Prevention and control of mycotoxins
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Maitree Suttajit, Ph.D
Serveral mycotoxins in agricultural products cause health hazards to people and animals and economical problem. Dangerous mycotoxins are naturally present in foods, feeds and our environment. They are pathologically classified as hepatotoxins, nephrotoxins, vomitoxin and neuro-musculotoxin, some of which are potentially carcinogenic and mutagenic (Table 1). Aflatoxin, for example, is the most potent hepatocarcinogen and mutagen among mycotoxins. Therefore, the contamination of mycotoxins should be minimized by designing a series of measures of prevention and control.
STRATEGIES FOR PREVENTION AND CONTROL OF MYCOTOXINS
To design strategies for the reduction or elimination of mycotoxins, knowledge about their fungal sources are needed. The growth of fungi in crops and agricultural products is the main cause of toxin formation and related to the concentration of the toxic substances. Many factors are involved in enhancing the formation of mycotoxins. They are plant susceptibility to fungi infestation, suitability of fungal substrate, temperate climate, moisture content and physical damage of seeds due to insects and pests.
Toxin-producing fungi may invade at pre-harvesting period, harvest-time, during post-harvest handling and in storage. According to the site where fungi infest grains, toxinogenic fungi can be divided into three groups: (a) field fungi; (b) storage fungi; and (c) advanced deterioration fungi. The first category includes species of plant pathogenic fungi, namely, genus Fusarium, e.g. F. moniliforme, F roseus, F. tricinctum and F. nivale. The "storage fungi" are principally the general Aspergillius and Penicillium, e.g. A. flavus and A. parasiticus. The "advanced deterioration fungi" normally do not infest intact grains but easily attack damaged ones and require high moisure content. The examples of the third group are A. clavatus, A. fumigatus, Chaetomium, Scopulariopsis, Rhizopus, Mucor, and Absidia.
The prevention of mycotoxins in our environment is a big task. In general, prevention of the contamination of fungi and their mycotoxins in agricultural commodities can be divided into these following three levels.
1. Primary prevention
The step of prevention should be initially carried out before the fungal infestation and mycotoxin contamination. This level of prevention is the most important and effective plan for reducing fungal growth and mycotoxin production. Several practices have been recommended to keep the conditions unfavorable for any fungal growth. These include:
Table 1: Some mycotoxins, their sources and potential toxicities (1).
|Aspergillus parasiticus||and fatty liver|
|Citreoviridin||Penicillium viridicatum||Cardiac beri-beri|
|Cytochalasin E||Aspergillus clavatus||Cytotoxicity|
|Patulin||Penicilliumc-expansum||Brain & lung hemmorrhage|
|Penicillium patulum||and carcinogenicity|
|PR Toxin||Penicillium requeforti|
|Rubratoxin||Penicillium rubrum||Liver hemmorrhage and fatty infiltration|
|Rugulosin||Penicillium islandicum||Nephrosis & liver damage|
|Tremorgens||Penicillium and Aspergillus|
|Vomitoxin (Deoxynivalenol)||Fusarium graminearum||Vomiting|
2. Secondary prevention
If the invasion of some fungi begins in commodities at early phase, this level of prevention will then be required. The existing toxigenic-fungi should be eliminated or its growth to be stopped to prevent further deterioration and mycotoxin contamination. Several measures are suggested as follows:
3. Tertiary prevention
Once the products are heavily infested by toxic fungi, the primary and secondary preventions would not be then feasible. Any action would not be as effective as the practices mentioned above, since it will be quite late to completely stop toxic fungi and reduce their toxin formation. However, some measures should be done to prevent the transfer of fungi and their health hazardous toxins highly contaminated in products into our daily foods and environment. For example, peanut oil extracted from poor-graded peanut seeds always contains very high levels of aflatoxins and the oil-soluble toxin has to be eliminated by absorption and alkalinization during oilrefining process. Only a few practices are recommended:
Since aflatoxin is the most well-known mycotoxin ever throughly studied and its prevention and control has been most successfully practiced in various countries, therefore, this paper will focus on such practices in certain detail for the prevention and control of aflatoxins mycotoxin contamination. Successful development will bring a great impact for the increased production of crops and safe and nutritious foods around the world. A number of researchers have been working on A. flavus-resistant or tolerant varieties of corn (2-3) and peanut (4-6)
It has been clear that the fungal-resistance of each variety is genotypic. However, the resistance to invasion of A. flavus has been attributed to several biochemical, environmental and physical factors. Uncontrollable factors could bring the failure in the utilization of selected fungal-resistant variety, as shown by laboratory screening, in the field.
Davis and his co-workers (7) reported the survey and comparison of aflatoxin contamination in upto 215 corn hybrids grown in Alabama, USA during 1976-81. Unfortunately, they could not find any hybrid tested resistant to aflatoxin formation. They were convinced that significant aflatoxin levels generally accompanied stress caused by high temperature, low rainfall, low moisture-holding capacity of sandy soils and insect infestation.
A differential pathogenic capacity of various toxigenic strains of A. flavus have been observed (8). Some strains would require physical damage for their infestation and others would not. The association of mycotoxin production and physical damage to grain and drought during grain ripening indicates that Aspergillus spp. are weak pathogens. During long grain storage, the biochemical activity of grain is much reduced, while invasion of storage fungi and mycotoxin contamination would increase. More data is needed on the biochemistry and pathogenesis of toxigenic fungi to understand and evaluate their genotype.
The germination and viability of maize seeds could be affected by attack of Aspergillus and Penicillium species and their fungal infestation have been found to be different among maize genotypes (9-10).
Similarly, genotypes of peanut and biochemical properties of its seed such as tannin content (11), thin pericarp (12), small amount of cuticular wax (13) and chemical composition of the pericarps and embryos (14) have been shown to inhibit fungal invasion by A. flavus and aflatoxin formation.
Recently, antifungal enzymes, chitinase (15) and B-1, 3-glucanase (16), found in a number of plant seeds, may act as defense against pathogenic fungi, since chitin and glucan are major polymeric components of many fungal cell walls. Such polysaccharides in fungal cell wall could be enzymically hydrolysed into smaller products resulting the damage or killing of fungal mycelia or spores. The role of these enzymes for genotype evaluation is now being studied. It is foreseen that seeds rich in such antifungal enzymes likely resist the infestation of fungi. If so, the seeds for breeding would be easily screened out and used a stock one.
Even there are many technical problems in searching for the "super" plant against pathogenicity, the development of fungal-resistant plant varieties utilizing genetic resistance to mycotoxin contamination is still possible and encouraged.
FUNGAL GROWTH INHIBITION
How to prevent growth and invasion of pathogenic fungi in agricultural commodities is very important in preventing mycotoxin contamination. The inhibition of fungal growth can be achieved by physical, chemical and biological treatments (17).
DECONTAMINATION OF MYCOTOXINS
Contaminated mycotoxins in foods and feeds should be removed, inactivated or detoxified by physical, chemical and biological means depending on the conditions. However, the treatment has its own limitations, since the treated products should be healthsafe from the chemicals used and their essential nutritive value should not be deteriorated. The following methods are suggested to be applied for effective decontamination of some mycotoxins.
Physically, fungi-contaminated seeds can be removed by hand picking or photoelectric detecting machines. The method would consume time and Iabor or expensive.
Organic solvents (chloroform, acetone, hexane and methanol) have been used to extract aflatoxins for agricultural products, but mainly in vegetable oil refining process (36).
Heating and cooking under pressure can destroy nearly 70% of aflatoxin in rice compared to under atmospheric pressure only 50% destroyed (37). Dry and oil roastings can reduce about 50-70% of aflatoxin B1 (38). We could show that only about 10% of total 1242 ppb of aflatoxin B. decreased in naturally contaminated peanut by heating at upto 100°C (39). Since aflatoxin resists to higher temperature upto 260°C, long-time cooking and overheating would destruct essential vitamins and amino acids in treated foods.
Ionizing radiation such as gamma-rays can stop growth of food spoilage organisms, including bacteria, molds and yeasts. It also inactivates pathogenic organisms including parasitic worms and insect pests. It has been reported that gammairradiation (5-10 M-rad) caused reduction of aflatoxin (40). The irradiation, however, could not completely destroy the toxin and its mutagenicity. In our laboratory, only about 30% of total 600 ppb at aflatoxin B1, either pure toxin or in contaminated peanut, was destroyed by 1 and 5 Mrad or gamma irradiation (23). The treatment combination of gamma irradiation and ammoniation should be therefore attempted for more aflatoxin decontamination.
Chemical treatment has been used as the most effective means for the removal of mycotoxins from contaminated commodities. The method should be sure that the detoxification system is capable of converting the toxin to a nontoxic derivative (s) without deleterious change in the raw product. Mutagenicity of the treated products should be assessed. The toxicity may be checked by feeding animals including bouts, egg embryos, chicken, ducklings and rats. Many common chemicals have been brought to test the effectiveness in detoxification of aflatoxin. These chemicals include the followings:
The chemical reactions of detoxification of aflatoxin are primary addition of the double bond of the furan ring and oxidation involving phenol formation and opening of the lactone ring. In the presence of acid, aflatoxins B. and G. will be converted into their 2-hydroxy derivatives, aflatoxins B2a respectively.
Figure 1 Proposed formation of aflatoxin-related reaction products following exposure to ammonia
Figure 2 Comparison detoxification efficiency of 3% ammonium bicarbonate and 3% ammonium carbonate at different length
Figure 3 Comparison of reduction of AFB1 content in different treatment periods and their mutagenicities
Figure 4 Effect of gamma irradiation with and without ammonium blbdrbondte (3%) on AFB1 in peanut.
Other mycotoxins which are like aflatoxin and have a lactone grouping in the molecule can be similarly destroyed by alkaline condition using ammonia, sodium hydroxide and sodium bicarbonate. These toxins are patulin, penicillin acid, citreoviridin, citrinin, cyclochlorotin, ochratoxin A, rubratoxin, trichothecenes and zearalenone.
Certain conditions such as moisture content, heat, ultraviolet or gamma irradiation, sunlight and pressure at different treatment-periods have been simultaneously combined with the chemicals for the enhancement of detoxification.
Inactivation methods can be achieved by mixing, packing, fumigation and immersion with the chemical used.
CONTROL OF MYCOTOXINS
Careful control of mycotoxins should be started and administered by the government of each country through ministries and organizations such as the Ministry of Health, the Ministry of Agriculture, Food and Drug Administration, National Environment Committee Board and Consumer Protection Committee Board. The control program may be set up by a special administrative committee and the legislative body who regulate the national policy of food safety and the maximum tolerance limits for mycotoxins. Farmers, middlemen, food and feed factories and exporters will be well educated about mycotoxins and encouraged to prevent and control the contamination of microflora and their health-hazardous mycotoxins in their commodities as much as possible.
International cooperation for the mycotoxin regulation in trading products or commodities is also needed. The countries should establish quality control limits for certain commodities intended for export or import. The producer countries would be stimulated to be aware of mycotoxin contamination in their exported susceptible commodities. For example, the European Economic Community (EEC) has already established certain maximum tolerance limits of aflatoxins for animal feeds, i.e. not more than 20 ppb for complete feed for gigs, poultry and feed supplements for dairy animal; 50 ppb for produce to be processed into mixed feed and complete feed for cattle, sheep and goats.
International organizations such as FAO, WHO and UNEP in the UN system are engaged in providing essential information on various aspects of prevention and control of mycotoxin problems to all the countries. Guidelines for international trade include: a) procedure of sampling and analysis, b) surveillance and food control inspection systems, c) use of contaminated produce in feeding of different animals, d) protocols for detoxification and the quality control of the products. Conferences, symposiums, trainings and workshops on current informations of mycotoxins should be promoted. Low-cost technology for assessment, prevention and control of environmental mycotoxins could be then transferred from developed countries to developing ones.
Several effective ways for prevention and control hazardous fungi and their dangerous mycotoxins have been presented. The methods include biological control and physical and chemical treatments. Selection of fungal resistant hybrids of crops are recommended and further experimented. Pre-harvesting preparation of the field and environments should be aware of. Drying of commodities after post harvest is the most economical and effective means for farmers or layment, but sometimes is not suitable during rainy season or wet condition. Thermal treatment or gamma irradiation is not effective or practically used by villagers. Chemical treatments such as alkalinization and ammoniation are well-recognized and industrially used. Some modifications of the application of effective chemicals to the detoxification of mycotoxins should be developed. International cooperations through authorized organizations should be promoted and supported aiming the benefits for the economics and health of people of all the nations.
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