Nutrient Composition and Aflatoxin Contamination of African Sourced Peanuts and Cashew Nuts: Its Implications on Health

Edible nuts are popular worldwide based on their varied attributes such as desirable taste, high nutritional value as well as some health benefits. Globally, the most popular edible nuts are groundnuts or peanuts, almond, cashew nut among others. Due to the rich nutritional composition of nuts, they tend to be prone to contamination by toxigenic fungi which could ultimately results in the release of fungal metabolites known as mycotoxins into nuts. In view of the nutritional composition of nut and its high susceptibility to fungal attack, this chapter looks at the nutritional profile, mycotoxigenic fungi and aflatoxins contamination of peanuts, cashew nuts and their products with a central focus on Africa where the effect of aflatoxin contaminations is more prominent.


Introduction
Edible nuts are popular worldwide based on their varied attributes such as desirable taste, high nutritional value as well as some health benefits. Moreover, production of edible nuts can be done under various growing conditions and climates [1], however edible nut production is mainly in the tropical and subtropical regions of the world.
Globally, the most popular edible nuts are groundnuts or peanuts (Arachis hypogaea) and are classified as legumes [2] with several other tree nuts such as almond (Prunus dulcis), cashew (Anacardium occidentale), Brazil nut (Bertholetia excelssa), hazelnut (Corylus avellana), macadamia (Macadamia integrifolia), pecan (Carya illinoinensis), pine nut (Pinus pinea), pistachio (Pistachia Vera), and walnut (Juglansregia) [1,3]. Although nuts are considered as food with numerous health benefits, they are prone to contamination by toxigenic fungi which could ultimately results in the release of fungal metabolites known as mycotoxins into nuts [4].  Various forms in which ground nut can be consumed, adopted from market insider with modifications [19].

Nutritional benefits of peanut on human health
The major components of peanut are Protein, fats, and fibre (Table 1), and are present in their most beneficial forms. The protein is plant-based, while the fat is unsaturated, and the fibre is composed of a complex carbohydrate that are beneficial for human nutrition.

Peanut protein
The nutritional value of a food protein is determined by its essential amino acid contents and its digestibility. The protein content in the cake could reach 50% after the peanut oil has been extracted [24]. Peanuts contain all the 20 amino acids in variable proportions [25]. According to its Protein Digestibility Corrected Amino Acid Score (PDCAAS) peanut proteins and other legume proteins such as soy proteins are nutritionally equivalent to meat and eggs and ideal for human growth and health [26]. The true protein digestibility of peanuts is comparable with that of animal protein [27].

Fatty acid composition of peanut oil
Although peanuts and tree nuts have high lipid contents, peanut oil is rich in unsaturated fat, predominantly, monounsaturated fats (MUFA which have been associated with lower cardio-vascular risk) [28]. The MUFA of the regular US peanuts is 49-57% while a medium (66-69%) and high oleic (78-80%) rich peanuts have been reported [29]. The consumption of MUFA promotes arteryclearing which keeps the flow of blood and lowers the risk of atherosclerosis, heart attack or stroke [30]. Clinical studies demonstrated that intakes of MUFAs and PUFAs are associated with low risk of cardiovascular diseases (CVD) and death, whereas saturated fat and trans-fat intakes are associated with high risk of CVD [28].

Dietary Fibre
There are soluble and insoluble dietary fibres which have health benefits such as lowering the risk of heart diseases, diabetes and maintenance of a healthy weight [31]. Other health benefits includes, the lowering of blood cholesterol, improvement of bowel movement and reduced risk of metabolic syndrome [32]. The dietary fibre content of dry roasted peanut was reported as 8.4 g per 100 g of peanut similar to that in soybean (9.3 g per 100 g) while the total dietary fibre of defatted peanut flour (15.8%) was comparable to that of defatted soybean flour (17.5%) [33]. This substantial amount of dietary fibre could help individuals reach their recommended daily allowance of 38 g for men and 25 g for women.

Vitamins and minerals
Peanut has been recognised as a great source of niacin, which is important for the proper functioning of the digestive systems, skin and the nerves. It helps in the conversion of food to energy and supposed to protect against Alzheimers disease and cognitive decline [34]. Peanut is an excellent source of vitamin E whose consumption in good quantities could lead to benefits against coronary heart disease [35]. Peanut also contains good amounts of folates which are important during infancy and pregnancy for the production and maintenance of cells. It was reported that a 100 g of peanut can provides the Recommended Dietary Allowance [36] levels of copper (127%), manganese (84%), iron (57%), phosphorus (54%) and magnesium (42%) intake which is associated with reduced inflammation [37, 38] decreased risk of metabolic syndrome [38] and type II diabetes [39]. They are also referred to as a nutrient dense food, rich in multiple natural micronutrients ( Table 1) including bioactive compounds such as resveratrol, phytosterols, phenolic acids and flavonoids that are beneficial to health. This makes them a viable option for improving the nutrition status of the malnourished, neonates, growing, or those in need of critical nutrients [40].

Health benefits of peanuts
Peanuts lipid profile is high in unsaturated fatty acids than the saturated fatty acids, trans-fat-free, cholesterol-free. Its low saturated fats thus qualify it as safe and desirable. Apart from basic benefits of daily nutrition, peanut consumption leads to long term health benefits such as the management of cancer

Cashew nut
Cashew (Anacardium occidentale L.) is one of the most traded processed tree nuts on the global market [45] and has become a source of income to people in the producing countries through export markets [13]. Cashew nuts are among tree nuts which are known to possess many health benefits such as reduced chances of cardiovascular diseases, diabetes, metabolic syndrome, weight gain and obesity as well as mental instability [46,47]. This is mainly because tree nuts contain several nutrients needed for the proper functioning of the human body such as unsaturated fatty acids, proteins, mineral, vitamins, phenolic compounds and fibre [47,48].
Following an analysis of raw cashew nuts from Brazil, India, Vietnam, East and West Africa, Rico, Bulló [47] highlighted fat as the major constituents of cashew nuts with an average of 48.3% of its total weight. However, in another study, Abubakar, Abubakar [46] reported a much higher percentage of total fats (56.4%) in raw cashew nuts from Nigeria. Nevertheless their findings were within the range (40-57%) reported in literature [49]. Fats are known to play several roles in the diet such as provision of energy, essential fatty acids as well as fat soluble vitamins [50].
The cashew tree produces the nut (kidney shaped) as the main fruit and an accessory fruit known as the cashew apple (Figure 3) [52,53]. The main traded cashew products are the raw nut, the seed and the cashew nut shell liquid (CNSL) whereas the apple is converted into various beverages [54].

Contamination of peanuts and cashew nuts by toxigenic Aspergillus species
Despite the fact that developing countries cover the world's greatest peanut producing area, yield is very low especially in Africa because of poor socioeconomic factors as well as the prevailing weather conditions [55]. The climatic conditions in the tropics and subtropics promote the proliferation of diverse pathogenic fungi capable of producing aflatoxins [56][57][58]. Extensive research on aflatoxins began in the 1960s with analysis on peanuts after poultry deaths in the United Kingdom, Kenya and India. This death was linked to the consumption of contaminated peanut meal [59]. The analysis of mould contaminants in peanut meals from various countries, implicated Aspergillus flavus [58] as the chief mould contaminant. Aspergillus species are ubiquitous group of soil borne fungi that can be found in agricultural soils, storage and food processing facilities as well as in the distribution systems [7, 60]. Aspergillus parasiticus is a soil inhabitant normally associated with pod and seed contamination whereas A. flavus thrives well in aerial environment and are therefore more likely to contaminate tree nuts [61]. Nuts can be colonised by fungi before harvest (Figure 4). Several authors have reported the presence of diverse Aspergillus species in the groundnuts and cashew nuts ( Table 2).
Initially aflatoxin production was linked to A. flavus [85], therefore most researchers use A. flavus as an indicator of possible aflatoxin contamination in food commodities. For example, Sultan and Magan, [11] isolated several Aspergillus species from Flavi, Circumdati and Nigri sections, but performed aflatoxin producing potential tests only on the isolates from section Flavi. Similarly, Oyedele and co-workers [77] also isolated three Aspergillus species (A. flavus, A. parasiticus and A. tamarii) from peanut samples. However, determination of the toxigenicity of the isolates was carried out on those identified as A. flavus only. In another survey by Riba and colleagues [64] isolated species belonging to Flavi, Circumdati, Terrei and Nigri sections from shelled peanuts. Despite the fact that species from section Nigri had the highest incidence of occurrence, toxigenicity tests were done only for isolates from Flavi section. However some authors have reported aflatoxin production by species outside the section Flavi [86][87][88][89]. Therefore, it is important for researchers to determine the aflatoxigenicity of all isolates even if they do not belong to the Flavi section.
The high prevalence of the black Aspergillus i.e. section Nigri has been reported in many peanuts samples. Mohammed and Chali [55] reported the prevalence rates of A. niger ranging from 35 to 66% in peanut samples from the fields, storages and

56-100 [84]
Keys: n.c -not counted, 0 = none was aflatoxin positive. strains [90,91]. On the other hand, Akinola and co-workers [92] highlighted the possibility of gene transfer between toxigenic and atoxigenic strains in feedlots resulting in strains previously known as non aflatoxigenic becoming toxigenic. Acquisition of aflatoxin producing genes by A. niger may become a threat to the fermentation industry where it is mainly used. The quality of the edible part of cashew nut is of great importance as it is used as either as food or ingredients in processed products. However, like others cashew nuts are also prone to fungal contamination with consequent mycotoxin production. Table 2 shows the presence of Aspergillus species especially the section Nigri being present in most of the samples. Lamboni and others [65] highlights the implication of this section in food deterioration. The same author reported production of mycotoxins by some isolates from cashew nuts in Benin belonging to the section Nigri such as Aspergillus tubingensis, A. niger, A. brasiliensis, A. carbonarius, A. luchuensis, A. aculeatus, A. aculeatinus, however none of the isolates were able to produce aflatoxins in culture. A similar observation was noted by El-Samawaty and colleagues [93] where the majority of the Aspergillus species in cashew nuts from Saudi Arabia belonged to section Nigri. Adeniyi and Adedeji [94], emphasised the importance of moisture in the kernels during storage as it may contribute to the colonisation of the nuts by mycotoxigenic fungi.

Aflatoxin contamination of groundnut and cashew nut
Aflatoxin B 1 is the most potent and commonly produced naturally occurring aflatoxin [95], hence its presence in most of the samples and the most reported by researchers ( Table 3). African countries especially those in Southern Africa namely Botswana [98], Democratic Republic of Congo [100,101], Malawi, Mozambique [56], South Africa [101], Zambia [123,124] and Zimbabwe [125][126][127][128] had the highest incidence of aflatoxins in the samples analysed. Most batches had more than 50% of the samples testing positive to one or more of the naturally occurring aflatoxins ( Table 3).
In samples from Northern Africa, aflatoxins were detected though incidences of contamination were less than those reported in Southern Africa. Most samples from the batches analysed had less than 50% aflatoxin contamination occurrence rate with a few exceptions like Sudan in peanut butter. Weather conditions in Northern African countries are characterised by high temperature and humidity [129], due to being surrounded by the Mediterranean Sea and the Atlantic Ocean which promotes growth, occurrence of toxigenic moulds and subsequent aflatoxin production [130].
From West African States, Nigeria and Benin had majority of their peanuts and cashew nuts contaminated with aflatoxins and with almost 100% incidence rates. Total aflatoxin contamination in peanut cake from Benin exceeded the stipulated EC regulatory limit (4 μg/kg) whereas in cashew nuts the concentration was below the limit of detection (LOD). Studies have shown that peanut production in Africa is faced with so many challenges such as poor resources and field management which resulted to the nut being neglected. Peanuts harvest and marketing are often delayed thereby exposing the peanuts to high levels of aflatoxin contamination [95]. Contamination of nuts by aflatoxin can either be before or after harvest and a gradual increase in aflatoxin content with prolonged storage [131] could be expected. In an analysis of aflatoxin levels in peanuts at farms and markets in Uganda, Kaya and co-workers [132] reported the presence of aflatoxins at farm level in ≥60% of the peanuts. Results from analysis of raw peanuts, peanut flour, roasted peanuts and peanut butter [ Keys: -= absent, + = present, n.d = not determined, LOD = limit of detection, LOQ = limit of quantification, Max = maximum, A* = kernels from government supply, B* = kernels from miscellaneous supply. Table 3.

Aflatoxin contamination of groundnuts, cashew nut and their products in African countries.
decrease in aflatoxin concentration in the order raw peanuts ˃ peanut flour ˃ roasted peanuts ˃ peanut butter. These results are in agreement with those of Siwela and colleagues [133] who reported a 51% reduction in aflatoxin after roasting of peanuts during large scale peanut production. Peanut oil is the most utilised oil by people in the tropics due to its affordability [134,135]. Analyses of peanut oil samples have shown the presence of aflatoxins especially in the unrefined oils. It has also been reported that oils extracted from peanuts often have higher aflatoxin contamination [136]. Abalaka [113], highlighted the use of crude oils by the majority of the Nigerian population hence their exposure to aflatoxins [113]. Most of the analyses in oils were from Nigeria and Sudan. Sudan is known as a major vegetable oil producer.
Cashew nut production in Africa is dominated by West African countries [137] hence the high number of reports from this quarters. Most of the studies in Africa were from Nigeria as it is one of the leading producers of cashew nuts worldwide. The result of analyses on aflatoxins in cashew nuts showed that they were within the EU and FDA regulatory limit of 15 μg/kg for total aflatoxin in nuts intended for further processing. However Milhome and associates [14] highlighted some samples from Brazil having total aflatoxins greater than this limit.

Impact of Aflatoxin on health
Mycotoxins finds their way into human and animal body through the consumption of mycotoxin contaminated foods [138]. Mycotoxins causes significant decline in animal productivity and general health performance. Out of the over 400 mycotoxins identified in food and animal feeds, those capable of causing significant health effect in humans and animals includes aflatoxin (AFs), fumonisins (FUM), zearalenone (ZEA), T-2/HT-2, deoxynivalenol [139] and ochratoxin A [15], they are of great concern for their effects on animal and human health [140]. Aflatoxins are naturally occurring chemical contaminants of foods such as cereals, legumes and nuts; groundnuts and cashew nuts [141], Aspergillus parasiticus and Aspergillus flavus are the primary producers of aflatoxin in crops [142]. The consumption of contaminated peanuts and cashew nuts which serves the function of food ingredients and as snacks could results in mycotoxicosis in humans and animals. Similarly, the detection of aflatoxin in animals carcases have been related to the ingestion of aflatoxin contaminated feed ingredients such as peanut [143]. Aside food substrates, mycotoxin occurrence have been reported in animal feed, animal feedlots and animal derived food products [92,144,145]. Aflatoxin is regarded as the chief of mycotoxins based on their degree of toxicities [146]. They have been classified as class 1 human carcinogens based on their deleterious effect on the health of both humans and animals [147,148].
Human exposure to aflatoxin occurs due to the consumption of contaminated agricultural produce and animal derived food product [143]. Aflatoxin ingestion have been reported to cause teratogenic, mutagenic, carcinogenic immunosuppressive, hepatotoxic, nephrotoxic, and genotoxicity effect in humans and animals [144]. The degree of toxicity of mycotoxins on health of animals or humans is a function of the aflatoxin type, species and sex [149]. The liver is the major target organs for aflatoxin toxicity and could show symptoms such as liver lesions and tumour upon exposure to low and moderate doses of aflatoxin [146,150]. The consumption of aflatoxin contaminated nuts could impair the immunity, feed efficiency and cause a teratogenic and mutagenic effect in animals [151][152][153]. The consumption of this nuts could pose a threat to consumer's health causing ill-health, immunosuppression, cancer and liver and kidney damage in humans and animals [150]. The consequences of aflatoxicosis accounts for more than 40% of the diseases in developing countries [9]. In tropical and subtropical countries with less or lack of regulatory activities governing the acceptable level of aflatoxin in food and feeds, the risk of human aflatoxicosis is huge. [154,155]. The study of Ibeh et al. [156] reported the effect of aflatoxin on male fertility. In their study, males with high aflatoxin levels in their serum had abnormal sperm morphology, motility and sperm count. There are also evidence to the transfer of aflatoxin from mothers to babies. Authors [157][158][159] have reported the detection of aflatoxin M1 in breast milk of mothers exposed to aflatoxin contaminated foods in Gambian and United Arab Emirates respectively. Neonatal jaundice was reported in foetus exposed to aflatoxin in Nigeria and Iran [160,161]. Studies have also shown the negative effect of aflatoxin on birth weight, gestational age, birth height, in blood samples obtained from mothers [158,159].
Yousef and Lamplugh [162] have also reported mobility and mortality cases as a result of aflatoxin ingestion in humans. Chronic aflatoxicosis can results from the continuous exposure to aflatoxin contaminated foods and could cause reduction in life expectancy, cancer, immunosuppression and stunting in children [154].

Aflatoxin regulation
After the discovery of aflatoxins and their effects on health of both humans and livestock, regulatory limits were set in the late 1960s [163]. The United States of America was the first country to set the aflatoxin limit of 20 μg/kg [164] and the EU limit (4 μg/kg) for total aflatoxin and 2 μg/kg for aflatoxin B1. However there was harmonisation of aflatoxin standards for the EU countries which took place in 1997 and implemented in 1998. Total aflatoxin for peanuts needing further processing which was previously set at 10 μg/kg was changed to 15 μg/kg and 4 μg/kg for nuts intended for human consumption. Aflatoxin B1 was set at 8 μg/kg and 2 μg/kg for nuts that required further processing and direct human consumption respectively [165]. Not all countries adopted the harmonised standards, for examples in Asia, China and the Philippines limit of aflatoxins set by these countries were 20 μg/kg while Indonesia set 15 μg/kg for aflatoxin B1 [166].

Conclusion
Nuts and nut products and specially peanuts and cashew nuts have long been recognised for their nutritional content and contribution to good health. One of the limitations to the role of these nuts in human nutrition and health is their susceptibility to Aspergillus species and related aflatoxins. Aflatoxins have continued to be a problem from the time of discovery as their presence in nuts especially in African countries is still above the limits. Nuts are sources of livelihood to most people in developing countries as they can be used in nutrition as well as for income generation but its high susceptibility to aflatoxin contamination poses a huge threat to the consumers as well as reducing its value economically especially at the International market [139]. As aflatoxins are not really a threat to the developed countries, because of their stringent rules on acceptable limits in foods meant for human consumption.
This chapter revealed that a larger percentage of the nuts produced in Africa are contaminated with aflatoxin concentration above the regulated permissible level, hence consumers of nuts especially peanuts in Africa are at risk of aflatoxicosis despite the nutritional importance of the nut. Hence, strategies to reduce the proliferation of aflatoxigenic fungi in nuts while on the field and at post-harvest level should be harnessed. Some suggested strategies to achieve this includes: • Good Agricultural practices such as planting of improved varieties of nuts that are resistant to drought and stress, good storage practices, proper drying of produce before storage, prevention of kernel damage during harvesting etc. should be encouraged.
• Appropriate controls of storage parameters that could aid impedes Aspergillus spp. growth, contamination and aflatoxin production.
• Application of biological techniques such as use of atoxigenic strains of Aspergillus to control the toxigenic strains.
It is recommended that future research should focus on the nutritional advantages of peanuts and cashew nuts and their related health benefits beyond the ones that have been identified in this chapter.