Control of Aflatoxin Contamination of Corn

Mr. Werawat Nilrattanakhun
Nakhonsawan Field Crop Research Center,
Takfah District, Nakhonsawan Province, 60190
Thailand
Tel: 665-624-1019 Fax: 665-624-1498, 2003-05-01

ContaminatiON with aflatoxin is a major problem in maize production in Thailand. Several studies have shown that maize often becomes contaminated with aflatoxin after harvest, and that the level of contamination increases during handling, especially by local merchants (Fig. 1). A huge amount of wet maize is harvested and distributed in Thailand during the rainy season. The control of aflatoxin contamination was developed for two kinds of maize storage: that of farmers and that of local merchants.

Storage of Maize by Farmers

With regard to storage by farmers, research has focussed on a suitable grain moisture content, and the best time for harvesting and storing corn, since it is impractical for Thai farmers to dry their corn after harvest. The recommendation is to harvest the corn cobs after they are fully mature, by leaving them for at least one week after all the leaves in the field have turned yellow (hybrid corn variety). At this stage, the moisture content is usually less than 23%, which is safe for cob storage.

If it is necessary to harvest the corn early, it should be harvested soon after all the leaves turn yellow. At this stage, the moisture content is usually less than 25%, which is considered the critical level for storing the cobs. If farmers are forced to harvest corn with a moisture content of more than 25%, the corn should be stored no longer than 15 days.

At local merchants, if grain with a high moisture content cannot be dried to a safe level, carbon dioxide (CO2) fumigation should be used to give temporary control of fungi and aflatoxins. Initially, the grain should be sealed inside a plastic bin and fumigated with CO2 at a rate of 0.5 kg/mt. If the air of the bin has first been removed with a vacuum blower, it can then be fumigated with CO2 at a rate of 0.3kg/mt (Fig. 2). These methods can preserve high-moisture corn for at least 10 days without any visible growth of mold. The temperature of the bulk grain remains unchanged. However, the anaerobic fermentation produces a sour smell. This smell should disappear when the maize is dried in the sun.

Fumigation of Wet Maize

Wet maize should be fumigated within two days after it is shelled. The moisture content of the grain should not exceed 30%.
Pile up the grain on a concrete floor. The volume of the pile should depend on the size of the plastic sheet.
Place a CO2 gas cylinder on a weight balance (to show how much gas is being used), next to the pile of grain.
Cover the pile with a plastic sheet. Anchor the sheet to the concrete floor by folded jute bags or sand snakes (i.e. rolls of cloth filled with sand) (Fig. 3).
Connect a flexible tube to the cylinder and insert the other end inside the sealed pile of grain. It should be inserted at the bottom of the pile, to a depth of 10 - 15 cm from the surface of the grain.
Two methods can be used for fumigation:
Either discharge the CO2 into the sealed pile at a rate of 0.5 kg/mt. (Fig. 4), or:
Use a vacuum machine to remove the air from the sealed pile before fumigation with CO2, at a rate of 0.3 kg/ton.

Removing Air from the Pile of Grain

To remove the air, connect a flexible outlet tube to the vacuum machine (Fig. 5). Cover the other end of the tube with a piece of plastic net, and place it at bottom of the sealed pile (Fig. 6). Vacuum the air out of the sealed pile until the plastic sheet is fully stretched, and the vacuum pressure inside the pile has reached its maximum. Then continue sucking the air out for another 20 minutes (Fig. 7). Withdraw the tube carefully from the sealed pile, not allowing the air to enter. Then, discharge the CO2 at a rate of 0.3 kg/ton (Fig. 8). Check the amount of discharged gas by checking the weight loss from the weight balance.

When fumigation is finished, withdraw the tube from the fumigated pile, taking care not to allow any air to enter to the bulk. The fumigated pile can be stored for at least 10 days.

Drying the Stored Grain

The grain can be dried to the required moisture content when drying facilities become available. Note that when fumigation is carried out of wet maize with a moisture content higher than 30%, a visible growth of yeast occurs on the surface of the fumigated grain during storage.

During storage, the higher moisture content of the fumigated grain may cause discoloration. The kernel of the grain may appear pale yellow, with a brown color at the tip. This discoloration is very obvious if the grain is compared to maize which has a lower moisture content.