Fungal contamination and subsequent production of aflatoxin can occur in crops in the field, at harvest, during postharvest operations and in storage. The rate and degree of contamination are dependent on temperature, humidity and soil and storage conditions. Prevention, particularly by excluding or reducing toxigenic mould growth and toxin production in susceptible food crops, is the most effective way to restrict mycotoxin contamination. In practice, this can be accomplished by reducing fungal infections in growing crops through rapid drying and correct storage of the harvested crops, the use of effective anti-mould preservatives and adherence to proper postharvest processing, transport and distribution practices.

Many mycotoxins, including aflatoxins, can form during the growing stages of certain crops. Climate, sources of fungal inoculum (or suitability of the fungal substrate), potential insect vectors and the plant response (or the plant susceptibility to fungal infestation) can interact to result in specific mycotoxin occurrence. Damage caused by insects can expose susceptible tissues to colonization by toxigenic fungi with subsequent mycotoxin formation.

Following harvest and during shipment and storage of agricultural commodities, toxigenic mould growth and potential mycotoxin production are influenced by many factors including moisture level, temperature, aeration, infestation by insects and other microorganisms, storage time, chemical treatments, spore infection density and storage conditions (especially leakage of water or condensation).

By far the most critical environmental factors determining whether a substrate will support mould growth are moisture content, temperature and time. Thus, drying, proper storage and suitable transportation are of prime importance in prevention.

The prevention of mycotoxin contamination in storage is largely a matter of strict moisture control of the crop. There must be no insect activity, as insects can create favourable microclimates for toxigenic fungal growth; no moisture migration; no condensation or water leaks; and no rodent activity, as the moisture level could be increased by urination. In summary, conditions which restrict fungal growth will almost invariably limit or exclude mycotoxin production.

Where harvesting occurs in dry weather, mycotoxin contamination does not usually reach alarming proportions. It becomes a problem where harvesting is done in very humid weather. In many developing countries, the combination of insufficient drying equipment coupled with humid atmospheric conditions results in unacceptable levels of aflatoxin in harvested groundnuts, tree nuts and other foods.

ref. http://www.fao.org/

1. Nuts are sorted, using visual sorting techniques, to eliminate those with defects.
2. Empty or partially empty pistachios are separated by an air stream, and unsplit pistachios by floatation in water.
3. Shells are cracked for extraction of kernels when desired.
4. Nuts are sorted to eliminate those with possible aflatoxin contamination.