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United States Department of Agriculture

Agricultural Research Service

Title: Walnut and Pistachio Hydrolyzable Tannins: Comparison of Chemical Structures and Aflatoxin Inhibiting Properties

item Mahoney, Noreen
item Molyneux, Russell
item Campbell, Bruce
item Kim, Jong Heon

Submitted to: Aflatoxin Workshop
Publication Type: Abstract Only
Publication Acceptance Date: September 8, 2004
Publication Date: October 25, 2004
Citation: Mahoney, N.E., Molyneux, R.J., Campbell, B.C., Kim, J.H. 2004. Walnut and pistachio hydrolyzable tannins: comparison of chemical structures and aflatoxin inhibiting properties. Aflatoxin Workshop.

Technical Abstract: Pistachio and walnut fruit tissues contain hydrolyzable tannins that inhibit the growth and aflatoxin production of Aspergillus flavus. Walnut hydrolyzable tannins, located in the kernel seed coat, are composed of glucose esterified with gallic acid and hexahydroxydiphenic acid, which spontaneously lactonizes into ellagic acid upon hydrolysis. Pistachio hydrolyzable tannins, located in the hull, are composed of quinic acid esterified with gallic acid. A. flavus produces an extracellular tannase which hydrolyzes tannins into their polyol and phenolic components. The hydrolyzed components of tree nut tannins are potent inhibitors of aflatoxin; ellagic acid, quinic acid, and gallic acid reduced aflatoxin by 70%, 84%, and 88%, respectively, when incorporated at 0.2% (w/v) in nut kernel media. These compounds did not affect A. flavus radial growth. Despite the fungal tannase activity, the tree nut hydrolyzable tannins are more powerful inhibitors of aflatoxin biosynthesis and fungal growth than any of the individual tannin components. When incorporated at 0.2% (w/v) in walnut kernel agar, walnut tannin reduced aflatoxin production by 99.9% and significantly reduced A. flavus growth. Most remarkable is the ability of pistachio tannin to completely inhibit the growth of A. flavus at 0.04% (w/v) in pistachio kernel agar.

Last Modified: 4/18/2015
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