"Peanut skin could provide an inexpensive source of natural antioxidants, such as catechins and procyanidin, for use in food and dietary supplements," said Jianmei Yu from North Carolina A&T State University, lead author of a new study on how peanut skin removal techniques affect polyphenolic content and anti-oxidant activity.
Procyanidins, polyphenolic compounds found naturally in green tea, fruit, vegetables and red wine, have been linked to a wide range of health benefits, including reduced risks of cardiovascular disease and certain cancers.
Peanut skins were the original source of oligomeric proanythocyanidins (OPCs) - dimers, trimers and tetramers of the procyanidin monomers - identified by French scientist Masquelier in the 1950s. But when import practices changed for the benefit of oil producers so that peanuts from Africa were more likely to be shipped ready-shelled, fewer peanut skins were available.
These days peanut butter manufacturers are more likely to use peanut skins as animal feed.
The main sources of polyphenols on the market today are green tea extracts, like DSM's Teavigo and Taiyo International's Sunphenon, grape seed extracts, like Masquelier's Original OPCs, Anthogenol, and pine bark extracts like Horphag's Pycnogenol.
According to Frost and Sullivan, the European polyphenols market is dominated by grape seed extracts, which account for over 50 per cent of the market. Green tea extracts account for between 20 to 25 per cent.
The new study, published in the Journal of Food Composition and Analysis (Vol. 19, pp. 364-371), investigated three different peanut skin removal techniques (direct peeling, blanching or roasting).
The total polyphenol yield for the three removal techniques were 130.8, 15.1, and 124.3 milligrams gallic acid equivalents (GAE) per gram of dry skin for the direct peeling, blanching or roasting techniques.
Blanching (boiling in water and then peeling) and roasting at 175 degrees Celsius followed by the standard ethanol extraction, significantly altered the procyanidin profiles compared to direct (untreated) skin removal.
Polyphenolics such as caffeic acid, chlorogenic acid, ellagic acid and resveratrol were no longer present as a result of the heat treatments.
The procyanidin compositions of the extracts were compared to those of grape seed varieties. The yield of catechins was significantly less for the peanut skin extract (untreated, 16.1 mg per 100 g dry sample) than, for example, the pinot noir grape seed (437 mg per 100 g) or a cabernet grape seed (125 mg per 100 g).
Interestingly, the concentration of OPCs was found in comparable quantities to those found in grape seed extracts. Indeed, the trimer concentration from the peanut skin extract (untreated, 221.3 mg per 100 g dry sample) exceeded those of the all the grape varieties tested (for example, Pinot noir, 84 mg per 100 g dry sample).
According to data from the US Department of Agriculture, a healthy diet should provide an adequate intake of OPCs, with studies showing a benefit of OPCs with doses in the range of 100 to 300 milligrams a day, but additional studies have suggested that intake of an average diet is only about 25 mg per day.
"Peanut skin contained higher concentration of procyanidin trimers and tetramers than grape seed which gives peanut skin a comparative advantage as a source of potent antioxidants," said the researchers.
The total antioxidant activity (TAA) of the peanut skin extracts "increased almost linearly with increasing concentration of total phenolics, and they were higher than those of Trolox [vitamin E analogue] and vitamin C with exception of TAA of crude extract from the blanched peanut skin," reported Yu.
The free radical scavenging capacity (FRSC) of the extracts was also greater than Trolox and vitamin C, with the extract of the direct peeled peanut skin having the highest FRSC of all the extracts.