US scientists have found a way to prevent the formation of the naturally occurring cancer-causing compound aflatoxin in nuts such as almonds, pistachios and walnuts, findings they hope will lead to the first environmentally friendly way to combat the toxin.
The research, which has so far focused on preventing aflatoxin biosynthesis in the laboratory, is due to be presented next week at the Fungal Genomics, Fumonisin and Aflatoxin Elimination Workshops in Ft Worth, Texas.
Aflatoxins are formed by certain moulds growing on some food crops - in warm, humid conditions - during production and storage. They have been shown to be carcinogenic in animals. Aflatoxin B1, the most toxic, is classified as both a human carcinogen and mutagenic - it damages DNA.
A fungus, or mould, known as Aspergillus flavus is a leading source of the toxin. When the A. flavus mold feeds on certain kinds of tree nuts, the nuts respond by forming compounds called"oxidants". The mould, now undergoing what is known as "oxidative stress," reacts by producing aflatoxin.
But according to scientists at the US Department of Agriculture's Agricultural Research Service (USDA ARS), the formation of the toxin can be prevented at plant level, by adding certain "antioxidants" , which can counter the mould's oxidative stress.
One of these, says Bruce Campbell, who heads up the ARS's Plant Mycotoxin Research Unit, is the naturally occurring compound caffeic acid.
However, caffeic acid has itself been linked to cancer.
According to the Food and Drug Administration's (FDA) Office of Food Additive Safety, the compound has been tested experimentally for use as an antimicrobial agent in food.
"Caffeic acid is not approved for direct use in food. It is considered to be a suspected human carcinogen based on testing in mice," it said.
A simple phenolic, caffeic acid is found in a wide variety of plants, including apples, bell peppers and pears. It proved to be the most effective out of all the compounds tested by the scientists, reducing aflatoxin production by more than 95 percent.
When it suppresses aflatoxin production, caffeic acid is thwarting the mould's aflatoxin genes. Pinpointing these guilty genes could widen the array of new tactics to undermine them. Using microarray technology, the scientists took successive snapshots that captured the actions of one form of the mould's genetic material, messenger RNA. Snapshots were taken before the caffeic acid was applied and at four and six days after. Their findings revealed that 48 genes responded to the antioxidants. About half a dozen of these genes were not known to be involved with aflatoxin but may in fact be key players, said ARS.
"We understand the genetic basis for how these compounds work. This information is the most recent we have uncovered using comparative gene expression analysis (microarrays). When treated with these antioxidants, an enzyme is triggered in the fungus to breakdown the signal that triggers aflatoxin biosynthesis," said Campbell.
Other compounds that prevent aflatoxin biosynthesis fall under the heading of being antioxidants. Like caffeic acid, most of these compounds are simple phenolics. These include such compounds as gallic, vanillic and chlorogenic acids. These are all antiaflatoxigenic when applied at appropriate levels, he explained.
"Hydrolyzable tannins, which have a phenolic component, are also antiaflatoxigenic and are found commonly in plants. The problem, however, is that we want these compounds to occur in plants where the fungus is infecting and doing its aflatoxin contamination."
"We have discovered how to prevent aflatoxin biosynthesis in the laboratory and that we understand how it works. Now, the next thing is to figure out how to apply it. Basically, this would essentially involve manipulating the oxidative stress response system of crop plants so that they do not trigger aflatoxin biosynthesis by the fungus infecting them," Campbell told FoodNavigator-USA.com.