Aflatoxins
Peanuts are particularly vulnerable to contamination by fungi Aspergillus flavus and Aspergillus parasiticus. These fungi produce aflatoxins, which in humans are known to cause cancers, increase incidents of hepatitis viruses B and C, lower the immune response, impair growth in children, and cause childhood cirrhosis. In poultry and livestock, aflatoxins can cause feed refusal, 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 raised temperatures or drought stress alone increase aflatoxin production, but high temperatures appeared to have a greater impact. Key requirements of postharvest management of aflatoxins are control of seed moisture and insects. Both A. flavus and A. parasiticus are xerophiles, and can grow and produce aflatoxins when relative moisture is around 85%. Storage at lower levels of moisture, or at 10°C, will prevent aflatoxin production (Payne, 2016).
In 1960, aflatoxin-contaminated peanut meal from Brazil killed thousands of turkey poults in the UK, drawing attention to these contaminants (FAO, 1979). New regulatory measures in importer countries, such as EU member states, led to a significant restriction of the trade of peanut and peanut products, particularly of those produced in sub-Saharan Africa (Waliyar et al., 2007; ICRISAT, 2016). The demand for peanut meal in industrialised countries evaporated in the 1970-1980s. France, which imported 400,000 t of peanut meal in 1973 (FAO, 1979), only imported 4000 t in 2014 (Oil World, 2015). Former UN Secretary-General Kofi Annan once claimed that "EU regulations on aflatoxins cost Africa $670 million each year in export revenue" (Wu et al., 2012). 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 improved 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 both to decrease aflatoxin exposure in the local population and to increase the export of peanut products that are negatively impacted by aflatoxin regulations (Waliyar et al., 2007; ICRISAT, 2016). As of 2016, aflatoxin contamination in peanut produced in tropical countries, and particularly in low-input systems, still remains a major problem, as shown by many 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 reached levels as high as 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/tolerant cultivars, the application of lime and manure, and better harvesting and drying techniques helped to reduce aflatoxin levels by 80-94% (ICRISAT, 2016).
Antinutritional factors
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 antitrypsic 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 regular conditions involved in peanut seeds and meal processing are enough to make peanut products safe for animal feeding. However, tannins may be a contributing factor for low protein digestibility of peanut meals (Chiba, 2001).