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Since the occurrence of mycotoxins is a consequence of biodeterioration, it follows that the mycotoxin problem is best addressed by controlling those agents - temperature, moisture and pests - which encourage spoilage.
The pre-harvest control of the agents of biodeterioration is somewhat compromised by Man's inability to control the climate! Both insufficient and excessive rainfall during critical phases of crop development can, for example, lead to mould contamination and mycotoxin production. The very substantial economic losses attributed to mycotoxins, on the North American continent, clearly illustrates the difficulties associated with the prevention of contamination, even in wealthy, developed nations.
Considerable effort has been expended on the development of crop strains which are resistant to mould growth and/or mycotoxin production. Breeding programmes have focused, for example, on the development of Aspergillus/aflatoxin resistant varieties of maize and groundnuts, with limited success. It has been suggested that wheat has three types of resistance to Fusarium graminearum; resistance to the initial infection, resistance to the spread of the infection and resistance to mycotoxin (deoxynivalenol) production. Attempts to exploit the resistance to mycotoxin production (through either the inhibition of synthesis or chemical degradation) may hold the most potential because of the limited number of genes which control this process.
The post-harvest handling of grains does, however, present many more opportunities for controlling mycotoxin production. Although many small farmers will not have access to artificial drying equipment, the importance of the utilisation of effective drying, and storage regimes cannot be overemphasised, and is covered extensively in later chapters. Drying to moisture levels which will ensure safe storage in tropical climates is especially important when grains are shipped from temperate to tropical climates.
However, despite the best efforts of the agricultural community, mycotoxins will continue to be present in a wide range of foods and feeds. Consequently, strategies are required for the removal of mycotoxins from grains. Currently, two approaches are utilised; namely, the identification and segregation of contaminated material and, secondly, the destruction (detoxification) of the mycotoxin(s).
The Segregation of Contaminated Grains
In the first instance, the identification and segregation of contaminated consigments is pursued through the implementation of quality control procedures by exporters, importers, processors and regulators. The consignment is accepted or rejected on the basis of the analysis of representative samples of the food or feed. Acceptable levels of mycotoxin contamination are specified by individual customers, commercial agreements and regulators. Currently, over 50 countries now regulate against the aflatoxins; 5 parts per billion (µg/kg) is the most common maximum acceptable level. Aflatoxin M1 in dairy products is regulated in at least 14 countries, the tolerances for infant diets being 0.05-0.Sppb milk. Regulations exist for other mycotoxins including, for example, zearalenone (1mg/kg in grains; the former USSR), T-2 toxin (0.1mg/kg in grains; the former USSR) and ochratoxin A (150ppb food, 100-1000ppb feed; numerous countries). Guidelines, advisory levels and 'official tolerance levels' for deoxynivalenol also exist in some countries. The guideline in Canada, for example, refers to 2mg/kg in uncleaned soft wheat, 1mg/kg in infant foods and 1.2mg/kg in uncleaned staple foods calculated on the basis of flour or bran. In the USA, 4mg/kg is advised for wheat and wheat products used as animal feeds.
The mycotoxin content of grains can be further reduced during processing. Automatic colour sorting, often in combination with manual sorting, is widely used to segregate kernels of abnormal appearance (which are considered more likely to contain aflatoxin) during the processing of edible grade groundnuts. Mycotoxins can also be concentrated in various fractions produced during the milling process. Zearalenone and deoxynivalenol, for example, are reportedly concentrated in the bran fraction during the milling of cereals. It can be argued, however, that all fractions will contain mycotoxins if the original grain is heavily contaminated. Ochratoxin A appears to be reasonably stable to most food processes. In general, the stability of mycotoxins during processing will depend upon a number of factors including grain type, level of contamination, moisture content, temperature and other processing agents.
A further segregation process involves the removal of aflatoxin, from animal feeds, after ingestion. Here, mycotoxin binding agents - hydrated sodium calcium aluminosilicate, zeolite, bentonite, kaolin, spent canola oil bleaching clays - included in the diet formulation, reportedly remove aflatoxin, by adsorption from the gut.
The Detoxification of Mycotoxins
Ammonia, as both an anhydrous vapour and an aqueous solution, is the detoxification reagent which has attracted (Park et al, 1988) the widest interest and which has been exploited commercially, by the feed industry, for the destruction of aflatoxin. Commercial ammonia detoxification (ammoniation) facilities exist in the USA, Senegal, France and the UK, primarily for the treatment of groundnut cake and meal. In the USA, cottonseed products are treated in Arizona and California whilst maize is ammoniated in Georgia, Alabama and North Carolina. Commercial ammoniation involves the treatment of the feed, with ammonia, at elevated temperatures and pressures over a period of approximately 30 minutes. Onfarm procedures, as practiced with cottonseed in Arizona, involve spraying with aqueous ammonia followed by storage at ambient temperature, for approximately two weeks, in large silage bags.
The nature of the reaction products of the ammoniation of aflatoxin is still poorly understood. However, many studies have been performed, on both isolated ammoniation reaction products and treated feedingstuffs, in an attempt to define the toxicological implications of ammoniation. Very extensive feeding trials have been performed with a variety of animals including trout, rats, poultry, pigs and beef and dairy cattle. The effect of diets containing ammoniated feed has been determined by monitoring animal growth and organ weights together with haematological, histopathological and biochemical parameters. The results of these studies, combined with the practical experience of commercial detoxification processes, strongly indicate that the ammonia detoxification of aflatoxin is a safe process. However, the formal approval of the ammoniation process by the USA Food and Drug Administration is still awaited.
Commercial processes have not been developed for the detoxification mycotoxins.