Comparative Evaluation of the Difco Disc Assay Method for Detecting Penicillin in Milk and Milk Products

The Difco disc assay method to detect antibiotic residues was evaluated and compared with the Bacillus subtilis disc assay and the Sarcinia lutea cylinder plate methods. The Difco method, using both color change and zone of inhibition as indicator, was able to detect penicillin to levels as low as 0.002 units per ml. Of 5200 raw milk samples analyzed for presence of penicillin in a commercial laboratory, the B. subtilis disc assay method identified and confirmed as positive 12 samples (0.239%). The Difco disc assay method identified 61 samples as positive for inhibitor, of which 48 (0.92%) were confirmed as positive for penicillin. Of 37 of these latter samples, analyzed also by the S. lutea cylinder plate method, 29 (78.4%) yielded positive results. The Difco procedure was found effective as a test method for detecting penicillin in finished fluid milk products.