**4. Frequency and levels of contamination in food**

**µg/kg Food**

20 Food addressed to humans consumption. Corn and grains for animal feeds.

Many relevant aspects of aflatoxins biosynthesis and molecular biology have been studied and extensively described. The first step in the biosynthetic pathway is considered the pro‐ duction of norsolorinic acid, an anthraquinone precursor, by a type II polyketide synthase. A series of about 15 post-polyketide synthase steps follows, yielding increasingly toxigenic metabolites (Bennett, Chang, Bhatnagar, 1997; Cleveland, Bhatnagar, 1992; Hicks, Shimizu, Keller, 2002; Payne, Brown, 1998; Townsend, 1997; Trail, Mahanti, Linz, 1995). Sterigmato‐ cystin, a related dihydrofuran toxin, mutagenic and tumorigenic but less potent than afla‐ toxin (Berry, 1988), is a late metabolite in the aflatoxin pathway, and is also produced as a final biosynthetic product by a number of species like *Aspergillus versicolor* and *Aspergillus nidulans*. Analysis of the molecular genetics of sterigmatocystin biosynthesis in the genetical‐ ly tractable species *Aspergillus nidulans* has provided a useful model system. The genes for the sterigmatocystin gene cluster from *Aspergillus nidulans* have been cloned and sequenced (Brown, Yu, Kelkar, Fernandes, Nesbitt, Keller, Adams, Leonard, 1996). Cognate genes for aflatoxins pathway enzymes from *Aspergillus flavus* and *Aspergillus parasiticus* show high se‐ quence similarity to the sterigmatocystin pathway genes (Payne, Brown, 1998; Yu, Chang, Bhatnagar, Cleveland, 2000; Yu, Woloshuk, Bhatnagar, Cleveland, 2000). Genes organization for *Aspergillus flavus, Aspergillus nidulans*, and *Aspergillus parasiticus* sterigmatocystin-aflatox‐ in pathway has been studied as reported by Cary et al. (Cary, Chang, Bhatnagar, 2001) and

*Aspergillus oryzae* and *Aspergillus sojae*, species that are widely used in Asian food fermenta‐ tions such as soy sauce, miso, and sake, are closely related to the aflatoxigenic species *Asper‐ gillus flavus* and *Aspergillus parasiticus*. Although these food fungi have never been shown to produce aflatoxin (Wei, Jong. 1986), they contain homologues of several aflatoxin biosynthe‐ sis pathway genes (Klich, Yu, Chang, Mullaney, Bhatnagar, Cleveland, 1995). Deletions and other genetic defects have led to silencing of the aflatoxin pathway in both *Aspergillus oryzae* and *Aspergillus sojae* (Takahashi, Chang, Matsushima, Abe, Bhatnagar, Cleveland, Koyama,

100 Corn and grains for breeding beef cattle, breeding swine, or poultry.

300 Corn and grains for finishing beef cattle, swine, poultry.

**Table 2.** Aflatoxins levels limits generally considered as safe.

200 Corn and grains intended for swine.

346 Aflatoxins - Recent Advances and Future Prospects

**3. Biosynthesis of aflatoxins**

Hicks et al. (Hicks, Shimizu, Keller, 2002).

2002; Watson, Fuller, Jeens, Archer, 1999; Bennett, Klich, 2003).

Aflatoxins have received greater attention than any other mycotoxins because of their dem‐ onstrated potent carcinogenic effect in susceptible laboratory animals and their acute toxico‐ logical effects in humans. Many countries have attempted to limit exposure to aflatoxins by imposing regulatory limits on commodities to be used as food and feed. The two species of *Aspergillus* fungi, aflatoxin producing, are especially found in areas with hot and humid cli‐ mate. Since aflatoxins are known to be genotoxic and carcinogenic, exposure through food should be kept as low as possible. Aflatoxins have been also associated with various diseas‐ es, such as aflatoxicosis. Aflatoxin B1 is the most common in food, and has the most potent genotoxic and carcinogenic effects. Aflatoxin M1 is a major metabolite of aflatoxin B1 in hu‐ mans and animals, which may be present in milk from animals fed with aflatoxin B1 conta‐ minated feed. Aflatoxins can occur in foods, such as groundnuts, treenuts, maize, rice, figs, grapes, raisins, and other dried foods, spices and crude vegetable oils, and cocoa beans, as a result of fungal contamination before and after harvest. The biosynthesis and the occurrence of aflatoxins is influenced by environmental factors; consequently the extent of contamina‐ tion varies with geographic location, agricultural and agronomic practices. The susceptibili‐ ty of commodities to fungal invasion during preharvest, storage, and/or processing periods is also important to assess the possible contamination.
