## 3 Bioactive Compounds in Peanut Skins  
### Chemical Antioxidant Activity  
The antioxidant activity of peanut skin extracts was first investigated as a side project from research on peanut hulls [16]. The total phenolic content was reported using the Folin-Coicalteu assay and the antioxidant activity in sunflower oil was reported to be less than that of BHT, a commonly used synthetic antioxidant ingredient for food oils. Isolated compounds from peanut skins using a series of extractions and chromatography were found to have free radical scavenging ability when evaluated using the DPPH assay. These included flavonoid glycosides [19] and proanthocyanidins [21]. A comparison study of several black seed-coated peanuts with red seeded ones evaluated the total phenolic content (TPC), content of several phenolic classes of compounds, and several antioxidant assays [28]. Extraction of the peanut skins was performed using 70% acetone in water acidified with acetic acid after defatting with hexane. The TPC was not found to vary widely between the varieties. The skins from red seed-coated peanuts were found to be higher in total flavonoid and total condensed tannin content than the black seed coated ones, although the black peanut skins had higher anthocyanin contents. Correspondingly, one of the red seed-coated peanut types had higher antioxidant power when measured using the FRAP assay but all the peanut skins from the seeds tested had similar free radical reducing power when measuring using the DPPH assay. Phenolic compounds in peanut skins are increased by stress conditions. Fungal attack as a source of stress was investigated [58]. Peanuts with skins of red, reddish-brown, and black colors were inoculated by the fungus, _R Rhizopus oligosporus_. The seeds were then germinated for 3 days in the dark. The germinated seeds and unperminated seeds used as the controls were extracted using a mixture of acetone, ethanol, and water (2:2:1 v/v/v) that had been acidified. From the extracts, 45 different compounds including phytoalexins were identified using HPLC-MS. In addition, the antioxidant activity was determined using total phenolic content (TPC), total flavonoid content (TFC), oxygen radical absorbance capacity (ORAC), hydroxy radical absorbance capacity (HORAC), superoxide radical absorbance capacity (SORAC), and DPPH. In general, the black skins had higher activity in most of the assays, with the exception of SORAC. The red and reddish-brown colored skins tended to be similar in strength. In most cases, the treated seeds had higher activity than the untreated controls. Aqueous methanol continued to be used in studies after defatting with hexane [59]. The extracts were found to have little antioxidant activity when incorporated into vegetable oils and lard but were found to test high in free radical activity testing using chemical tests. The extracts were found to have metal-chelating activity. The same extraction scheme was used to compare the effectiveness of skins from high oleic peanuts with those from peanuts with normal oleic acid levels [60]. No significant differences were found in the antioxidant power of the extracts from the two different types of peanuts. The use of several solvents individually without the addition of water was studied to determine the yield of soluble material by weight [61]. The highest recoveries were from ethanol, followed by methanol and acetone. Although the amount of solids recovered from these solvents was equivalent (ca. 200 mg from 5 g peanut skin), the acetone extract had only 3% of the antioxidant activity compared to the alcohol extracts (3% vs. 9.8% for ethanol and 93.5% for methanol) using a \(\beta\)-carotene-based assay. Hexane and ethyl acetate yielded much less solid material (35 mg and 90 mg, respectively) and comparable activity to the acetone extract (3%). This proved the active components are polar compounds. Differences between peanut skin extracts from raw and roasted peanuts were determined using a series of solvents and solvent mixtures [62].