Released: August 28, 2003

Black Light Test for Aflatoxin Is Questionable Process

MANHATTAN, Kan. – Hot, dry weather has once again provided favorable conditions for an ear mold of corn known as Aspergillus flavus, but a Kansas State University plant pathologist says the black light test often done to find it is not reliable and should not be the determining factor in detecting it.

The Aspergillus fungus is an olive or yellowish-green mold that grows on the kernels of corn ears. It’s usually found at or near points of insect feeding. Colonies can vary in size, but typically range from dime- to quarter-sized, said Doug Jardine, state plant pathology leader for K-State Research and Extension.

While Aspergillus can be responsible for yield loss, its greatest threat is that it also produces the potent toxin known as aflatoxin. Aflatoxin has been identified as a carcinogen, and acceptable levels in corn and other agricultural products, such as peanuts and cotton seed, are highly regulated.

Farmers’ exposure to the potential problem usually occurs first at local elevators where the so-called “black light” test is sometimes performed. In this test, cracked corn kernels are passed under a short-wave ultraviolet light. The basis for the test is that Aspergillus can produce flourescent by-products. “The accuracy of the black light test has long been called into question,” Jardine said. “In an Iowa study, less than half of the samples fluorescing under a black light actually had detectable levels of aflatoxin present. More importantly, in a study conducted in Brazil, of 296 samples that showed no fluorescence under the black light, 96 were confirmed to be contaminated with aflatoxin, based on more sophisticated tests.”

This can be bad news for both farmers and elevator operators, the scientist said. For farmers, corn that is aflatoxin-free may be rejected or heavily discounted. For elevators, operators may be accepting corn that potentially could have levels of aflatoxin that may limit its use and thus their ability to resell the commodity.

The problem with the black light test, Jardine said, is that the fluorescence produced by Aspergillus is actually caused by a secondary by-product known as kojic acid, not the aflatoxin itself. Kojic acid is a chemical used in the cosmetics industry.

Because the two products are produced by separate pathways in the fungus, there can be high levels of kojic acid and little or no aflatoxin. In this case, the corn will fluoresce and be rejected when it actually is safe.

In the reverse scenario, there can be little or no kojic acid, but high levels of aflatoxin. In this case, the grain will pass inspection, but could lead to feeding problems with livestock later on.

The only sure way to know whether aflatoxin is present in a corn lot is to have a sample analyzed, using a quantitative test kit, he said. Most elevators are not able to do this test on-site. In Kansas, most samples are analyzed by the Kansas Grain Inspection Service, Inc., located in Topeka.

“If the safety of a corn lot is in question, a quantitative test should be performed,” Jardine said.

Growers can help themselves, he added, by scouting corn fields for the presence of Aspergillus before harvest. If the mold is absent from the field, the corn can be safely harvested and used. If the fungus is found, it should be tested immediately following harvest to determine the level of toxin present – which will dictate its potential uses.

Dairy farmers, in particular, need to be concerned, Jardine said. Aflatoxin is not allowed in milk. Its presence can cause milk to be dumped and the herd quarantined for a period of time until its milk is deemed safe for use.

More information on aflatoxin is available at every local K-State Research and Extension office in publication MF-2061, “Mycotoxins in Feed Grains and Ingredients.” That resource also is posted on the Web (http://www.oznet.ksu.edu/agronomy-block2/aflatoxin.pdf).

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K-State Research and Extension is a short name for the Kansas State University Agricultural Experiment Station and Cooperative Extension Service, a program designed to generate and distribute useful knowledge for the well-being of Kansans. Supported by county, state, federal and private funds, the program has county Extension offices, experiment fields, area Extension offices and regional research centers statewide. Its headquarters is on the K-State campus, Manhattan.

Story by:
Mary Lou Peter, Interim News Coordinator
mlpeter@oznet.ksu.edu
K-State Research& Extension News