Formation of Mycotoxins

As the world’s population grows, access to a safe food supply will continue to be a global priority. In recent years, the world has experienced an increase of mycotoxin contamination in grains due to climatic and agronomic changes that encouraged fungal growth during cultivation. A number of the molds that are plant pathogens produce mycotoxins, which are known to cause serious human and animal toxicosis.

Although there is a harmful effect on humans and animals exercised by many toxins from fungi, a small group of these toxins, play activities beneficial to man. Are compounds that have activity as antibiotics (eg. penicillin), and ergot alkaloids group, widely used for controlling migraines.

Fungi, also called mold or molds are multicellular organisms and filamentous, whose structure is best observed under a microscope and is therefore designated microfungi, instead of mushrooms, which are macrofungi. And when these fungi infest grains and foods produce toxic or non-toxic components. It is known that the secular activity of fungi causes changes in food quality. Sometimes these changes are desirable, for example, to develop characteristic taste in certain cheese. However, in most cases, fungi cause undesirable changes, producing unpleasant tastes and smells caused by different degrees of deterioration.

Fungal secondary metabolites constitute a wide variety of compounds which either play a vital role in agricultural, pharmaceutical and industrial contexts or have devastating effects on agriculture, animal and human affairs by virtue of their toxigenicity.

Mycotoxin is the term used to describe toxic substances formed during the growth of fungi, which is associated with physical changes in the nature of the food flavor, odor and appearance. Fungi mainly of the genus Aspergillus, Penicillium and Fusarium are widely distributed in nature and therefore often also contaminate fresh and processed foods.

The mycotoxin terminology includes a diversity of compounds from different precursors and metabolic pathways, assembled according to the degree and type of toxicity to higher animals and human.

Intoxication may be carried out directly or indirectly. The direct form occurs when the product is directly used in human food or animal. While the indirect form when contaminated by-products and derivatives are used.

Cereals and oilseeds are often affected by these secondary metabolites of fungi during harvesting, storage and industrialization.

The products that may generally be carriers of mycotoxins to humans or animals are: (1) agricultural products: cereals, seeds, nuts, fruits, vegetables; (2)food industrialized; (3) products of animal origin: milk and dairy products meats, sausages; (4) cheeses with fungi; (5) Oriental fermented foods; (6) products obtained by fermentation: beer, food additives and vitamins.

Food contamination by fungi can cause, in addition to health problems, economic irreparable losses, comprising: direct losses of agricultural products; loss of animals due to death; human diseases and decreased productivity of animals; indirect costs of control systems of some mycotoxins; costs of detoxification in order to make the product acceptable; rejection of the product by export markets.

The five most important mycotoxins in relation to food: aflatoxin, deoxynivalenol, nivalenol, zearalenone and fumonisins.

Currently, a series of aflatoxins are known and well-studied. In addition, other mycotoxins were investigated, isolated and characterized biochemically. Among these are patulin, citrulline, fusaric acid, citrinin, penicilic acid, ochratoxins, rubratoxins, sterigmatocystin and trichothecenes.

Within the wide range of fungi exist, there are few who play an important role in the fields livestock, agricultural and biological. The fungi that produce mycotoxins are divided generally into two groups: those that attack before harvest, commonly called field fungi, and those which occur only after harvest, called storage fungi.

There are three types of fungi toxicogenic in the field:

•• Plant pathogens such as Fusarium graminearum (deoxynivalenol, nivalenol).

•• Fungi growing on senescent or stressed plants, such as Fusarium verticillioides (fumonisins) and occasionally Aspergillus flavus (aflatoxins).

•• Fungi that previously harvesting have predisposed to the production of mycotoxin contaminants, such as Penicillium verrucosum (ochratoxin) and Aspergillus flavus (aflatoxin).

In all these cases there is an association relatively well defined between the fungus and the host plant as the genus Aspergillus and Fusarium that are probably the most significant fungi producers of mycotoxins in the field of tropical countries.

Kernel root caused by Fusarium is one of the most important diseases of the corn ear in hot growing areas. It is associated with heat, dry season and/or insect damage. There is a strong relationship between damage caused by insects and seed rot caused by Fusarium. It has been found during research work in the field, for example, that the incidence of corn borer in Europe, increases the diseases caused by Fusarium verticilioides with concentrations of fumonisin.The intensity of the temperature during the plant’s growth is also important, the occurrence of fumonisin in corn hybrids grown across the area of cereal planting in the United States, Europe and Africa, indicate that hybrids grown outside their range of temperature adaptation have higher concentrations of fumonisin.

After the harvest, when the grains or seeds become dormant as a result of the drying process, disappear associations among fungi-plants, however physical factors determine whether members of other groups, just as storage fungi, establish and produces mycotoxins or not. The primary factors influencing fungal growth in storage food products are moisture content (more precisely, the water activity) and temperature. In practice, in the tropics, the temperature is almost always suitable for storage fungi. For this reason, is the action of water which becomes the main determinant of invasion and fungal growth. Once mycotoxins are produced they will prevail, even if the fungus will be destroyed by heat sterilization.

Biological materials such as grains, seed and feed, have the characteristic of being hygroscopic, therefore, between the air and water changes are established mainly as a vapor. Thus, on the surfaces of products are set microclimates that their situations have been mainly influenced by moisture content of products. In this microclimate, the amount of water available is expressed by the water activity factor (aw factor), which varies from 0 to 1. This factor is defined as the ratio between the pressure of water vapor present in the vapor pressure and microclimate at the surface of a portion of pure water, which is the vapor pressure of the air saturated condition. Thus, the moisture content sets the values of vapor pressure and a factor on the surface of the product.

AW = Ps/PO

Thus, in the space formed between the grains, known as intergranular space, during the storage period is set an environment that has its status conditions mainly affected by the moisture content of the grain mass. What may or may not favor the development of microorganisms, this fact will depend on the water factor.

Bacteria develop in products whose water activity is above 0.90, while for fungi values range from 0.65 to 0.90, the range may have grain moisture content 14-22%. Therefore, a drying process is used for preservation of grains aiming to reduce the moisture content of the products to levels that the water activity is not conducive to the proliferation of fungi.

In situations of hygroscopic equilibrium of relative humidity of the intergranular air corresponds to 100 times the value of water activity. For this situation, the relative humidity is named as equilibrium relative humidity and grain moisture.

The fungi, to develop and produce mycotoxins, require favorable conditions. Besides the humidity, that is the most relevant, there are others factors that should be considered:

1. Moisture - the absence of water available in the food prevents fungal growth, must, therefore, be avoided situations where food is stored under conditions of high humidity.

2. pH - fungi grow in a pH ranging from 2 to 8. Aflatoxins have optimum growth at pH 5-7.

3. Food composition - in general, foods with high carbohydrate content are more favorable to high yields of aflatoxin than, grains with high amount of oil (except peanuts) or foods with high protein content.

Another factor that promotes its formations is the presence of glycerol and metals trace ( Mo, Zn, Mg and Fe). The production of aflatoxin is also favored by NaCl in concentrations of 1.0-1.5%, decreasing with increasing of concentration. Pyruvate has its influence providing the growth of fungus, but not formation of the toxin.The exception to the formation of toxins occurs when there in sucrose in concentrations of 50%.

4. Redox potential - aerobic organisms and fungi, so any change in the atmosphere could affect the production of the toxin, although, Aspergillus flavus at 50 ° C can grow in the presence of 20% CO2 for 14 days in high humidity. Conditions of reduced oxygen atmosphere retard the growth of fungi and can reach completely inhibit the development.

5. Temperature - the various species of fungi have different temperature ranges for possible development. The optimum temperature for growth is generally the highest temperature of toxin production. In general, the optimum temperature is 20-30 ° C, may be considered the minimum temperature of 3-7 ° C. If during the storage of agricultural products, the temperature is low, fungi psychrophiles (which develop at low temperatures) could grow, and thereby raise the temperature at certain points, providing conditions for a secondary contamination.

6. Microbial interaction - toxigenic fungi rarely occur alone in natural foods, they coexist with other fungi and some yeasts and bacteria.

 

Boehm, J. Occurrence and noxiousness of mycotoxins in European Foods. J.Food and Nutrition Sciences. v.4, n.2, p. 3-7, 1995.

Bottalico, A. Mycotoxins in foods with possible human health implication. Part I - AFLATOXINS.IGIENE-MODERNA, v.111, n.2, p. 133-169, 1999.

Diekman, M.A.; Green, M.L. Mycotoxins and reproduction in domestic livestock.  J. Animal Sci. v.70, p. 1615-1627, 1992.       

Diniz, S.P.S.S. Mycotoxins – Biochemical Approach. Ed. Albatroz, Rio de Janeiro, 2015. 186p.            

Juvvadi, P.R.; Seshime, Y.; Kitamoto, K. Genomics reveals traces of fungal phenylpropanoid-flavonoid metabolic pathway in the filamentous fungus Aspergillus oryzae. J. Microbiol., v.43, n.6, p. 475-486, 2005.

Keller, N.P.; Turner, G.; Bennett, J.W. Fungal secondary metabolism – from biochemistry to genomics. Nat. Rev. Microbiol., v. 3, n.12, p. 937-947, 2005.

Pozzi, C.R.; Correa, B.; Gambale, W.; Paula, C.R.; Chacon-Reche, N.O; Meirelles, M.C.A. Postharvest and stored corn in Brazil: Mycoflora interaction, abiotic factors and mycotoxin occurrence. Food Additives and Contaminants.v.12, n.3, p. 313-319, 1995.