Submitted to: International Journal of Food Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/11/2007
Publication Date: 11/21/2007
Citation: Molyneux, R.J., Mahoney, N.E., Kim, J.H., Campbell, B.C. 2007. Mycotoxins in edible tree nuts. International Journal of Food Microbiology. 119:72-78
Interpretive Summary: Edible tree nut crops are extremely valuable to the state of California with much of the income deriving from exports. These exports are threatened by the very strict standards for toxic compounds produced by molds that sometimes grow on the nuts. Natural methods are required to prevent mold infections or the formation of toxic compounds. It has been shown that walnuts in particular have compounds in the seed coat that prevent toxins from being produced. These findings have been extended to pistachios and almonds which also have toxin-preventative compounds but that are less effective. The formation of these protective compounds may be enhanced by selective breeding.
Technical Abstract: Tree nuts (almonds, pistachios, and walnuts) are an exceptionally valuable crop, especially in California, with an aggregate value approaching $3.5 billion. Much of this economic value comes from overseas markets, with up to 60% of the crop being exported. The product can be contaminated with aflatoxins or ochratoxins, with the former being of special concern because of the strict regulatory levels (4 ppb total aflatoxins) applied by the European Community (EC). Natural, consumer-acceptable control methods are therefore required to conform to such limits. Research has shown that aflatoxin production is markedly decreased by the presence of natural antioxidants that occur in tree nuts, including hydrolysable tannins, flavonoids and phenolic acids. In vitro testing of individual compounds showed that the antiaflatoxigenic effect correlated with the structure and concentration of such compounds in individual nut varieties and species. This lead to the hypothesis that aflatoxin biosynthesis is stimulated by oxidative stress on the fungus and that compounds capable of relieving oxidative stress should therefore suppress or eliminate aflatoxin biosynthesis. Oxidative stress induced in A. flavus by addition of tert-butyl hydroperoxide to the media stimulated peak aflatoxin production and maintained high levels over time. However, aflatoxin formation was significantly inhibited by incorporation into the media of the antioxidant, tannic acid. Measures to increase antioxidant natural products in tree nuts may thereby reduce or eliminate the ability of A. flavus to biosynthesize aflatoxins, thus ensuring levels at or below regulatory limits and maintaining export markets for U.S. tree nuts.