Improved microbiological assay procedures for dihydrostreptomycin residues in milk and dairy products

Because dihydrostreptomycin can remain as a slowly removed antibiotic residue in dairy animals and because of the need for more sensitive procedures with which to provide information concerning antibiotic residues in food products, procedures were developed in more sensitive assays of dihydrostreptomycin in milk and some representative dairy products. The cleanup procedures that aided these improvements were (i) precipitation of milk proteins by acidification and (ii) centrifugation to extract cheeses and to remove physical barriers to diffusion in the cylinder plate assay. In addition, a thin, single seeded assay layer was used to maximize diffusion. Levels of dihydrostreptomycin as low as 0.06 microgram/ml in milk and 0.2 to 0.4 microgram/g in cheeses were measurable; these levels were some fourfold more sensitive than those presently recommended by the Food and Drug Administration.