## 2 Extraction of Peanut Skins
### Compound Identification
Increasingly, previously unidentified compounds have been reported from peanut skin extractions. Researchers have described an A-type procyanidin (epicatechin-(2\(\beta\)\(\rightarrow\)\(\alpha\)\(\rightarrow\)7,4\(\beta\)\(\rightarrow\)8)-[catechin-(6\(\rightarrow\)4\(\beta\))]-epicatechin) that had not been reported before after extraction of peanut skins with 70% acetone in water, followed by fractionation on Sephadex LH-20 and elution with ethanol [35]. Research to differentiate between the classes of procyanidins present in peanut skins has used the technique of hydrogen/deuterium exchange (HDX) to elucidate the structural differences between isomers [36]. The technique was able to differentiate between the A-type procyanidins in peanut skin extracts containing up to three linkages. The extracts were prepared using 70% acetone in water containing 0.1% formic acid after defatting of the peanut skins. The filtered extracts were dried under nitrogen gas and then reconstituted in the deuterated mobile phase. As a follow-up to a previous study of the composition of whole peanuts, the authors used the same extraction system of 80% methanol in water after defatting with hexane to compare peanut skins from two different peanut market types [37; 38]. The analysis using HPLC-MS of the extracts found that the Valencia market type peanut skin extracts had higher levels of flavonoids, quercetin, and its methylated analog, isorhamnetin. One of the most complete studies of the phenolic type compounds in peanut skins used the extraction procedure developed for the analysis of grains to prepare samples for instrumental analysis [39]. The peanut skins analyzed were obtained after commercial blanching, which requires mild heat treatment. As heat treatment has been reported to liberate smaller phenolic compounds from larger polymers, this proved advantageous in allowing for a range of identifications [40]. Acidified water was used for the initial extraction followed by a partition into diethyl ether to capture the free phenolic compounds. A solvent exchange of methanol for the ether was performed before the analysis by HPLC-MS. From this, 88 individual phenolic type compounds were found, although some could only be identified by their class (Table 2). In addition, 60 proanthocyanidins were found, with most being of the A-type. The same group used the same extraction scheme with fractionation to determine the bound phenolics [41]. After the extraction and portioning of the free phenolics into diethyl ether, the aqueous phase was then base-hydrolyzed and acid-hydrolyzed to convert the ester derivatives to their carboxylic acid or flavonoid analogs. The analysis was then done with the same LC-MS system. An additional 78 compounds that existed as esters or glycosides were tentatively identified. The reader is referred to these publications for the listings of these compounds. \begin{table}
\begin{tabular}{c c} \hline
**Free Phenolic Compounds a** & **Content b (mg/100 g)** \\ \hline Protocatechic acid & 3.43 \(\pm\) 0.04 \\ \hline p-Hydroxybenzoic acid & 1.03 \(\pm\) 0.06 \\ \hline Caffaric acid & 51 \(\pm\) 0.12 \\ \hline cis-Coutaric acid & 10.1 \(\pm\) 0.52 \\ \hline trans-Coutaric acid & 2.11 \(\pm\) 0.08 \\ \hline p-Coumaroyl-O-pentoside & 5.52 \(\pm\) 0.23 \\ \hline p-Coumaric acid & 0.53 \(\pm\) 0.06 \\ \hline Chicoric acid & 3.44 \(\pm\) 0.12 \\ \hline p-Coumaroylcaffeoyltartaric acid & 2.26 \(\pm\) 0.13 \\ \hline Chicoric acid & 3.12 \(\pm\) 0.13 \\ \hline di-p-Coumaroyltartaric acid & 13.8 \(\pm\) 1.53 \\ \hline p-Coumaroylsinapoyltartaric acid & 6.32 \(\pm\) 0.94 \\ \hline p-Coumaroylerfuloyltartaric acid & 5.87 \(\pm\) 0.71 \\ \hline trans-Resveratrol & 0.36 \(\pm\) 0.05 \\ \hline Quercetin & 2.11 \(\pm\) 0.27 \\ \hline Isorhamnetin & 1.51 \(\pm\) 0.02 \\ \hline Diosmetion & 0.40 \(\pm\) 0.01 \\ \hline \end{tabular} a
[FOOTNOTE:a]Footnote a: footnotetext: Caffaric acid and
[ENDFOOTNOTE]  
\end{table}