Walnuts' Anti-Aflatoxin Ally: Gallic Acid
Chemist Russell Molyneux
prepares walnut pellicle
samples for analysis of
gallic acid content.
Rich, crunchy walnuts are always delicious, whether sprinkled on top
of a waffle at breakfast, added to brownies you've whipped up for an
after-lunch treat, or tossed with a crisp green salad for dinner. Walnuts
are a good source of omega-3 fatty acids, thought to reduce risk of
cancer. They also provide protein, several essential vitamins and minerals,
and antioxidants, yet are free of trans fats and cholesterol. Today,
they're ranked as America's third most popular tree nut.
U.S. walnut growers and processors meticulously inspect walnuts so
that only the best make their way from orchards to your kitchen. Part
of their quality control involves making sure the nuts don't exceed
federal food-safety standards for aflatoxin, a naturally occurring compound
that can be converted into chemicals harmful to us.
Two kinds of fungi, Aspergillus flavus and A. parasiticus, are perhaps the best known of the microbes capable of forming aflatoxin. They have the ability to infect not only walnuts, but also almonds, pistachios, peanuts, corn, and cotton.
The thin outer coating
or pellicleof Tulare
walnuts is removed and
later analyzed for gallic
But ARS scientists in the Western Regional Research Center's Plant Mycotoxin Research Unit at Albany, California, have found that a commercial walnut variety, Tulare, is remarkably resistant to being contaminated by the fungus. The quest to discover Tulare's secret has led the researchers to gallic acid, a natural compound that's locked up in walnut tannin.
Enzyme Disrupts Aflatoxin-Production Process
Ironically, Aspergillus itself helps free up the gallic acid
in walnut tannin. An Aspergillus protein or enzyme known as tannase
breaks down the tannin and unlocks the gallic acid.
In Tulare walnuts, the gallic-acid-containing tannin occurs in levels sufficient to render the fungus incapable of making aflatoxin. The nuts' ability to inhibit aflatoxin production increases as the walnut matures, notes ARS chemist Russell J. Molyneux, who led the research.
Research associate Jong
Kim (left) and research
leader Bruce Campbell
bioassays to determine
effects of gallic acid
on genes that control
Comparing Gallic Acid Levels
Molyneux and ARS chemist Noreen E. Mahoney at Albany tracked down gallic
acid as the key player in Tulare's aflatoxin-defense strategy. Their
petri-dish tests revealed that this variety's anti-aflatoxin activity
was better than that of the other commercial walnut varietiesand
other tree nutsthey screened.
The assays encompassed nearly a dozen leading varieties of English
walnuts, the kind most widely marketed in the United States and Europe
today, and two species of black walnuts, not as widely grown because
their thicker shells are harder to open and their nutmeats don't loosen
as easily from the shells.
The Albany scientists collaborated in the screening with Charles A. Leslie, Gale H. McGranahan, and James McKenna of the University of California, Davis. They found that Tulare walnuts contained one and a half to two times more gallic acid than, for instance, Chicothe most aflatoxin-susceptible variety they examined.
Chemist Noreen Mahoney
performs analysis of
aflatoxin from a petri
dish (center) containing
Tulare walnuts inoculated
with Aspergillus flavus.
The dish in the foreground
(green) contains no walnut
tissue and displays
extensive growth of
Researchers elsewhere had already shown, in laboratory experiments,
that gallic acid had antimicrobial effects. But the investigations Molyneux
directed are likely the first to find that an agricultural cropsusceptible
to infection by an aflatoxin-producing Aspergillus speciescan
actually prevent the Aspergillus from making aflatoxin.
The work paves the way for moving Tulare's gallic-acid-producing prowess into vulnerable walnut varieties. Conventional walnut breeding is one way to accomplish this.
Discovering More About Gallic Acid
Molyneux and Mahoney pinpointed Tulare walnuts' pelliclethe nutmeat's
thin outer coatingas the only source of the gallic acid. Exactly
how gallic acid disrupts the fungus's ability to produce aflatoxin isn't
clear yet, says Bruce C. Campbell, who heads the Albany research unit.
Preliminary research by Campbell and molecular biologist Jong H. Kim
suggests that Aspergillus may make aflatoxin in response to environmental
stress, such as drought.
More details about the gallic acid research appeared in the Journal
of Food Science.
The aflatoxin investigations were funded in part by the California-based
Walnut Marketing Board. California produces nearly all of the nation's
walnut crop. The 2003 harvest of 326,000 tons of in-the-shell walnuts
was worth about $355 million to growers.
This research may be good news for growers, as it might lead to new,
environmentally friendly strategies to undermine aflatoxin production
not only in walnuts and other popular tree nuts but also in other crops.By
Agricultural Research Service Information Staff.
This research is part of Food Safety, an ARS National Program (#108)
described on the World Wide Web at www.nps.ars.usda.gov.
Russell J. Molyneux, Noreen
E. Mahoney, Bruce C. Campbell,
and Jong H. Kim are in the USDA-ARS
Plant Mycotoxin Research Unit, Western
Regional Research Center, 800 Buchanan St., Albany, CA 94710; phone
(510) 559-5812 [Molyneux], (510) 559-5971 [Mahoney], (510) 559-5846
[Campbell], (510) 559-5841 [Kim], fax (510) 559-5777.
"Walnuts' Anti-Aflatoxin Ally: Gallic Acid" was published in the March 2005 issue of Agricultural Research magazine.