Mold is widely distributed in nature, and it is easy to pollute food because it can form various microspores. After food is contaminated by mold, it can not only lead to corruption and deterioration, but also some molds can produce toxins, which can cause mycotoxin poisoning in people and animals by eating by mistake. Mycotoxin is a toxic secondary metabolite produced by mold. Since the discovery of highly carcinogenic aflatoxin in 1960s, people have paid more and more attention to the pollution of food by mold and mycotoxin. Mycotoxins usually have the characteristics of high temperature resistance, no antigenicity, mainly attacking parenchymal organs, and most mycotoxins also have carcinogenic effects. The effects of mycotoxins include reducing cell division, inhibiting protein synthesis and DNA replication, inhibiting DNA from forming complex with histone, affecting nucleic acid synthesis and reducing immune response. According to the target organ of mycotoxin, it can be divided into hepatotoxicity, nephrotoxicity, neurotoxicity and photoallergic dermatitis. Once people and animals eat food containing a large amount of mycotoxin, they often suffer from acute poisoning, while long-term intake of food containing a small amount of mycotoxin will lead to chronic poisoning and cancer. Therefore, mildew of grain and grain will not only cause economic losses, but also cause acute or chronic poisoning of people and animals and even lead to cancer.

3. 1 toxin production characteristics of mold

1) The toxin production of molds is limited to a few toxic molds, and only a part of the toxic strains produce toxins. 2) The virulence of virulent strains also showed variability and variability. After several generations of culture, the virulent strain can completely lose its virulence, while the non-virulent strain can produce virulence under certain conditions. Therefore, we should always consider this issue in practical work. 3) A strain or strain can produce several different toxins, and the same mycotoxin can also be produced by several molds. 4) The toxin-producing strains need certain conditions, mainly the type of substrate, moisture, temperature, humidity and air circulation.

3.2 Main toxic molds

At present, there are the following kinds of molds that are known to pollute cereals and foods and have toxic strains:

3.2. 1 Aspergillus

Aspergillus has developed mycelium, which is multicellular and septate. Its asexual reproduction produces conidia, the conidiophore is unbranched, and the top of conidia expands into a spherical or wooden stick shape, which is called the apical sac. One or two petioles radiate from the apical sac, and a series of conidia are attached to the top of the petiole. Conidia have different colors, such as black, brown and yellow. The sexual generation of Aspergillus produces a closed capsule containing many spherical ascospores. Aspergillus is widely distributed in nature and has a strong ability to decompose organic matter. Some Aspergillus species, such as Aspergillus Niger, are widely used in food industry. At the same time, Aspergillus is also an important food contamination mold, which can lead to food spoilage and some strains can also produce toxins. The species that can produce toxins in Aspergillus are Aspergillus flavus (Aspergillus flavus), Aspergillus ochraceus (Aspergillus versicolor), Aspergillus fumigatus, Aspergillus nidulans (Aspergillus parasitica) and so on.

Penicillium

Penicillium mycelium is colorless or light-colored, with many branches and septa. From hypha to conidia with diaphragm, the top branches 1~2 times. These branches are called secondary branches and peduncle bases, where many peduncles are produced and conidia are found at the top of the peduncle. This structure is called a broom. Conidia can have different colors, such as cyan, grayish green, yellowish brown and so on. And the broom body are provided with a single wheel, a symmetrical multi-wheel and an asymmetrical multi-wheel. Only a few species of Penicillium form closed shells and produce ascospores. Penicillium is widely distributed and varied, and often exists in soil, grain, fruits and vegetables. Some species have high economic value and can produce a variety of enzymes and organic acids. On the other hand, Penicillium can cause spoilage of fruits, vegetables, grains and foods, and some species and strains can also produce toxins. For example, Penicillium islandicum (P.islandicum), Penicillium citrinum (P.citrinum), Penicillium yellowish green (P.citreo-viride), Penicillium rubrum (P.rubrum), Penicillium expansum (P.expansum), Penicillium arc, Penicillium expansum, Penicillium expansum and Penicillium expansum.

Fusarium

The aerial hyphae of this genus are developed or underdeveloped, and conidia can be divided into two types: large conidia with 3~7 septa, which are produced on the short claw-like processes of hyphae or in myxomycetes and have various shapes, such as sickle shape and spindle shape. Microconidia have 1~2 septa, which are oval and oval in shape and are produced on the conidiophore. The aerial hyphae, myxospores and sclerotia can be of various colors, and the substrate can be dyed in various colors.

Fusarium includes many species, most of which are plant pathogens and can produce toxins. Such as Fusarium graminearum, Fusarium graminearum, Fusarium poa, Fusarium aseptic, Fusarium oxysporum, Fusarium moniliforme, Fusarium moniliforme, Fusarium equisetum, Fusarium australis, Fusarium pink, etc.

Alternaria

The hyphae have transverse septa, creeping growth, short conidiophore, solitary or clustered, and most of them are unbranched. The conidia at the top of conidia vary in shape and size, including mulberry, oval and oval, and conidia are vertical and horizontal.

The septum and apex are elongated into beaks and multicellular. Spores are brown, and the number is constant and connected into chains. No sexual generation was found.

Alternaria alternata is widely distributed in soil and air, some of which are plant pathogens, which can cause fruits and vegetables to rot and produce toxins.

Other genera

Trichophyton farinosus, Trichoderma, lettuce, Aureobasidium nigrum, etc.

3.3 Main mycotoxins

3.3. 1 aflatoxin

Aflatoxin (AFT or AT for short) is the metabolite of Aspergillus flavus and Aspergillus parasiticus. All strains of parasitic Aspergillus can produce aflatoxin, but parasitic Aspergillus is rare in China. Aspergillus flavus is a common fungus in food and feed in China. Aflatoxin has attracted much attention because of its strong carcinogenicity. However, not all Aspergillus flavus are toxin-producing strains, and even toxin-producing strains can only produce toxins under suitable environmental conditions.

3.3. Characteristics of1.1aflatoxin

The chemical structure of aflatoxin is dihydrofuran and xanthone. Up to now, more than a dozen have been isolated, such as B 1, B2, G 1, G2, B2a, G2a, M 1, M2, P 1 and so on. Among them, B 1 is the most toxic and carcinogenic, and its toxicity is 0/00 times greater than that of potassium cyanide, second only to botulinum toxin, and it is the strongest mycotoxin. Carcinogenesis is stronger than all known chemical carcinogens and 75 times stronger than dimethylnitrosamine. Aflatoxin has the characteristics of heat resistance, its cracking temperature is 280℃, its solubility in water is very low, and it can be dissolved in oil and various organic solvents.

3.3. 1.2 conditions for aflatoxin production

The temperature range of Aspergillus flavus growth and toxin production is 12~42℃, the optimum temperature for toxin production is 33℃, and the optimum Aw value is 0.93~0.98. When Aspergillus flavus grows on corn, rice and wheat with water content of 18.5%, it begins to produce aflatoxin on the third day, reaches the peak on the tenth day, and then gradually decreases. When bacteria form spores, the toxins produced by mycelium are gradually discharged into the matrix. This delay in toxin production of Aspergillus flavus means that if the grain with high moisture content is dried within two days, the moisture content of the grain will drop below 13%, and even if Aspergillus flavus is polluted, it will not produce toxin.

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Aflatoxin pollution can occur in a variety of foods, such as grains, oilseeds, fruits, dried fruits, condiments, milk and dairy products, vegetables, meat and so on. Among them, corn, peanuts and cottonseed oil are the most vulnerable to pollution, followed by rice, wheat, barley and beans. Grains such as peanuts and corn are suitable substrates for the growth of aflatoxin-producing strains and the production of aflatoxin. Peanut and corn may be contaminated by Aspergillus flavus before harvest, which makes mature peanuts not only pollute Aspergillus flavus but also carry toxins. When the ear of corn is ripe, not only Aspergillus flavus can be isolated from the ear, but also aflatoxin can be detected.