Nuts and seeds may be subject to mould growth during storage if the conditions are inappropriate. Certain moulds produce secondary metabolites which are toxic to humans and animals, known as the mycotoxins. Of these mycotoxins, the aflatoxins, notably aflatoxin B1, are produced by three closely related species of mould: Aspergillus flavus, A. parasiticus, and A. nomius. These moulds may contaminate various food commodities in tropical and subtropical regions, including tree nuts, but one of the most important crops to be affected is the peanut. Aflatoxins are acutely toxic to the liver and may also be involved in the etiology of human liver cancer in certain parts of the world. Ochratoxins, which are produced by other Aspergillus species, have also been found to contaminate nuts.

Some species of mould are able to proliferate within growing crops even before they are harvested, forming an endophytic relationship with the plant. This relationship has been found to exist between Aspergillus parasiticus and peanuts. It appears that when the plant is growing normally, no aflatoxin is produced by the mould, but when the plant is stressed, as occurs in drought conditions, then the mycotoxin may be produced. The concentrations of aflatoxins produced in this way are lower than would ensue from poor postharvest storage, but the economic consequences still may be considerable.

There are regulatory limits for the aflatoxin levels in foods. In the UK, the sale of nuts for direct consumption is prohibited if the aflatoxin content exceeds 4 mgkg-1 or 10 mgkg-1 for nuts which are to be subjected to further processing before being sold. A proportion of nuts imported into the UK, especially peanuts, are contaminated with aflatoxin. In 1994, 3% of samples examined under a European surveillance program were found to exceed the UK limit. Nonetheless, such findings should be kept in perspective: The numbers are low and their significance in public health terms, relative to other diet-related risks, is small.

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