الفهرس 
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Abstract
AFs consist of a group of approximately 20 related secondary fungal metabolites,
although only aflatoxins B1, B2, G1 and G2 are normally found in foods (Wogan et al., 2012). Being intensely fluorescent in ultraviolet light the four are designated by letters B1,
B2, G1 and G2 representing their blue and green fluorescence in UV light. Two other familiar AFs are M1 and M2 representing their presence in milk of animals previously exposed to B1 and B2. Of all the above-named aflatoxins, aflatoxin B I (AFB1) is the most acutely toxic to var ous species. Toxigenic A. flavus isolates generally produce only AFB1 and AFB2, whereas A. parasiticus isolates generally produce AFB1, AFB2, AFG1 and AFG2 (Davis and Diener, 1983). Other metabolites B2a, aflatoxicol, aflatoxicol H1 and AFs P1 and Q1 have been identified (FDA, 1979). AFM1 is a metabolite of AFB1 in humans and animals. AFM2 is a metabolite of aflatoxin B1 in the milk of cattle fed on contaminated foods. Although aflatoxins B1, B2 and G1 are common in the food sample, AFB1 predominates (60-80% of the total aflatoxin content). Generally, AFB2, AFG1 and AFG2 do not occur in the absence of AFB1. In most cases AFG1 is found in higher concentrations than AFB2 and AFG2 (Weidenborner, 2001). AFs are heat-stable and invisible to the naked eye (Phillips et al., 2002). They are intensely fluorescent in ultraviolet light (Devero, 1999).
The formation of aflatoxins is influenced by physical, chemical and biological factors. The physical factors include temperature and moisture. The chemical factors include the composition of the air and the nature of the substrate. Biological factors are those associated with the host species (Hesseltine, 1983). The rate and degree of contamination depend on different factors such as temperature, humidity, water activity, concurrent mycobiota, physical damage, and other storage conditions (EFSA, 2004).Specific nutrients, such as minerals (especially zinc), vitamins, fatty acids, amino
acids and energy source (preferably in the form of starch), are required for aflatoxins
formation (Wyatt, 1991). Large yield of aflatoxins is associated with high carbohydrate concentrations, such as are found in wheat and rice while a lesser extent in oilseeds is associated with cotton seed, soya beans and peanuts (Diener and Davis, 1968). Contamination of crops and toxin production are particularly likely to occur in
subsistence farming communities in tropical and sub-tropical regions with high
temperatures and humidity. These environmental conditions, in addition to the moisture
content of plants, are important factors in determining the growth, and toxin production by,
these molds (Williams et al., 2004; Strosnider et al., 2006). Crops that are particularly
likely to be affected are those either grown domestically or purchased at local markets. (Kew, 2013).The limiting temperatures for the production of AFs by A. flavus and A. parasiticus.