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Aflatoxin contamination of groundnuts: What can be done?

The fungi (Aspergillus flavus and A. parasiticus), responsible for the production of aflatoxin are soil-borne and therefore practically difficult to control. With the growing awareness of consumers to safety and quality of foodstuffs, the aflatoxin content of groundnuts consignments is becoming a serious issue for importers as well as exporters for first choice groundnuts worldwide.

Legislation is in place regarding aflatoxin levels for import/export and local groundnuts, but what can local producers, processor, and the marketing companies do in order to minimize aflatoxin contamination of groundnuts?

Before the harvest
The basic rules are as follows: when the groundnuts are in the ground, keep them wet. When they are out of the ground, keep them dry. Aflatoxin contamination usually takes place in the field long before it becomes apparent in the laboratory tests. As mentioned above, the fungi that cause infestation survive in the soil. These fungi produce millions of spores (reproductive propagules) that can spread by wind, water or agricultural practice. The fungi are very weak pathogens and do not kill the groundnut plant but simply colonise the groundnut kernels below the soil surface.

Aflatoxin is formed as a by-product resulting from metabolism (fungal growth) and is stimulated by conditions of heat and drought-stress late in the season. In the year when there has been a late-season drought with high daily temperatures, risk of aflatoxin contamination is increased significantly. Dryland groundnuts therefore are a greater risk for aflatoxin contamination than irrigated groundnuts. If it is possible dryland groundnuts should be harvested before conditions of drought and heat stress sets in. Irrigated groundnut should never be allowed to go under stress, especially in the late season.

Since the fungi are soil-borne, chemical control is not practical. Deep ploughing is, however, effective in reducing the inoculum in the soil. The fungi are concentrated in the top 5cm of the soil and deep ploughing is effective in burying fungal inoculum to certain extent, reducing the chance of contamination late in the season. The new groundnut varieties, Akwa and Kwarts, are more resistant than Sellie. Even though the resistance is not significant, the new varieties should preferable be planted ahead of Sellie when possible.

After the harvest
Sorted groundnuts are of lower aflatoxin risk than unsorted groundnuts. Also, hand-sorted groundnuts are of a lower aflatoxin risk than machine-sorted ones. In the process of sorting, visually contaminated kernels are removed from the seedlot and the level of contamination of the consignment is reduced significantly.

However, a visually sound kernel may contain high levels of aflatoxin, indicating that neither machine nor hand-sorting is 100% effective. It has also been shown that the aflatoxin fraction is highest in the smaller kernels (those which were immature at harvest). Sized groundnuts are therefore also a lower risk.

Harvested groundnuts may also be contaminated once they have been harvested, depending on the handling procedures. Harvested groundnuts should never be allowed to get wet once they have been harvested. Conditions of heat and moisture in post-harvest phase of production will enhance fungal growth and cause uninfected kernels to become contaminated. It may happen that the stacks are improperly covered and become wet from rain, or that condensation of moisture on the cold roof of a store may cause water to drip back onto the stack of groundnuts below. Such situations should be avoided at all costs. Groundnuts should also be stored at the correct moisture content (7%) to minimize the risk of fungal growth on kernels.

Groundnuts are a high-return cash crop, which is not produced without a fair amount of problems. However, if the correct procedures are followed, a high-quality groundnut may be produced which will fetch excellent prices locally and on the export market.
Dr Andre Cilliers, ARC-Grain Crop Institute - FARMER'S WEEKLY, March 2002

Published by the Directorate: Food Control


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