Applicability of the Bacillus stearothermophilus Disc Assay for Penicillin Residues in Casein/Caseinates

Six buffer systems were examined as hydrating solutions for assaying antibiotic residues in bovine casein or caseinates by the qualitative Bacillus stearothermophilus disc assay method. Formic acid and 1% potassium phosphate buffer systems were suitable in that they did not react adversely with the B. stearothermophilus spores or cause degradation of penicillin. With the formic acid buffer, a 20% casein slurry and a 10% caseinate slurry were sufficiently fluid to allow capillary saturation of a 12.7-mm paper disc. Casein and caseinate rehydrated with 1% potassium phosphate buffer were too viscous to permit saturation of a disc. The detectable level in a 20% casein slurry and a 10% caseinate slurry was ≥0.004 IU penicillin G/ml. Casein and caseinate prepared from potassium or procaine penicillin G-contaminated skim milk contained no detectable level of antibiotics as determined by the B. stearothermophilus disc assay method.