**2. AFB1 occurrence**

#### **2.1 Toxic fungi and their classifications**

Toxic fungi often live in the human crops and produce mycotoxins such as aflatoxins. Toxic fungi in crops can be divided into two categories according to whether their mycotoxins are produced before or after crop harvest. The first category is termed as field fungi, which often invade crops and produce mycotoxins before harvest. The another, also called storage fungi, mostly occurs in the storage of crops after harvest. The sources of both types of toxigenic fungi are affected by environmental factors. Crops before harvest, fungi can invade crops to produce toxins by interacting with other organisms, such as insects. The harvested crops are regulated by factors such as nutrients, temperature and humidity in the air, and biological agents (insects, competitive interference). Furthermore, toxigenic fungi can be divided into four types according to their effects on crops: *A. fungi* acting as plant pathogens, such as grass fungi; *b.* fungi producing fungal toxin and stressing plants, such as *Candida* and *A. flavus*; *c.* fungi acting as colonizers (such as *Aspergillus flavus*), which first colonize in harvest plants and subsequently, produce mycotoxin and contaminate crops; and *d.* fungi decomposing plants (such as *Penicillium chrysogenum* and *Aspergillus oryzae*), which often live in the soil [6, 7].

The crop fungi inoculate the growing crop kernels in the field and proliferate in storage under suitable conditions. Among known crop fungi, *Aspergillus*, *Fusarium*, and *Penicillium* have identified as toxin-producing fungi. Although many compounds produced by these toxin-producing fungi are known as their toxins, there have been only five important agricultural mycotoxins until now: deoxycinol, praurone, ochratoxin A, fumarin and aflatoxin. Mycotoxins produced by *Fusarium* include fumonisins, deoxycinol and zerneone. Although *Penicillium* and *Aspergillus* are storage fungi, they can also invade field stress plants and produce toxins. Increasing evidence has shown that *Penicillium* can produce ochratomycin, citrinin and patron, and that *Aspergillus* can produce aflatoxin, citrinin and baturin [8, 9].

#### **2.2 AFB1 occurrence**

Several previous reviews have fully summarized the occurrence and biosynthesis of AFB1. Briefly, AFB1 are an important class of mycotoxins mainly produced by *Aspergillus flavus* and *Aspergillus parasiticus*. This term is so named and concerned because of the following several reasons: *a.* this mycotoxin has been identified in the *A. flavus* and regarded as pathologic agent of "turkey X" disease; *b.* this mycotoxin is the first B-type aflatoxin which can produce fluorescent characteristic under UV light; and *c.* AFB1 often display its severe toxic effects on human and animals. Usually, it is synthesized through 18 biological steps under the regulations of a huge neighbor gene cluster consisting of about 60–70 kb in original fungi. This biosynthesis at least involves in the three stages, consisting of the formation of primary product hydroxyversicolorone (the first to eighth step), middle product versicolorin B (the ninth to twelfth step) and ultimate product AFB1 (the thirteenth to eighteenth step). During the biosynthesis of AFB1, several key enzymes, including nicotinamide-adenine dinucleotide, nicotinamide-adenine dinucleotide phosphate reduced form, and 2S-adenosylmethionine, are required for the biosynthesis limitation [3, 10, 11].

**193**

*The Toxification and Detoxification Mechanisms of Aflatoxin B1 in Human: An Update*

*A. flavus* is widely present in the soil, causing pollution to many crops such as corn, peanuts, rice, etc., and using these crops as a host produces aflatoxin, which in turn contaminates crop fruits. Aflatoxin contamination of food and animal feed has now become a major problem that threatens food safety. Crops can be contaminated with fungi in the field, harvested, and stored, making crop contamination control difficult. The Food and Agriculture Organization of the United Nations (FAO) estimates that 25% of the world's food crops are contaminated with mycotoxins [12]. Aflatoxins, the most harmful toxins, are the most difficult to deal with because they are commonly found in corn, peanuts and their products, cottonseed, peppers, peppers, pistachios and other foods. Studies have shown that the type of mold and its concentration of conidia, as well as the moisture content of corn, play a key role in the process of mold infection, spoilage and AFB1 production in corn. In the field, Aspergillus flavus enters the plant primarily by vaccination or secondary inoculation, which in turn infects the seed to produce aflatoxin. In spring, the source of inoculation of Aspergillus flavus spores is mainly from the propagules in the soil, the plant debris in the soil, and the wintering mycelium, insects or the Aspergillus flavus nucleus in the soil in the litter. In corn fields, spores are mainly derived from the spore-derived sclerotium conidia of Aspergillus flavus. Before the harvest, the insects damage the corn kernels to form the sclerotia. When harvesting, the sclerotia is dispersed in the soil, and the spring conidia are exposed on the surface of the sclerotium. A secondary inoculation source was found in the cotton field, and *A. flavus* was isolated from the leaves, flower buds and leaf discs of cotton, the content was 15, 94, or 56%, respectively. Most of the colonies were mainly distributed on the calyx tablets. Conidia are the main source of secondary inoculation. Fungal spoilage and mycotoxin contamination are major problems in crop contamination. Grains are affected by storage conditions and the environment after harvesting, such as storage containers, oxygen content in the air, water activity, temperature, and insects, all of which are factors of toxin contamination [7]. If stored poorly, it will increase the contamination of mycotoxins. As one of the main crops for human food and livestock feed, corn is planted annually at 120 million hectares and is one of the most polluted toxins. Aspergillus flavus is the main fungus that is infected after corn harvest. The drying and storage conditions of corn before storage are extremely important. Moisture can accumulate from the activity of pests, which provides ideal conditions for the proliferation of fungi and the accumulation of mycotoxins. In order to reduce the effects of mycotoxins on food and feed chains, it is necessary to control pest and fungal contamination [13, 14]. Humidity and temperature have important effects on mold growth and mycotoxin production. Therefore, humid and hot climates in tropical and subtropical regions provide favorable conditions for mold growth. The moisture content of the grain is generally expressed in terms of water content. Pathogenic fungi that invade crops prior to harvest typically require higher moisture levels (200–250 g/kg) to infect, while fungi that can proliferate during storage (130–180 g/kg) require higher moisture levels. Therefore, most feeds with a water content above 130 g/kg are prone to mold growth and formation of mycotoxins [15]. Therefore, the control of the moisture content in the grain becomes particularly important, especially in the control of moisture at the harvesting point, while the drying and storage of the grain before storage and the frequency of grain drying and plowing, as well as

*DOI: http://dx.doi.org/10.5772/intechopen.89221*

**3.1 The effects of AFB1 on the food chain**

**3. AFB1 toxification and toxic mechanisms**

*The Toxification and Detoxification Mechanisms of Aflatoxin B1 in Human: An Update DOI: http://dx.doi.org/10.5772/intechopen.89221*
