Aflatoxin: Toxicity to Dairy Cattle and Occurrence in Milk and Milk Products - A Review

Abstract

Aflatoxins are toxic and carcinogenic secondary metabolites produced bysome common aspergilli duringgrowth on feeds, foods or laboratory media. Aflatoxin Bj (AFB,) is a decaketide (Cjo-polyketide) which is synthesized by the mold from acetate units via the polyketide pathway. Methionine contributes the methoxy-methyl group. Sixknown intermediate compounds in the biosynthesis of AFBi include norsolorinic acid, averantin, ^ averufin, versiconal hemiacetal acetate, versicolorin A and sterigmatocystin. Other aflatoxins (Bz, Bj^, G,, Gj and Gja) appear to be conversion products of AFBi. When aflatoxins, and in particular AEBj, occur in feed and are consumed by dairy cattle, a variety of symptoms can occur, which includes unthriftiness, anorexia and decreased milk production. Changes in amounts of enzymes and other blood constituents also result from ingestion of AFB,. The hepatic microsomal mixed-function oxidase systemof the cowconverts some of the ingested AFBj into aflatoxin Mj (AFMi), which is excreted in milk. AFMi retains thetoxicity of,but is less carcinogenic than AFBj. Certain heat treatments associated with milk processing appearto inactivate a portionof the AFMj in milk. If rawmilk contains AFM,, products (fluid products, nonfat dried milk, cultured milks, natural cheese, process cheese, butter) made from such milk also will contain AFM,. AFM, appears to be associated with thecasein fraction ofmilk, hence concentrating the casein in the manufacture of products (e.g. cheese, nonfat dry milk) is accompanied by concentrating of the AFM,. Methods involving thin-layer or high-performance liquid chromatography are commonly used to detect and quantify AFM, in milk and milk products.