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.