Quantitative Assay of Beta-Lactam Residues in Raw Milk Using a Disc Assay Method

Numerous methods have been developed to determine presence of antibiotics in raw milk. Until recently, major effort had been placed on qualitative considerations, and primarily for detecting presence of penicillin (beta-lactam) residues. Only one method, the Sarcina lutea Cylinder Plate (CP) procedure, has been modified to provide for quantitative estimates. The CP method is a rather long, tedious test, requiring considerable technical skill. Need for a simpler, faster quantitative method was apparent. This paper describes a method for making quantitative estimates of beta-lactam residues around a fixed reference standard. The method uses Bacillus stearothermophilus in a disc assay test. Quantitative estimates above or below the reference level of antibiotic are computed through a paired-t statistical analysis. The test can be completed within 3 h.