ARS | Publication request: SAMPLE PREPARATION AND PRESAMPLING OF PISTACHIOS

Submitted to: Journal of Agriculture and Food Chemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: July 14, 2003
Publication Date: September 3, 2003
Citation: Schatzki, T.F., Toyofuku, N. 2003. Sample preparation and presampling of pistachios. Journal of Agriculture and Food Chemistry. 51(20):6068-6072.

Interpretive Summary: Tree nuts, such as pistachios, contain rare, but highly aflatoxin contaminated kernels. Because of their rarity, large (20 lbs and more) samples must be tested to get a measure of the average aflatoxin content. Since such large samples can not be handled in a laboratory, they are ground for homogeneity, following subsamples (say, ½ on.) is taken for analysis. This sub-sample, in turn, must be representative of the 20 lbs. sample. We have developed a theory for computing the uniformity of such sub-samples, based on their size, the fineness of the grind and the frequency of contamination. This theory has been tested on dry-ground and wet-slurried pistachios with excellent results. Surprisingly, water slurrying results in aflatoxin levels are 30% higher than what is seen for dry grinding, the traditional method. The results have direct implications for testing lots in the U.S., prior to overseas shipment, since demanding markets have started using slurrying methods. The result will probably be that for overseas sales, tests will have to be done by slurrying. Similar results are expected for almonds and peanuts for major U.S. exports.

Technical Abstract: A theory has been developed to quantify the reduction of sub-sample variance of aflatoxin contamination, which is observed when granular materials are wet slurried, rather than dry ground, during sub-sample homogenization. A coefficient of variation, based on particle size distribution, sub-sample size and probability of contamination, is predicted. The theory is tested with dry ground and with wet slurried pistachios and excellent quantitative agreement is obtained. A 31% increase in the mean aflatoxin level is observed as well when wet slurrying is applied. While no statistical explanation for this effect can be found, it is suggested that it is related to physio-chemical binding between the nut matrix and aflatoxin which is (partly) broken by wet slurrying, making the extraction of more toxin possible. Other parameters which may affect slurrying results have been investigated as well.