Moulds and mycotoxins
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The interactions described above, within granular ecosystems, will support the growth of a succession of micro-organisms as the nutrient availability and microenvironment changes with time. In the field, grains are predominantly contaminated by those moulds requiring high water activities (at least 0.88 aw) for growth, whereas stored grains will support moulds which grow at lower moisture levels. The rate of mould growth is also determined by the ability of the micro-organism to compete with other species. Some species, including those of Aspergillus, Penicillium and Fusarium, can occur both in the field and in storage.
Secondary metabolites are those compounds, produced by living organisms, which are not essential for growth. Some secondary metabolites produced by moulds are highly toxic to animals, humans and plants. These so-called 'mycotoxins' have been extensively studied since 1961, when a group of highly toxic Aspergillus flavus toxins - the aflatoxins - were isolated from a consignment of groundnut meal which had been imported into the UK (Coker, 1979).
Any activity which disturbs the stability of an ecosystem will increase the production of secondary metabolites, including mycotoxins. Such activities include the widespread use of fertilizers and pesticides, high yielding plant varieties and the cultivation of a limited number of plant species with restricted genetic variation. The normal practices of harvesting, drying and storage also, of course, significantly disturb the ecosystems of grains established before harvest.
The major mycotoxin-producing moulds include (Miller, 1991) certain Aspergillus, Fusarium and Penicillium species. Toxigenic (mycotoxin-producing) Aspergillus moulds can occur both before and after harvest, whereas Fusarium and Penicillium moulds occur predominantly before and after harvest respectively. In general, Aspergillus is associated with the tropics and Penicillium with temperate climates, whereas Fusarium moulds occur world wide. However, because of the complexity and variety of ecosystems supporting mould growth in grains, the nature and extent of the worldwide occurrence of moulds and mycotoxins cannot, as yet, be confidently defined. About 300 mycotoxins have been reported, produced by a wide variety of moulds. A few of the major moulds and mycotoxins are listed in Table 2.1 and discussed in the following sections of this Chapter.
Table 2.1. The major moulds and mycotoxins.
|Mould species||Mycotoxins produced|
|Aspergillus parasiticus||Aflatoxins B1, B2, G1, G2|
|A. flavus||Aflatoxins B1, B2|
|Fusarium sporotrichioides||T-2 toxin|
|Penicillium verrucosum||ochratoxin A|
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