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 incidents of hepatitis viruses B and C, lower the immune response system, impair growth in children and cause childhood cirrhosis. In poultry and livestock, aflatoxins 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. Either elevated temperatures or drought stress alone increased aflatoxin production, but high temperatures appeared to have a greater impact. Key requirements of post-harvest management to control aflatoxins are the level of seed moisture and insect infestations. Both A. flavus and A. parasiticus are xerophiles that can grow and produce aflatoxin at relative moisture around 85%. Storage below this moisture level or at 10°C will prevent aflatoxin production in storage (Payne, 2016).
In 1960, aflatoxin-contaminated peanut meal from Brazil fed to poultry killed thousands of turkey poults in the UK, drawing attention to these contaminants (FAO, 1979). New regulatory measures in importer countries, such as the EU member states, led to a significant restriction of the trade in peanut and peanut products, and particularly in those produced in sub-Saharan Africa (Waliyar et al., 2007; ICRISAT, 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).
Since the 1980s, the reduction of aflatoxins in peanut products has been the subject of considerable research focused on the following areas:
Increase awareness of local populations about aflatoxin issues
Implementation of pre-harvest, post-harvest and storage practices, and technologies for mitigating contamination
Identification/development and use of resistant peanut cultivars
Biological control agents
Methods for detoxifying peanut products.
The objective of aflatoxin management in producing countries is, on the one hand, to limit the aflatoxin exposure in their population, and, on the other hand, to increase exports by decreasing the aflatoxin content of peanut products to a level that meets international requirements (Waliyar et al., 2007; ICRISAT, 2016). As of 2016, aflatoxin contamination in peanut production in tropical countries, and particularly in low-input systems, still remains a major problem, as shown by surveys conducted in China (Wu et al., 2016), Zambia (Bumbangi et al., 2016), Ethiopia (Chala et al., 2016), Nigeria (Ezekiel et al., 2013), Cameroon (Kana et al., 2013), India (Kolhe, 2016a) and Brazil (Oliveira et al., 2009). Aflatoxin contamination of peanut meal in these surveys occasionally reached 100%, with values well beyond international standards.
Progress in breeding for aflatoxin resistance has been limited (Holbrook et al., 2016). Biological control using non-aflatoxigenic strains of Aspergillus has been shown to be effective in Argentina, Australia, the Philippines, and the USA, with reductions ranging from 85% to 98% (Payne, 2016). In Mali, a combined approach using resistant and tolerant cultivars, the application of lime and manure, and better harvesting and drying techniques, helped to reduce aflatoxins by 80-94% (ICRISAT, 2016).
Like other legume seeds, peanuts contain substances with potentially antinutritional effects, such as tannins which are present in the seed coats (Sanders, 1979), lectins, and trypsin inhibitors (Ahmed, 1986; Ahmed et al., 1988a; Ahmed et al., 1988a). These substances have received little attention, perhaps because the antinutritional factors in peanut seem less deleterious than those of other legumes. For instance, a comparison of soybean and peanut flour in rats showed that raw peanut flour was much better tolerated than raw soybean flour, even though lectin concentration and antitrypsin activity were similar in both seeds (Sitren et al., 1985). Peanut lectins can be fully inactivated by heat (moist heat being more effective than dry heat) (Ahmed, 1986) so it is possible that the normal conditions involved in peanut seed and meal processing are enough to make peanut products safe for animal feeding. However, tannins may be a contributing factor for the low protein digestibility of peanut meals (Chiba, 2001).