# Extraction of Antioxidants from Grape and Apple Pomace: Solvent Selection and Process Kinetics
## 3 Results and Discussion
### Kinetic Study of Extraction Processes
Acetone and a mixture of acetone (80% _v/v_) and water, when compared to ethanol 70% in water at 25 \({}^{\circ}\)C (12.08 \(\pm\) 1.39 TEAC/gds in 60 min) are slightly better extractants (12.06 \(\pm\) 1.28 TEAC/gds and 11.45 \(\pm\) 1.39 in the same time, respectively), although water is the worst solvent in this case, with an antioxidant capacity of the aqueous extract at 60 min of 6.75 \(\pm\) 0.58 TEAC/gds. However, AC values at zero time are very interesting for acetone (4.5 mg TEAC/gds and 3.5 mg TEAC/gds) compared to the benchmark ethanol 70% solution (3 mg TEAC/gds). In general, these results show again the superiority of ethanol:water 70/30 _v/v_ mixture for the extraction of antioxidant power from AP, although pure acetone allows for a very fast washing, suggesting a good solvation power or a certain role of viscosity--very low for the acetone at 25 \({}^{\circ}\)C--in this first phase. Moreover, the ratio AC/TPC is 1.8 for ethanol 96% and 1.5 for ethanol 70%, showing a more specific extraction for the hydroalcoholic solvent richer in ethanol. When applying these solvents and conditions to the extraction of GP components (Figures 7-10), the better extraction process in terms of TPC and antioxidant capacity yields is performed again with the mixture ethanol/water 70% (_v/v_), increasing yield values as the operating temperature increases. Therefore, as expected, temperature affects the solubility of phenolics and the rate of the extraction process from the matrix, in this latter case due to the increase in the mass transfer rate and/or solvation power (viscosity reduction, increment in solubility). The GP extraction yields ten times higher TPC than when extracting AP. Moreover, the process is more affected by temperature: it increases from 44.05 \(\pm\) 1.78 mg GAE/gds at 25 \({}^{\circ}\)C to 68.46 \(\pm\) 1.03 mg GAE/gds at 90 \({}^{\circ}\)C (a 55.4% increment in comparison to a 24.2% increment for AP with the same temperature variation). This fact could suggest that AP porosity structure is less affected by this variable, so the effective diffusivity that depends on it changes only slightly. Figure 7: Kinetic analysis of the extraction process from grape pomace (GP) with a hydroalcoholic solution (ethanol/water 70%/30% _v/v_) at several temperatures: (**A**) 25 \({}^{\circ}\)C, (**B**) 50 \({}^{\circ}\)C, (**C**) 70 \({}^{\circ}\)C, and (**D**) 90 \({}^{\circ}\)C. Data, in points, reflects the total phenolic content (TPC) obtained by the Folin–Ciocalteu method. The lines indicate the fitting of the second-order kinetic model presented in Equation (1). Figure 8: Total phenolic content results, and their kinetic analysis, obtained during the extraction processes applied to GP using several solvents: (**A**) acetone at 25 \({}^{\circ}\)C, (**B**) acetone 80%: water 20% at 25 \({}^{\circ}\)C, (**C**) ethanol 96% at 70 \({}^{\circ}\)C, (**D**) water at 25 \({}^{\circ}\)C, (**E**) ethylene glycol at 70 \({}^{\circ}\)C, and (**F**) propylene glycol at 70 \({}^{\circ}\)C. The lines show the second-order kinetic model (Equation (1)) fitting to data. Figure 9: Antioxidant activity by DPPH method in points (data) and fit of the second-order kinetic model in lines. Results from grape pomace (GP) with a hydroalcoholic solution (ethanol/water 70%/30% \(\times\)/_v_) at several temperatures: (**A**) 25 \({}^{\circ}\)C, (**B**) 50 \({}^{\circ}\)C, (**C**) 70 \({}^{\circ}\)C (green line and points: ethanol 70%; orange line and points: ethanol 96%), and (**D**) 90 \({}^{\circ}\)C.  
Again, there is a notable difference between the TPC and antioxidant capacity (AC) results: a 30% water content leads to a slightly better extraction of phenolics, but, for GP extractions, their AC is more than double that of the phenolics extracted with ethanol containing 4% water at 70 \({}^{\circ}\)C. Curiously, the AC/TPC ratio is worse for ethanol 96% (0.5) than for ethanol 70% (1.83, the best one for GP extraction), contrary to what is observed for AP extraction.