Submitted to: Journal of Agricultural and Food Chemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/29/2004
Publication Date: 3/13/2004
Citation: Mahoney, N.E., Molyneux, R.J. 2004. Atoxigenicity of aspergillus flavus induced by phytochemical constituents of walnut (juglans regia). Journal of Agricultural and Food Chemistry. 52(7):1882-1889.
Interpretive Summary: Foodstuffs infected by molds can contain compounds which are harmful to humans and are strictly regulated. Such regulations can limit the export market for California almonds, pistachios and walnuts. The walnut variety 'Tulare' has been shown to be extremely resistant to formation of aflatoxin by molds growing on the kernel. The source of resistance is in the seed coat and analysis of the compounds present in this tissue have shown that it is due to a group of compounds known as tannins, comprised of organic acids and sugars. The fungus degrades the tannins to produce a compound known as gallic acid, which has now been shown to prevent formation of aflatoxins. The levels of gallic acid in 'Tulare' walnut have been shown to increase rapidly as the nut matures and to maintain a high level until harvest. Other walnut varieties have lower levels of gallic acid, or the level declines as the nut matures. It should be possible to breed new walnut varieties for levels of gallic acid sufficient to prevent aflatoxin formation, ensuring that the product is safe for human consumption and conforms to regulatory limits.
Technical Abstract: 'Tulare' walnut, a cultivar highly resistant to aflatoxin formation, was investigated for endogenous phytochemical constituents capable of inhibiting aflatoxigenesis by Aspergillus flavus. The activity, located entirely in the seed coat (pellicle), was extractable to varying degrees with polar solvents, while some activity remained unextractable, indicating that the bioactivity resided in a complex of hydrolyzable tannins. These tannins can be hydrolyzed by a fungal tannase present in A. flavus, yielding gallic acid and ellagic acid, testing of which showed that only gallic acid had potent inhibitory activity towards aflatoxin biosynthesis. Comparison of pellicle content of gallic and ellagic acids in 'Tulare' and 'Chico' cultivars over the 2002 and 2003 growing seasons, showed that the gallic acid content increased rapidly with maturation of the nut and was 1.5-2 times higher in 'Tulare' than in 'Chico'. Gallic acid content in pellicle at maturity of a series of commercial English walnut cultivars, and two black walnut species, correlated well with resistance to aflatoxigenesis on the basis of in vitro assays. Regulation of gallic acid levels in the hydrolyzable tannins of walnuts by conventional breeding or genetic manipulation has the potential to provide new cultivars with high resistance to aflatoxigenesis.