New Test Speeds Search for Aflatoxin Biocontrol

Scientists can now quickly identify whether a natural yeast strain has the potential to control aflatoxin on tree nuts, thanks to a new lab test in which red means no and white means yes.

Aflatoxin, produced by certain fungi, can be a threat to food and feed safety. The scientists, with USDA’s Agricultural Research Service in California, are seeking to identify natural, harmless yeasts with the best potential for suppressing the fungi.

So far, they have screened dozens of yeasts with the new test. They presented their results on May 7 at the American Society for Microbiology meeting in Miami, Fla.

The toxin-producing fungi--Aspergillus flavus and A. parasiticus--can infect tree nuts such as almonds, walnuts and pistachios along with peanuts, corn and cottonseed. Aflatoxins can cause disease if consumed in large quantities. To safeguard people and animals, government agencies monitor and strictly limit aflatoxin levels in feed and food products.

Last year, the scientists first discovered that some yeasts can reduce Aspergillus populations and toxin production on nuts. But finding the most effective yeasts among hundreds of natural strains has been a challenge requiring costly, time-consuming chemical analyses.

ARS plant physiologist Sui-Sheng Hua (schwi-shing hwa) and colleagues developed the new test at ARS’ Western Regional Research Center, Albany, Calif. With this test, scientists need only see what happens after they put into a laboratory dish both a candidate yeast and a special Aspergillus strain. This strain, developed by other researchers in the 1980's, has a genetic mutation. It forms a red-orange pigment, norsolorinic acid, or NOR, as a nontoxic precursor to making aflatoxin.

If a candidate yeast blocks aflatoxin synthesis, the fungi won’t make the colored acid. Instead, only the white color of spreading yeast appears. That tells the researchers the yeast is worth a closer look. This summer, they hope to conduct greenhouse tests of the most effective yeast strains. The best so far are yeasts in the Pichia genus.

Scientific contact: Sui-Sheng Hua, Plant Protection Research Unit, ARS Western Regional Research Center, Albany, Calif., phone (510) 559-5905, fax (510) 559-5777, ssth@pw.usda.gov.