# Extraction of Antioxidants from Grape and Apple Pomace: Solvent Selection and Process Kinetics
## 3 Results and Discussion
### Kinetic Study of Extraction Processes
Considering this functional parameter, the extraction driven by an aqueous solution of ethanol at 70% (\(v/v\)) shows differences depending on the temperature value. We can see that the maximum antioxidant activity, according to a DPPH test, is obtained at 70 \({}^{\circ}\)C and 90 \({}^{\circ}\)C, with results of 24.05 \(\pm\) 1.94 and 24.02 \(\pm\) 1.89 mg TEAC/gds at 60 min, respectively. It is interesting to observe that these results at both temperatures are identical from a statistical perspective, while a slightly higher initial extraction rate is observed at 90 \({}^{\circ}\)C, indicating that this temperature, even in the absence of light, can be deleterious of the antioxidant activity of the extract (as suggested before by TPC analysis) or that an asymptotic value for antioxidant activity is being reached. While the average slopes of tangents to the curve along the extraction period (slow phase) indicate an increasing extraction rate with temperature, the value of TPC at zero time increases slowly from 25 to 70 \({}^{\circ}\)C: 3 mg TEAC/gds (25 \({}^{\circ}\)C), 4.5 mg TEAC/gds (50 \({}^{\circ}\)C), 5.5 mg TEAC/gds (70 \({}^{\circ}\)C), and 7 mg TEAC/gds (90 \({}^{\circ}\)C), with a sudden increment from  
Figure 2: Kinetic analysis of the extraction processes of AP employing (**A**) acetone at 25 \({}^{\circ}\)C, (**B**) acetone 80% at 25 \({}^{\circ}\)C, (**C**) water at 25 \({}^{\circ}\)C, and (**D**) ethanol 96% (\(v/v\)) at 70 \({}^{\circ}\)C obtained by the Folin–Ciocalteu method. Fitting lines correspond to the second-order kinetic model shown in Equation (1). 70 to 90 \({}^{\circ}\)C coherent with a certain physical modification of the structure near the boiling point observed for the solvent at atmospheric pressure. Figure 4: Kinetic analysis of the antioxidant activity as measured by the DPPH method during the extraction processes of AP employing a 70%/30% _v_/_v_ ethanol/water mixture at several temperature values: **(A)** 25 \({}^{\circ}\)C, **(B)** 50 \({}^{\circ}\)C, **(C)** 70 \({}^{\circ}\)C, and **(D)** 90 \({}^{\circ}\)C. The lines show the fitting of a one-term second-order kinetic model (Equation (1)). Figure 3: Kinetic analysis of the extraction process of AP employing **(A)** ethylene glycol 90% at 70 \({}^{\circ}\)C and **(B)** propylene glycol 90% at 70 \({}^{\circ}\)C obtained by the Folin–Ciocalteu method. The lines show the fitting of the second-order kinetic model shown in Equation (1). In the case of propylene glycol and ethylene glycol, despite being the best extractives in terms of TPC, the antioxidant extraction capacity is lower, being higher in the case of EG (19.29 \(\pm\) 1.58 TEAC/gds in 60 min), while the nontoxic PG scarcely extracts 11.41 \(\pm\) 1.30 TEAC/gds in the same time. A deep eutectic solvent (DES) composed of CHCl and EG is as good as EG (17.83 \(\pm\) 1.49 TEAC/gds in 60 min). This may be caused by the nature of the phenolics extracted with these solvents in comparison to those dissolved in mixtures of water/ethanol, which show an almost-double antioxidant-specific activity in terms of AC/TPC (1.5 for ethanol 70% at 70 \({}^{\circ}\)C versus 0.77 for ethyleneglycol and 0.46  
Figure 5: Kinetic analysis of the extraction processes of AP employing (**A**) acetone at 25 \({}^{\circ}\)C, (**B**) acetone/water 80%/20% v/o at 25 \({}^{\circ}\)C, (**C**) water at 25 \({}^{\circ}\)C, and (**D**) ethanol 96% at 70 \({}^{\circ}\)C: antioxidant activity as obtained by DPPH method. Kinetic model fitting is presented in lines. Figure 6: Kinetic analysis of the extraction processes of AP with glycols and a glycol-based DES at 70 \({}^{\circ}\)C. (**A**) ethylene glycol 90%, (**B**) propylene glycol 90%, and (**C**) ChClGly 90%. The kinetic model fitting is presented in lines. for propyleneglycol), showing that, on top of being greener and less expensive, ethanol-water mixtures are more specific for phenolics with a high antioxidant activity when valorizing AP.