1.09 mg GAE/gds, respectively), so, in the extraction of AP, water content seems not relevant for this process.