# Extraction of Antioxidants from Grape and Apple Pomace: Solvent Selection and Process Kinetics
## 3 Results and Discussion
### Kinetic Study of Extraction Processes
This could be attributed to the fact that as AP is not a dry solid (AP here used had a dry solid content of 33.5 \(\pm\) 0.9%), the residue confers a local content of water to the mixture that is relevant to equalize, at pore scale, the amount of water irrespective of the percentage of water in the solvent, thus making an extraction without water not feasible. Ethanol 96% extraction is similar to acetone: the final solvent will be a mixture of water contained in the residue and ethanol, so the final yield (10.33 \(\pm\) 1.29 mg GAE/gds) was similar to the one obtained in the ethanol 70% extraction. The effect of water can be observed in the slower initial extraction rate. This can be attributed to the effect of the high polarity of water with yields at 5 min in the range of 4 mg GAE/gds in comparison to 5, 6, and 7 for aqueous mixtures of ethanol 70% and acetone 80%, and pure acetone, respectively. The difference between solvents may also be due, in part, to the viscosity at 25 \({}^{\circ}\)C for the solvents under comparison: 0.91 cP (water), 1.07 cP (ethanol), and 0.31 cP (acetone). One should keep in mind that, according to the Einstein-Stokes equation, and most empirical equations for the calculation of molecular diffusivities in liquids' diffusivity, and, thus, according to the first Fick law on diffusional mass transfer, mass flow rates are inversely proportional to viscosity. Acetone has, most evidently, the lowest viscosity. In the case of an almost pure ethanol (96%) at 70 \({}^{\circ}\)C, viscosity equals 0.5231 cP and cannot explain the low extraction rate (6 mg GAE/gds) achieved in the first 5 min, a fact that is further supported by a relatively low final yield slightly over 10 mg GAE/gds. Thus, the water content in hydroalcoholic mixtures seems critical to achieve high extraction yields, improving solvation power of the mixture in comparison to pure ethanol, while medium to high temperatures will result (Figure 1) in fast to very fast extraction processes. Possibly, in view of the results of acetone:water  
Figure 1: Results of total phenolic content (TPC) during extraction processes using AP and ethanol/water 70/30 (% \(v/v\)) at different temperatures: (**A**) 25 \({}^{\circ}\)C, (**B**) 50 \({}^{\circ}\)C, (**C**) 70 \({}^{\circ}\)C, and (**D**) 90 \({}^{\circ}\)C. Data were obtained by the Folin–Ciocalteu method. Kinetic analysis corresponding to the second-order kinetic model is displayed as lines. and ethanol:water mixtures, this observation can be extended to acetone:water mixtures applied at medium-high temperature and pressure-values. Figure 3 displays the results for TPC extraction when using glycol solutions with a small amount (10%) of water. From the perspective of TPC final yields, these glycol/water mixtures seem to be optimal solvents to extract phenolics in the case of AP at 70 \({}^{\circ}\)C, as TPC values reach 25 mg GAE/gds in 60 min and 13 mg GAE/gds in 15 min (equal to the TPC value for ethanol 70% aqueous solution in 60 min). Moreover, the apparent TPC value at zero time (the result of washing or dissolution of readily available phenolics on or near the waste particles' surface) is 6 mg GAE/gds for the ethyleneglycol solution and almost 5 GAE/gds for the propyleneglycol solution (comparing to 4.3 GAE/gds for the ethanol 70% watery solution). These numbers are a further confirmation of the fast nature of the dissolution or washing process: as the stirring speed was set to a high value (500 r.p.m.) for a low volume of liquid-solid suspension, no external mass transfer hindrances are expected, thus favoring the solubilization phenomena taking place in the first minute of contact. Here, mass transfer is immediate and solvent viscosity plays no role, being that its solvation power is the most important factor to achieve high TPC values. The kinetic trends and model curves reflecting the antioxidant capacity extracted from AP are displayed in Figures 4-6.