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Addressing Mexico's aflatoxin challenge LAB STORY

In the small village of Huexotla, Mexico, staff at Grulin Tortilleria are cooking maize kernels at 95 degrees Celsius. They leave them to soak overnight in vats of calcium hydroxide solution before grinding them to make masa dough, the base ingredient of tortillas and numerous other foods.

Populations in Mexico and Central America have used this traditional maize processing method – known as nixtamalization – for centuries, and though heat treatments and soaking periods may vary between communities, the overall process remains largely unchanged.

In addition to easing kernel processing, enhancing the taste of the end product and increasing its shelf-life, a number of health benefits have been associated with the process. These include increased calcium content in the final product, increased bioavailability of iron and niacin, reduced risk of pellagra disease, and reduced presence of mycotoxins such as fumonisins and aflatoxins. Aflatoxins are poisonous food toxins produced by Aspergillus fungi. Overexposure to these toxins can have a severe impact on consumer health and is fatal at high doses.

From left to right: sediment floats to the top of a vat of calcium hydroxide; moist maize kernels are filtered down to a mill for grinding; tortillas are formed from processed dough.

"The increasingly high prevalence of aflatoxins is a growing problem in Mexico," says Natalia Palacios, maize nutritional quality specialist at the International Maize and Wheat Improvement Center (CIMMYT). Crops are particularly vulnerable to fungi growth in production areas exposed to high temperatures and humidity levels, both in the field and during post-harvest storage. With the country’s limit for safe human aflatoxin consumption set low at 20 micrograms per kilogram (20 ppb), even a small amount of grain contamination can significantly reduce farmers’ ability to sell their crop.

CIMMYT is placing more emphasis than before on aflatoxin research. The focus shifting from work on genetic resources such as aspergillus flavus-tolerant germplasm to attempting biological control of the fungi in the field, post-harvest management and maize processing alternatives. Through different approaches, researchers are trying to understand and control aflatoxin prevalence and its effect on maize kernels in both Mexico and the wider region.

Not all kernels are fit for testing. Aide Molina carefully selects grains for nixtamalization, before grinding them in a small mill in the lab.

Aide Molina has spent nearly six years working on aflatoxin analysis at CIMMYT’s Maize Quality Lab. She has recently started investigating the impact of nixtamalization on aflatoxins.

The process takes two days in the lab. After assessing hardness, color and size, Molina cooks batches of selected maize grain for up to 45 minutes in a solution of water and calcium hydroxide and leaves it to steep for roughly 16 hours. Later, she lightly washes the grain to remove the pericarp – the thin peel covering the kernel – before milling it to make masa dough or flour. Samples of these are freeze-dried for up to three days to avoid contamination.

“As you can see here,” she says, holding up a contaminated kernel, “the fungus is on the outside of the grain. So when we remove the pericarp during nixtamalization we’re actually removing a large concentration of the aflatoxin.”

Removal of the pericarp is thought to help reduce aflatoxin contamination levels in maize kernels by 30 to 60 percent when the load is not already highly contaminated, and Palacios’s team is assessing which small modifications to the process will give the best results. However, once the toxin is already inside the germ it cannot be completely removed through nixtamalization, though the grain may have a slightly reduced concentration of aflatoxin.

Molina tests the processed grain and flour using a number of different methods. The most basic, qualitative method is known as the "box method" and uses UV light to gauge contamination levels, as contaminated kernels emit a blue or green fluorescence under UV.

While most quantitative testing methods are expensive, the UV lamp is widely commercially available. CIMMYT is providing smallholder farmers and small processing industries in and around the city of Texcoco, outside Mexico City, with training on how to use this method to determine the presence or absence of aflatoxins in their grain..

“This is a fast test,” Molina explains. “We can see from the fluorescence that the sample is highly contaminated but we don’t know exactly what the concentration of aflatoxin is, so it’s usually necessary to carry out further chemical analysis in the lab.”

Green fluorescence under UV lighting indicates that this sample is highly contaminated with aflatoxin G1.

Molina uses a commercial kit for aflatoxin quantification. Often referred to as the "test strip" method, this single-step, lateral flow immunochromatic analysis gives a more accurate reading of aflatoxin contamination.

After combining processed grain with a reagent and water, she mixes an extract with a developer and places it on a small strip of paper. Each strip contains two faint lines. The first is a control and will always develop color. The second indicates aflatoxin contamination, and will show up as pink if the toxin concentration is low, or not at all if the concentration is very high. Once the extract has been absorbed, the strip is read and photographed by a machine that measures the exact quantity of aflatoxin present in the sample.

Molina demonstrates use of the "test strip" method.

Aflatoxin contamination is a problem across the world, with countries as diverse as China, Kenya all suffering heavy maize production losses as a result. While training farmers in grain drying and storage techniques reduces post-harvest losses, nixtamalization technology could also have the potential to prevent toxin contamination and significantly increase food safety when used appropriately.

Text and photos: Emma Orchardson/CIMMYT.

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