Aflatoxins
Peanuts are particularly vulnerable to contamination by fungi (Aspergillus flavus and Aspergillus parasiticus). These fungi produce aflatoxins, which are known to cause cancers in humans, increase the incidents of hepatitis viruses B and C, lower the immune response, impair growth in children and cause childhood cirrhosis. In poultry and livestock, aflatoxin can cause loss of appetite, loss of weight, reduced egg production, and contamination of milk (ICRISAT, 2016). Aflatoxin contamination may occur in the field, after peanuts are lifted but before harvest, during transport, and during storage (Payne, 2016). As of 2016, the maximum authorised content in the EU for aflatoxin B1 in feed materials is 0.02 mg/kg (20 ppb or µg/kg) (European Commission, 2003). The risk of aflatoxins in peanut products is discussed more extensively in the Peanut meal datasheet.
Peanut skins, like other parts of the peanut fruit, can be contaminated with aflatoxins, and a study has found that the blanching process, which consists of removing the skins, decreased the amount of aflatoxins by 27%, indicating that a significant amount of the toxin is in the peanut skin (Siwela et al., 2011). However, the risk of contamination is reduced for peanut skins produced in countries where regulatory agencies and the peanut industry enforce strict norms for quality and safety throughout the supply chain, from harvest to the transportation of peanut end-products (Cowart et al., 2016).
Tannins
Peanut skins are particularly rich in tannins. Reported values range from 20-25% (Hill et al., 1986a; Hill et al., 1986b) to more than 45% (Sanders, 1979). It is possible that the latter extreme values refer to "pure" skins while the former and lower values refer to the mixture of skins and kernel fragments marketed as peanut skins. Peanut skin tannins are procyanidins, which interfere with protein digestion and absorption, and hence adversely affect animal performance (Constanza et al., 2012).
Low density
Peanut skins have a very low bulk density that makes them difficult to mix in livestock diets (Palmer, 2010).