Observations on antibiotic residues in milk

Abstract

The excretion of oxytetracycline and penicillin in bovine milk following their intramuscular administration was demonstrated by bromocresol purple test ("Intertest”). Dosages of 1200 mg of oxytetracycline hydrochloride and 5 mega units of penicillin (crystapen) per cow both resulted in detectable residues in milk for 24-32 hours following administration,. The minimum concentration of oxytetracycline in the serum which resulted in detectable residues in milk was 2.5 mg/ml while that of penicillin was 0.16iu/ml. Antibiotic concentrations in the serum were determined by the tube dilution method.

The effect of heat (boiling at 100°C for 10 minutes) on known concentrations of oxytetracycline and penicillin added to milk was determined using a modified disc assay method with the Oxford (Heatley) Strain of Staphylococcus aureus (NCTC 6571) as the test organism. Oxytetracycline was partially inactivated with a mean antibiotic activity loss of 37.8% whereas penicillin was not inactivated to any noticeable extent.