Saturday, 15 July 2006

Sorption of Aflatoxin by Clays and Modified Clays.

William F. Jaynes, Richard Zartman, and Wayne Hudnall. Texas Tech Univ, Plant and Soil Science Dept, P.O. Box 42122, Lubbock, TX 79409-2122

Aflatoxins are toxic fungal metabolites produced by Aspergillus flavus and other molds. Cancer and deaths in humans and animals can result from aflatoxin ingestion. Worldwide, 4.5 billion people may suffer chronic non-lethal toxicity. Of the 16 naturally occurring aflatoxins, B1, B2, G1, and G2 are commonly detected in food and feedstuffs. Aflatoxin and the B and G toxin names were derived from A.flavus and the blue and green fluorescence of these compounds. Fluorescence can be used to screen crops for aflatoxin contamination. Clays such as montmorillonite are commonly added to animal feeds as anti-caking agents to improve flow characteristics. As an added benefit, montmorillonite has been shown to sorb aflatoxins and reduce toxicity to animals. Other researchers have shown that a commercial montmorillonite clay product, Novasil, effectively sorbs aflatoxins from water and in feeding studies greatly reduces toxicity to animals. U.S. Food and Drug Administration (USFDA) regulations limit the use of grains with >20 g aflatoxins/kg and mandate the disposal of grains with >1000 g/kg. USFDA regulations allow the addition of up to 2% clay in animal feeds, but the claim that clay additives reduce aflatoxin toxicity has not been approved. Of the common aflatoxins, B1 is the most toxic. In this study, aflatoxin B1 (AfB1) retention by clays and modified clays was measured using aqueous batch sorption isotherms and by direct mixing with AfB1-contaminated corn meal. The objectives were to identify more effective clay additives and examine the nature of aflatoxin binding to clays. The reference clays, SWy-2, SAz-1, SepSp-1, were obtained from the Clay Minerals Society and used for comparison with Novasil. Organo-clays were prepared by treating SWy-2 with a quaternary alkylamine (HDTMA) and proteins (cotton seed, casien). Aflatoxin concentrations were measured using enzyme-linked immunoassay and UV absorption. Based on aqueous batch sorption isotherms, the reference montmorillonites, SWy-2 and SAz-1 (300 to 350 g/kg), were slightly more effective than Novasil (150 g/kg) in sorbing aflatoxins. Sepiolite, SepSp-1, was much less effective (~50 g/kg) than the montmorillonites. The HDTMA-SWy-2 (~25 g/kg) and protein-SWy-2 (100 to 200 g/kg) organo-clays were less effective than untreated SWy-2 (~350 g/kg). A synthetic hydrotalcite-like mineral was the most effective clay tested with AfB1 loadings of >1000 g/kg. Aflatoxin retention from corn meal by the clays was 100 times less than from water. Corn meal samples contaminated with ~900 to 2000 g AfB1/kg were mixed with clays and a small amount of water. Later, these samples were extracted with 60% methanol using a procedure modified from a method for aflatoxins in grain. The synthetic hydrotalcite that most effectively sorbed AfB1 from water was least effective in binding AfB1 (~0.5 g/kg loadings) in corn meal, whereas, the sepiolite (SepSp-1) was most effective with aflatoxin loadings of ~5.0 g/kg. The protein-SWy-2 organoclays were more effective than the untreated clay, whereas, the HDTMA-SWy-2 clay was much less effective. Minor AfB1 sorption by HDTMA-SWy-2 suggests that clay surface adsorption was more important than partitioning. Addition of 2% SWy-2, SAz-1, and Novasil to 2000 g AfB1/kg corn meal retained 90 to 95% of AfB1 and reduced extractable AfB1 to ~100 g/kg. A poor sorbent for aqueous AfB1, 2% sepiolite (SepSp-1) effectively sorbed 99% of AfB1 from corn meal and reduced extractable AfB1 content to ~20 g/kg. Montmorillonites and other clays effectively sorbed aflatoxins from water and very high loadings of sorbed aflatoxin to clays were achieved. In contrast, aflatoxin sorption from corn meal was 100 times less than from water. The relative amounts of aflatoxin sorbed from water compared to sorption from corn meal were reversed in several instances. Soluble corn meal components and methanol extraction probably account for the much lower aflatoxin sorption in the corn meal/clay experiments. The sorbed aflatoxin retained by clays in the corn meal experiments was more strongly retained than the much larger aflatoxin amounts sorbed from water. Aflatoxin sorption from corn meal is a more conservative measure of aflatoxin binding and might correlate more closely with animal exposure and toxicity reduction than sorption from water. Tests of aflatoxin sorption from grain might more effectively screen potential additives than sorption from water. Aflatoxin sorption data suggest that more effective clay feed additives might be developed using sepiolites, montmorillonites, or modified clays.

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