News about aflatoxin contamination in foods is on the rise. Already peanuts and maize products have been banned due unacceptable levels of aflatoxin. Aflatoxins are an odourless, colourless, flavourless toxin produced by the fungi strain Aspergillus flavus and are highly toxic to humans and animals. On maize, the aflatoxin-causing fungi is characterised by greenish yellow to yellowish brown, powdery mold growth on or between the maize kernels.
Initially it was believed that the growth of aflatoxin-causing fungi occurs on maize only in storage. But it has since been established that the infection can start from the field.
This fungus invades the maize through the corn silks. The damage could be due to insect damage or weather conditions such as hail that cracks the kernels.
Temperatures ranging from 26-35 degrees centigrade and relative humidity of 85 per cent are ideal conditions for the fungi. Periods of drought and heat stress during the growing season, especially during pollination and as kernels mature favour the growth of the fungi.
Several strategies can be implemented at different stages of crop growth during the growing season to prevent or minimise the risk of aflatoxin contamination. Select planting dates appropriate to your area. Plant at the right time to enable the crop disease escape, and have enough rain for growth and maturity towards the end of the season.
Do not plant too early to avoid crops maturing during the rains. Use fertilisers to supplement the nutrients. Crops grown under stress are more susceptible to infestation by the aflatoxin-producing fungi that cause the contamination. Applying fertilisers and other key inputs reduces crop stress.
Follow recommended management practices to minimise damage by insects. Control insects particularly stem borer, during crop growth. Insects create wounds on the crop that lead to invasion by fungi. Sometimes insect attack may completely damage the crop.
Timely harvesting of crops limits aflatoxin contamination. Harvest immediately after the crop is mature. Pluck the cobs off the plant and dry to less than 13 per cent moisture content without delay. Remove cobs damaged by birds or rodents. Damaged crops may have been infected by aflatoxin-producing fungi and may contain high aflatoxin concentrations.
During harvest, exposure to aflatoxin can happen because of certain practices. One such practice is harvesting during the rains which encourage the growth of the fungi. Because the aflatoxin-causing fungi produce aflatoxins when kernel moisture goes below 28 per cent, time of harvesting should be planned accordingly, while taking into account the growers needs to limit drying costs. Kernel moisture below 14 per cent during storage and moderate temperature and dry environment should be maintained to limit contamination. Metal silos and hermetic bags are recommended to protect against losses and reduction of aflatoxin contamination.
Sanitation is key in preventing post-harvest aflatoxin contamination. Clean out all harvesting, handling and drying equipment and storage bins prior to harvest. Remove all broken maize, dust and foreign material that can be source of contamination. Check and repair storage bins prior to prevent moisture leaks from faulty joints or other problems.
Use a moisture metre to determine the wetness of your maize. Alternatively, farmers can test for properly dried maize by cracking kernels between the teeth. If it shatters, then kernels are dry. If it is sticky then kernels are not dry.
Farmers can also throw maize up and down their palms or inside a bottle. Dry maize will produce a rattling noise, compared to maize that is not well dried. Place the grains in dry and well aerated storehouses.
The use of wooden pallets, and away from the walls, rather than placing bags in direct contact with floor surfaces and walls limits aflatoxin accumulation. Winnowing and grain sorting of grains to remove low density materials and grains that tend to harbour high proportions of the contaminated materials.
[The writer is an expert on sustainable agriculture and agricultural solutions]