Aflatoxigenesis induced in Aspergillus flavus by oxidative stress and reduction by phenolic antioxidants from tree nuts

Almonds, pistachios, and walnuts grown in California have an aggregate value of over $3.3 billion, with a large proportion of the crop being exported. Mycotoxin contamination affects this value, because importing nations, and in particular the European Community (EC), apply strict regulatory levels for aflatoxins, formed by infection of the product by Aspergillus species. Conformance with such limits can best be achieved by capitalising on natural resistance factors inherent to the nuts. Differential resistance of tree nut species to aflatoxin formation, with walnuts being least susceptible, led to research which established that the hydrolysable tannins in walnut pellicle were responsible for this effect. This observation has now been extended to show that the addition to Aspergillus flavus growth media of natural antioxidants present in tree nuts, such as hydrolysable tannins, flavonoids and phenolic acids, decrease maximum aflatoxin by as much as 99.8%. There is evidence that aflatoxin formation is exacerbated by oxidative stress on the fungus and therefore compounds capable of relieving such stress should limit aflatoxin accumulation. The addition of tertiary-butyl hydroperoxide to the media induced oxidative stress in A. flavus and stimulated peak aflatoxin production with high levels being maintained, rather than declining as in control samples. Aflatoxin formation was significantly inhibited by incorporation into the media of the antioxidant, tannic acid. A similar effect was seen for drought-stressed cultures, with increased aflatoxigenesis being reduced in the presence of gallic or caffeic acids. Increasing antioxidant natural products in tree nuts may restrict the ability of A. flavus to biosynthesise aflatoxins, thus achieving conformity with regulatory limits.