## 2 Extraction of Peanut Skins
### Extraction Optimization  
The microwave-assisted extraction was another method used for peanut skins [42]. In this case, 30% ethanol in water was chosen as this had previously been proven to be the optimum for total phenolic extraction [43]. Surface response methodology was used to determine the power and time to extract the highest total phenols without decreasing the antioxidant effect. It was reported that 30 s at 90% microwave power produces the optimum product. As the interest in recovering phenolic compounds from peanut skins increased, studies to optimize their recovery were done. Most of these studies concentrated on achieving the highest values using the Folin-Coicalteu assay rather than any defined class of polyphenols [44]. Factors such as particle size of the skin material, the proportion of solvent to the mass of skins, contact time with the skin material, maceration or shaking, and the number of extractions using 70% ethanol in water were evaluated. The efficiency of the extraction was evaluated using the 2,2-diphenyl-1-picrylhydrazyl (DPPH) assay for free radical scavenging power, which indicated that the method of agitation was not significant and that 10 minutes of extraction at 40 mL per 5 grams of peanut skins using 4 extractions was optimum. An optimization study examining proanthocyanidins from peanut skins used aqueous ethanol and ultrasonic extraction [45] (an aqueous solution of ethanol (55%) at an outlet power of 120 W at 35 \({}^{\circ}\)C). This resulted in the extraction of proanthocyanidins with a yield of 12.1%. Water only was used to extract primarily the procyanidins that were then used to evaluate the potency to suppress allergenic response [46]. Further purification of the extracts was done with a reverse-phase resin to bind the polyphenols. Successive elutions were made with increasing concentrations of acetone in water and finally with 80% ethanol in water. The final eluate was then further purified using a size-exclusion resin, followed by a gel filtration resin and finally with silica gel to isolate specifically the procyanidin A-1 for further testing. When compared with the unpurified extract from the peanut skins, the isolated compounds were not as potent in suppressing immunoglobulin synthesis and regulating systemic T helper cytokine production. Studies dedicated to the optimization of the extraction of phenolic compounds from peanut skins used response surface methodology and various chemical assays for comparisons [47,48]. Ethanol and methanol at concentrations of 30%, 60%, and 90% in water were compared with pure water and pure ethyl acetate. The total phenolics recovered (TPC) were optimized in ethanol at 30% (118 mg Gallic Equivalents per gram skins) and in methanol at 60% (112 mg Gallic Equivalents per gram skins). Pure water recovery was lower at 81 mg and ethyl acetate recovery was an order of magnitude lower. Using the Oxygen Radical Absorbance Capacity (ORAC) activity as the measure of antioxidant activity, the optimized alcohol extracts were similar (2050 \(\upmu\)mol Trolox equivalents per gram for ethanol and 2149 \(\upmu\)mol Trolox equivalents per gram for methanol). Pure water was less than half as effective. Varying the temperature of the extraction was investigated in this study. Increasing the temperature from 30 \({}^{\circ}\)C to 60 \({}^{\circ}\)C using 30% ethanol in the water had no effect on the TPC recovery but when using 30% methanol, increasing the temperature to 60 \({}^{\circ}\)C resulted in a 20% increase in the TPC. This was attributed to methanol being better able to solubilize more polar compounds. The use of microwave-assisted extraction was compared with the solvent extractions using conventional heating and mechanical shaking with the optimized solvent mixtures. The microwave procedure increased the ORAC activity to 2789 \(\upmu\)mol Trolox equivalents per gram when using the 30% ethanol in water mixture as the extraction solvent.