Abstract
Abstract A microbiological assay procedure with increased sensitivity was developed for milk and some of its products. The procedure is based on the classical cylinder-plate assay procedures with the following modifications: closer adherence to a consistent pH of 4.5, centrifugation of samples to remove the physical barriers to diffusion of the antibiotic, and use of a thin, single layer of seeded agar. Recoveries in milk by this procedure averaged 65.3% versus 37.9% for those obtained using FDA methods. Improvements in recoveries were noted with the milk products studied. Of the chlortetracycline added to milk, 25% was converted to the epimer form which has negligible microbiological inhibitory properties. Binding and/or the inability to account for the antibiotic was found to be negligible.
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