## 2 Extraction of Peanut Skins
### \"Green\" Extractions  
Supercritical extraction uses pressure to condense a solvent or solvent mixture with temperatures above the boiling point of the solvents so that the dielectric constant is decreased [53]. This results in a change in the polarity so that it becomes similar to solvents such as methanol, ethanol, and acetone at room temperature. This allows for the use of more \"green\" or less toxic chemicals to be used with enhanced solubility of phenolic compounds in the case of peanut skins. A study using this technology optimized the total phenolic content of extracts from peanut skins to be 10 min at 220 \({}^{\circ}\)C using 60.5% ethanol in water [53]. This shows the potential of using a more environmentally friendly technique. Further study with this technique to extract peanut skins determined the effect of the size of the particles of peanut skin extracted [54]. Peanut skins milled to particle sizes of 300, 355, 425, and 500 \(\upmu\)M were extracted using both traditional Soxhlet extraction and supercritical extraction using carbon dioxide. Optimum yields per weight were obtained by both methods when the particle size was 425 uM and pure ethanol was the solvent compared to either pure water or n-hexane. Although the yield from the Soxhlet extraction using ethanol was higher (36.38% by weight) than carbon dioxide using the supercritical system (15.53%), the antioxidant activity using the DPPH assay was higher for the supercritical system (93.43%) than for the Soxhlet extraction (62.11%). This was attributed to less temperature degradation of the polyphenols by the lower temperatures of operation for the supercritical system. The catechin recovery from this system was higher (139.92 ug/g peanut skin) compared to Soxhlet extraction (31.79 ug/g peanut skin) when ethanol was used as the solvent [55]. More recently, this system has been used to prepare extracts for investigations into the antioxidant protection in cell systems [56]. The extracts were compared to the activity of quercetin, which was thought to be the main compound present. After proving the additives were not cytotoxic up to 250 ug/mL, O2- scavenging activity was assayed. Quercetin was assayed to be seven times higher than the peanut skin extract, which was attributed to the pro-oxidant properties of the catechins present in those extracts. The addition of the quercetin and the peanut skins along with an oxidative stressor was proven to be protective against oxidative stress, whereas pretreatment was not.  
A system that used ultrasound was applied to peanut skins with the specific goal of recovering the stillbenoid, resveratrol was reported [57]. _Yeast_ CICC1912, _Apergillus oryzae_ 3.951, and _Apergillus niger_ 3.3148 were immobilized onto cellulose beads. Peanut skins were extracted with 80% ethanol in water and the extract was dried. Optimization of the recovery of resveratrol was performed using several surfactants, liquid-solid ratios, ultrasonic powers, and culture temperatures and times. At the optimized conditions, recovery of 96.58 ug/g of resveratrol was obtained, which was four-fold higher than from an untreated sample.