Submitted to: Meeting Abstract
Publication Type: Abstract Only
Publication Acceptance Date: 8/15/2011
Publication Date: 9/10/2011
Citation: Shier, W.T., Abbas, H.K., Weaver, M.A., Horn, B.W. 2011. Cultural methods as an alternative approach to assessing mycotoxin contamination in crops. The 1st International Symposium on Mycotoxins in Nuts and Dried Fruits, Damghan-Iran 10-12, Sep, 2011, page 54.
Technical Abstract: Contamination with aflatoxins and other mycotoxins is a major problem worldwide for numerous types of crops in addition to tree nuts. Aflatoxins are the major determinant of crop quality in several crops in developed countries, where government regulation is extensive, and the cost of monitoring mycotoxin levels is a small but significant part of the overall cost of production. Measuring aflatoxins is expensive whether done by ELISA (high unit costs) or HPLC (high capital cost to purchase equipment, but lower unit costs). In developing countries aflatoxin assay costs can be prohibitive for all but the export market. In developed countries mycotoxin levels are determined once post-harvest, and it is assumed by regulatory agencies that they never change. This assumption is reasonable in developed countries, where grains are stored under near-ideal conditions of low moisture (<15%), which prevents fungal growth and fungal metabolism, including mycotoxin production. However, in developing countries storage conditions are often suboptimal, so that substantial proliferation of Aspergillus flavus and aflatoxin production can occur during storage, making a single aflatoxin measurement near harvest time of limited value. Cultural methods offer an alternative approach to crop monitoring, which has the advantage of providing a measure of the potential for additional post-harvest mycotoxin accumulation during storage. Total contamination with A. flavus can readily be assessed by colony counts on media containing dichloran and rose bengal to restrict fungal colony diameters. Colony counts are measured per unit weight of harvested crop following dilution of extracts through sterile water. However, not all A. flavus isolates are aflatoxigenic and studies on biocontrol strains of A. flavus are consistent with non-aflatoxigenic isolates suppressing aflatoxin production in co-culture with toxigenic isolates. Thus, considerable effort has gone into adapting existing empirical cultural methods for distinguishing aflatoxigenic from non-aflatoxigenic A. flavus isolates. The most effective method has proven to be the empirical method of Machida and Saito, in which colonies turn from yellow to red on exposure to ammonia vapor. We have shown that the color change is due to the presence in aflatoxigenic colonies of anthraquinone intermediates on the biosynthetic pathway to aflatoxin which undergo pH-induced color change. The total A. flavus contamination colony counts can then be adjusted by the per cent aflatoxigenicity. Studies on the applicability of ammonia-induced color changes to aflatoxin detection in tree nuts by this laboratory have been limited to studies in coconut (copra), which is subject to substantial crop losses due to aflatoxin contamination. In the case of coconut, the nut is large enough that hand sorting to eliminate aflatoxins is an option in a developing country setting. Ammonia vapor exposure induces similar color changes in nut meat contaminated with aflatoxigenic A. flavus as were observed in culture medium.