Introduction

1.1 What are aflatoxins?

Many agricultural commodities are susceptible to several diseases caused by fungi, viruses and insect pests. These diseases may also contaminate the crops with deadly toxins. Fungi are of particular concern. Fungi are free-living organisms capable of surviving in the environment (soil, air and water) and can easily find their way into crop products especially when the weather conditions are suitable. Many of these fungi are free-living organisms and can survive in the environment even without the crops we grow. The toxins produced by fungi are called mycotoxins (myco=fungus; toxin=poison). Approximately 25% of the world’s agricultural commodities are contaminated by aflatoxin and other mycotoxins, resulting in nearly one billion tons of food loss every year1. Although there are multiple types of mycotoxins, aflatoxins are of particular concern because of how their high toxicity affects human health and the significant economic losses associated with contaminated staple crops.

1.2 Effects of aflatoxins

Food products that are commonly contaminated by aflatoxins include cereals (maize, sorghum, pearl millet, rice and wheat), oilseeds (groundnut, soybean and sunflower), spices (chillies, black pepper, coriander, turmeric and ginger) and milk from cattle that have eaten contaminated feeds. Aflatoxin cannot be destroyed by cooking. Aflatoxin contamination of food products have two major effects. Firstly, eating contaminated food has several harmful health effects and secondly, aflatoxin contamination affects the quality of the grain which leads to reduced markets for contaminated grains.

Health effects: Eating food contaminated with aflatoxins leads to aflatoxin poisoning also called aflatoxicosis. Regular consumption of low dosages for a long time (Chronic exposure) leads to liver damage, immune suppression, malnutrition and stunted growth in children, while sudden high-level ingestion of the toxin (Acute exposure) can lead to death.

Income losses: Groundnut, maize and other crop products are important as a source of income when sold locally or exported. To protect humans from the harmful effects of mycotoxins, especially aflatoxin, the European Commission has proposed maximum allowable limits for aflatoxin in certain foods. The permissible limit of aflatoxin in foods for human consumption is 4-30 ppb, depending on the country involved (FDA 2004; Henry et al. 1999). In the United States, 20 ppb is the maximum aflatoxin residue limit allowed in food for human consumption (FAO 2003). Due to aflatoxin contamination the food products cannot be exported leading to financial losses. For example Malawi lost as much as 42% of its groundnut exports in 2005 (Diaz-Rios and Jaffe 2008).

Aflatoxin Management in Groundnut (Oilseeds)

2.1 How crops get contaminated

Crops are particularly susceptible to infection by Aspergillus following prolonged exposure to a high-humidity environment or damage from stressful conditions such as drought, a condition that lowers the barrier to entry (Harris et al. 1976). The fungi can enter (colonize and contaminate) at any point of the crop value chain stage. The entry or infection points for these aflatoxin-producing fungi could be at any/all of these three stages:

•    Pre-harvest entry of fungus (during plant growth).

•    Entry of fungus during harvest.

•    Postharvest entry of fungus (after harvest and during processing).

2.1.1 Preharvest contamination

Fungus growth and aflatoxin contamination that occurs in the field during crop growth is called pre-harvest contamination. Preharvest infection is usually influenced by farming practices that make crops susceptible to infection, including the following:

•    Repeated cultivation of host plants: Repeated cultivation of the same crop or susceptible crops species on the same piece of land supports rapid build-up of A.flavus populations leading eventually to pre-harvest contamination of crops in field.

•    Late planting: Late planted crops will usually be affected by end-of-season drought as well as insect pest attacks, especially termites. Such insect-damaged pods create easy entry points for the fungus.

•    Drought: Under drought stress the groundnut pods crack and facilitate entry and growth of Aspergillus.

•    Termite attack and poor field hygiene: Using fields with a history of termite infestation provides an environment that supports pod damage by termites and eventual easy entry by the fungus. Unwedded fields also support termite and pest damage of developing pods, increasing chances of Aspergillus infection.

2.1.2. Contamination during harvest

The manner in which a crop is handled during harvesting will determine the extent to which it is predisposed to infection. Some of the predisposing conditions that favour fungal infection during harvesting include:

•    Poor harvesting techniques: Groundnuts are often harvested using hand hoes that can easily damage the nuts, creating easy entry points for the fungus. Groundnuts and Bambara nuts can get infected with Aspergillus from the soil if they are harvested with soil adhering to the pods. Crops like maize, sorghum, millet and sunflower, usually harvested and dried on bare ground, May also easily get attacked by fungi present on the ground.

•    Premature harvesting: Immature crops have high moisture content, a condition that favors fungal infestation. Harvesting immature nuts increases opportunity for infection by fungi.

2.1.3. Postharvest contamination

The predisposing factors to infection after harvesting the crop include:

•    Improper drying: Drying on roofs or on the floor exposes the grains to moisture that leads to growth of fungus.

•    Improper shelling: Practices like sprinkling water on pods to soften the shells to make shelling easy and also to improve the weight of the nuts to get more market value lead to Aspergillus flavus infection and aflatoxin contamination. Another bad practice is threshing of groundnuts in a sack.

•    Poor curing techniques: Overdrying nuts results in the cracking of the pod and seed coat, thus exposing the nuts to infection.

•    Poor stripping: Stripping groundnuts together with soil carries the fungus into storage and that provides a conducive environment for fungal infection and aflatoxin contamination.

•    Sorting: Poor grading especially the wounded nuts before storage is a source of contamination. Wounded, broken, shrivelled and cracked kernels need to be separated from healthy kernels before storage.

•    Poor storage conditions: Storing groundnuts with high moisture content and poor storage (at household level, market/shops) that expose grains to winter rains, high humidity during the night and poor air circulation that enhances high temperatures leads to fungus growth.

•    Use of airtight containers: Using non-porous nylon bags and other airtight storage materials decreases insect pest attack and subsequently infection by fungus.

•    Poor transport: Transporting the grains in vehicles with open roofs can expose the grain to sudden rain and moisture which leads to fungus growth.

How to reduce aflatoxin contamination

Fungal growth and aflatoxin contamination in crops occur due to several factors: some are environmental in nature while others are crop management related. Farmers have minimal control on environmental factors; however, they can improve agricultural practices used in crop production to reduce fungal infection, growth and aflatoxin production.

2.2.1. Crop management practices that reduce infection in the field

These approaches target minimization of pre-harvest infection by fungi. They aim at providing crops with the best possible growth environment so as to avoid infestation by insects, drought and subsequently infection by fungi.

•    Early planting: Early planting helps plants to escape end of season drought that in general predisposes pods to cracking and entry by A. flavus.

•    Maintaining field hygiene: Timely weeding helps to retain soil moisture needed for proper plant growth and avoidance of dry conditions that predisposes developing pods to cracking. Termite control is also very important to prevent damage to developing pods, especially as the crop matures.

•    Harvesting of water in the field: Drought conditions while the crop is growing in the field is a prerequisite to fungal contamination and subsequent aflatoxin contamination. Farmers should ensure that they retain moisture in their fields. The use of tied ridges (box ridges) can improve water penetration into the soils, thereby reducing exposure of the developing crop to A. flavus infestation. Box ridges should be put in place early in the cropping season to capture enough rainwater and reduce effects of end-of-season drought. Mulching also helps to retain water in the field.

•    Soil amendments: Application of lime to the crop supports development of strong shells (pod resistance). Strong shells provide the first line of defence against pest and fungal attack.

2.2.2. Crop management practices that reduce infection during harvesting

These are management practices that reduce contamination of pods and grain by reducing and/or avoiding exposure of grain to fungi.

•    Harvesting at the right stage: Premature kernels have high moisture content and support fungal infection and growth, and aflatoxin contamination. Therefore, harvesting at the right time when the crop is mature will minimize the exposure of the crop to extreme heat, sudden rain or drought, which also influence infection.

•    Avoiding injuries to pods: Care should be taken when using hand hoes to avoid injury to the pods. Proper harvesting of groundnuts ensures that the hoe is focused at the plant’s rooting zone to avoid damaging pods or kernels.

     •    Removal of soil: It is important to remove all the soil attached to the pods during harvesting to avoid carrying the fungus into stores and processing facilities.

Postharvest handling of crops is another major point of contamination that if well managed will minimize infection of harvests by fungus and minimize aflatoxin contamination. Management practices may be implemented at two levels.

At the household level

•    Proper drying: Drying on roof or on soil leads to the absorption of moisture and supports fungal growth that leads to aflatoxin contamination. The use of Mandela Corks (ventilated stacking) is one of the best methods for curing the groundnuts and is specifically preferred for its ability to minimize the direct exposure of groundnuts to the sun. Mandela Corks should be stacked on a raised platform, leaving a hollow space in the middle to allow air circulation inside the structure.

•    Proper shelling: Sprinkling water on pods to shell the groundnut leads to fungal accumulation and aflatoxin production. It is therefore always better to avoid wetting pods during shelling. The use of mechanical shellers can also reduce labour cost.

•    Grading and sorting: Fungi readily invade kernels with cracked or damaged pods or seed coats. Removal of such damaged, discoloured, small and shrivelled pods reduces the amounts of infected produce in the lot.

•    Proper storage: Entry of insects and moisture into storage lots allows the entry of fungus and eventually aflatoxin contamination. Grains should be stored in a dry and secure place where the entry of insects and moisture is not possible.

Tips to minimize storage contamination

• Store grains in cool dry conditions.4
• Stack gunny bags on wooden planks to avoid dampness.
• Ensure the storage has good roofing to prevent groundnuts from getting wet if it rains. Ensure the storage rooms are hygienic to avoid the contamination from the previous lot.
• Using good packaging materials for storage, gunny bags are most appropriate for groundnuts. If nylon bags are used additional perforations need to be made to facilitate air circulation or store in them only for a short time.
• Ensure groundnuts are not exposed to pest and rodents to minimize damage.
• Ensure good aeration in the storage room.