Dusanee Thanaboripat, Department of Applied Biology, King Mongkut’s Institute of Technology, Ladkrabang Bangkok, Thailand
Mycotoxins are toxic secondary metabolites produced by some certain strains of fungi. The contamination of mycotoxins in various foodstuffs and agricultural commodities is a major problem and may vary with geographical conditions, production and storage practice, and also with the type of food. The most recognized and intensively researched mycotoxin in the world is aflatoxin. This article reviews the importance and occurrence of mycotoxins and their controls.
OCCURRENCE OF MYCOTOXINS
Mycotoxins have been reported to be carcinogenic, tremorogenic, haemorrhagic, teratogenic, and dermatitic to a wide range of organisms and to cause hepatic carcinoma in human (Refai, 1988; van Egmond, 1989, Wary, 1981). More than a hundred species of filamentous fungi are known to produce mycotoxins and to cause toxic responses under naturally occurring conditions. Some mycotoxins produced by toxigenic fungi and their occurrence are listed in Table 1. Mycotoxins can enter the human and animal food chains by direct contamination when the food has been contaminated by toxigenic fungi while growing or after harvest, or indirect contamination, for example in milk from cows fed with contaminated food (Carlile et al., 2001). Mycotoxins contaminate up to 25% of the world’s food supply. More than 300 mycotoxins are known, of which about 20 are serious contaminants of crops used in human foods and animal feeds. Some mycotoxins are considered to be of major significance, responsible for widespread pathological effects, while others are of minor importance (Moore-Landecker, 1996). The problem of mycotoxin contamination of foods or animal feeds has been widely recognized since the discovery of aflatoxins (Golinski, 1984). The problem is more serious in developing and under-developed countries and in those countries where mycotoxin containing commodities can not be destroyed on account of scarcity of agricultural products (Sinha, 1993). Mycotoxin contamination of foods and feeds depends highly on environmental conditions that lead to mould growth and toxin production (van Egmond, 1989).
Aflatoxins are the most notorious mycotoxins because outbreaks of aflatoxicosis in farm animals have been reported from many areas of the world (Smith and Moss, 1985). Aflatoxin contamination of foods and feeds occurs when aflatoxigenic species of the A. flavus group successfully colonize and grow in a commodity, and subsequently produce the aflatoxin secondary metabolites. The species of the A. flavus group that produce aflatoxins include A. flavus, A. parasiticus, A. nomius, A. tamarii and A. bombycis (Goto et al., 1996; Peterson et al., 2001, Wilson and Payne, 1994). Some strains of A. flavus have been re-identified as A. parasiticus and A. nomius (Pitt, 1993). Aflatoxins can be produced only under particular environmental conditions. Therefore, the actual growth of aflatoxigenic fungi on the food does not necessarily mean that aflatoxins are also present. Moisture, temperature, and insect or other injury as well as the A. flavus isolate, the crop and the environmental conditions are particularly important factors in determining whether aflatoxins are actually produced as the fungus grows within the seeds or grains (Moore-Landecker, 1996; Wilson and Payne, 1994). Aflatoxins can be produced in preharvest as well as in stored products. Spores of A. flavus can be introduced into the plant through insect wounds, or they can germinate on the pistil of the flower. Its spores also contain aflatoxin (Thanaboripat, 1988).
Various agricultural commodities and industrial products have been contaminated by either aflatoxin producing fungi or aflatoxins. Aflatoxins have been found in food crops and foods such as peanut butter and other peanut products, breakfast cereals, corn and cornmeal, and a variety of other foods and feeds (Smith and Moss, 1985; Wilson and Payne, 1994). Soybean appears to be less susceptible to aflatoxin contamination than other crops (Pinto et al., 1991). The contamination of aflatoxin in soybean and soybean products is rare in commerce in the USA but detectable levels have been demonstrated in edible beans in Africa and Thailand (Smith and Moss, 1985). The highest risks of aflatoxin contamination are corn, peanut and cottonseed. Milk and milk products, eggs and meat products are sometimes contaminated because of the animal consumption of aflatoxin-contaminated feed. After consumption by animals, the B aflatoxins are metabolized to the M aflatoxins and secreted in the milk (van Egmond, 1994). Aflatoxin M1 is of special interest because it can be transmitted to a newborn offspring in the human’s milk (Moore-Landecker, 1996). Aflatoxin M1 has been reported in mother’s milk. 99.5% of breast milk from 445 people in Abudabi were found to contain 2-3 ng/l of M1 (Saito et al., 1991). However, 43 samples of human breast milk collected from three hospitals in Bangkok were not contaminated with aflatoxin M1 (Thanaboripat and Sukchareon, 1997).
Ochratoxins are produced by Aspergillus species, notably A. ochraceus in the tropics growing on cocoa and coffee and P. verrucosum in temperate regions growing on cereal such as barley (Carlile et al., 2001). Ochratoxin A has been shown to be a potent nephrotoxin in all species of animal tested, including fish, bird and mammal (Krogh, 1977). Ochratoxin A contaminates a variety of plant and animal products but is most often found in stored cereal grains (Abarca et al., 1994).
Patulin is produced by species of Penicillium, Aspergillus and Byssochlamys. Patulin may occur in fruits and fruit juices such as apple juice and grapefruit juice (van Egmond, 1989). Citrinin and penicillic acid are toxic antibiotics produced by several species of Aspergillus and Penicillium. Citrinin has been detected from peanut, tomato, corn, barley and other cereals (Sinha, 1993).
Trichothecene and zearalenone (F-2) are primarily produced by species of Fusarium on corn, wheat and other cereals (Moore-Landecker, 1996). Zearalenone, an oestrogenic mycotoxin, causes problems with the reproductive organs of farm animals, especially swine (van Egmond, 1989). Zearalenone is particularly occurred in corn and wheat and often found together with deoxynivalenol (vomitoxin). The other Fusarium toxin is Fumonisin, produced by F. moniliforme and related fungi, has been found most frequently in corn (Chu and Li, 1994). F. moniliforme is one of the most common fungi colonizing corn throughout the world. A number of mycotoxins produced by fusaria are found in the corn collected from China and southern Africa where high incidence of oesophageal cancer in humans are reported (Carlile et al., 2001).
CONTROL OF MYCOTOXIN IN FOODS
Every year a significant percentage of the world’s grain and oilseed crops is contaminated with hazardous mycotoxins such as aflatoxins (Phillips et al., 1994). Control of mycotoxin producing fungi and mycotoxin contamination in foods and feeds has been proved difficult. Many biological and climatic factors influence mycotoxin contamination in agricultural commodities and these factors are difficult to control. Detection, removal and diversion are reasonable means for preventing the entry of mycotoxins into the food chains (Figure 1). The best way of controlling mycotoxin contamination is by prevention and can be accomplished by reducing fungal infection in growing crops through the adoption of suitable cultural practices, by rapid drying or by the use of suitable preservatives (Sinha, 1993; Smith and Moss, 1985). If contamination can not be prevented, a way to either remove or destroy the toxin will allow consumption of the commodities with reduced adverse effect (Krogh, 1987). Physical, chemical and biological methods have been investigated in order to prevent the growth of mycotoxin producing fungi, eliminate or reduce the toxin levels, degrade or detoxify the toxins in foods and feeds. Mycotoxins can be eliminated or detoxified by physical, chemical or biological techniques. Many chemicals including numerous acids, alkalis, aldehydes, oxidizing agents and several gases have been tested for their ability to degrade or inactivate aflatoxin and many other mycotoxins (Samarajeenwa et al., 1990; Smith and Moss, 1985; Thanaboripat, 2002). Most of the monitoring for mycotoxins in foods have focused on aflatoxins.
Chemical treatment by ammoniation has been found to be an effective method to detoxify aflatoxin-contaminated corn and other commodities. Sunflower meal, an excellent source of protein supplement in poultry and animal feeds in Pakistan has also been tested for aflatoxin detoxification by ammoniation (Ahmad et al., 1995). Butylated hydroxyanisole (BHA), a phenolic antioxidant, has been reported to inhibit the growth of toxigenic species of Aspergillus, Fusarium, and Penicillium (Thompson, 1996).
The application of salt for controlling A. flavus in peanut was investigated. The result indicated that low concentrations of sodium chloride stimulated aflatoxin production whereas high concentrations inhibited fungal growth and aflatoxin production (Thanaboripat et al., 1992). High concentrations of sodium chloride may adversely affect the water activity required for growth and toxin production or it may be that sodium ions interfere with ion transport in the organism.
Natural compounds from plants have been used traditionally to preserve foods in countries like Japan, India and Russia (Wilson and Wisniewski, 1992). The extracts of some local plants show the ability to suppress the growth of toxigenic fungi and hence, the toxin production. Essential oils of cinnamon, peppermint, basil, origanum, the flavoring herb Epazote, clove, and thyme caused a total inhibition of A. flavus on maize kernels (Montes-Belmont and Carvajal, 1998). Essential oils from some Thai herbs are under investigation in the author’s laboratory for their inhibitory effect on growth and aflatoxin production of A. flavus and A. parasiticus in Potato Dextrose Agar (PDA) and corn (unpublished data). Preliminary results indicated that out of 25 plants tested, only essential oil from Betel Vine (Piper betle Linn.) showed the highest inhibitory effect on fungal growth (Figure 2). Natural plant extracts may provide an alternative way to protect foods or feeds from fungal contamination (Yin and Cheng, 1998). While dealing with grain protection, fumigation is the preferred method for applying substances into the bulks in order to control the biotic factors which damage the grains (Paster et al., 1995).
Various investigators have reported that a number of microorganisms affected the production of aflatoxin in a competitive environment. A mixture of Lactobacillus species has been reported to reduce mould growth and inhibit aflatoxin production by A. flavus subsp. parasiticus (Gourama and Bullerman, 1995). Rhizopus oligosporus, a fungus used in the preparation of tempeh, was reported to inhibit the growth of A. flavus and A. parasiticus and also aflatoxin (Ko, 1978; Thanaboripat et al., 1996). Ganoderma is a medicinal fungus and has been treasured for this value in China for more than two thousand years (Liu, 1993). Ganoderma lucidum (Lingzhi mushroom) can be produced in large quantities by solid state fermentation and submerged fermentation. Effect of mycelial growth of Ling Zhi mushroom on the growth and aflatoxin of Aspergillus parasiticus was studied. When growing G. lucidum as mycelium on sorghum seeds for 3 days or more before inoculating spores of A. parasiticus, the results showed that aflatoxin production was inhibited (Thanaboripat et al., 2002).
Trichoderma species have been reported to inhibit fungal pathogen growth and development (Elad et al., 1983). The ability of these antagonists to attack fungal pathogens has led to the use of Trichoderma spp. as potential biocontrol agents. Possible antagonism by Trichoderma have been suggested to involve antibiotics and/or enzyme production, as well as parasitism (Elad et al., 1983; Benhamou and Chet, 1993). Trichoderma viride and T. harzianum have been reported to inhibit the growth of A. flavus and F. moniliforme (Calistru et al. 1997). However, mycoparasitism is not the mechanism involved in the inhibitory interaction of either A. flavus or F. moniliforme with Trichoderma spp.
It has been realized that mycotoxins are very important because the contamination of mycotoxins pose serious problems in public health, agricultural and economic aspects. Prevention is still the best method for preventing mycotoxin production. Thus, all efforts have to be made in order to prevent the mould growth and mycotoxin production along the entire food chain.
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Professor Dusanee Thanaboripat is currently Associate Professor in the Department of Applied Biology, King Mongkut’s Institute of Technology, Bangkok, Thailand and Visiting Professor of the Harkin Institute of Technology, China.
Professor Thanaboripat gained her PhD at the University of Strathclyde, UK and has published widely on mycotoxins.