Testing for Natural Aflatoxin
In the United States, corn with more than 20 parts per billion (ppb) of
aflatoxin--which is the equivalent of just 1 ounce in 3,125 tons--is not
considered fit for feeding to animals that produce meat or milk for humans.
A known carcinogen, aflatoxin is the metabolic byproduct of Aspergillus
flavus fungi. Grain with more than 5 ppb gets thumbs down for making
food-grade corn products. And in the South and in areas where occasional
drought stresses corn and increases A. flavus levels, farmers may lose
opportunities to produce corn valued for export markets.
Finding natural compounds in corn that affect the toxin-producing machinery
of A. flavus is a first step toward identifying corn genes that might be
modified to make the microbe less harmful. The strategy could be joined with
efforts to breed corn that discourages growth of the fungus.
Now, a faster, cheaper test is helping researchers detect genetically
regulated compounds in corn that inhibit or promote the ability of A.
flavus fungi to produce aflatoxin. Agricultural Research Service chemist
Robert A. Norton developed the new procedure at the National Center for
Agricultural Utilization Research in Peoria, Illinois.
"We can now realistically test a much wider range of compounds for
toxin-producing activity--including lipids--using 1 milligram [thousandth of a
gram] or less of the test compound," he says.
Norton purchases the compounds for testing, some of which cost up to
hundreds of dollars per milligram, though most cost less. Despite the expense,
Norton says that it's cheaper to buy the compounds than to tediously extract
them from corn.
"And with the new testing method, we don't have to use as much of
them," he says.
His procedure involves placing the test compound, along with about 29
microliters [millionths of a liter] of a nutrient medium and A. flavus
spores, on a small disk. The disk is hung by a pin from a Teflon cap inside
a bottle containing a small amount of water. After 5 days, researchers measure
fungal growth on the disk. They use a small amount of solvent to extract
aflatoxin from the fungus; high-performance liquid chromatography measures the
amount. The method saves time, nutrient medium, and solvent.
Norton currently tests up to 200 samples per week. So far, he has pinpointed
several aflatoxin-synthesis inhibitors, including carotenoids that impart
yellow color to modern corn hybrids and a colorless benzoxazolinone compound.
He also plans to test colorless anthocyanin-related compounds that could be
bred into yellow corn.--By Ben
Hardin, Agricultural Research Service Information Staff.
Robert A. Norton is in the
USDA-ARS Bioactive Agents
Research Unit, National Center for Agricultural Utilization Research, 1815
N. University St., Peoria, IL 61604; phone (309) 681-6251, fax (309) 681-6693.
"Testing for Natural Aflatoxin Inhibitors" was published in
the July 1998 issue of Agricultural Research magazine. Click here to see this
issue's table of contents.