Heat inactivation of beta-lactam antibiotics in milk.

The presence of residues of antimicrobial substances in milk is one of the main concerns of the milk industry, as it poses a risk of toxicity to public health, and can seriously influence the technological properties of milk and dairy products. Moreover, the information available on the thermostability characteristics of these residues, particularly regarding the heat treatments used in control laboratories and the dairy industry, is very scarce. The aim of the study was, therefore, to analyze the effect of different heat treatments (40 degrees C for 10 min, 60 degrees C for 30 min, 83 degrees C for 10 min, 120 degrees C for 20 min, and 140 degrees C for 10 s) on milk samples fortified with three concentrations of nine beta-lactam antibiotics (penicillin G: 3, 6, and 12 microg/liter; ampicillin: 4, 8, and 16 microg/liter; amoxicillin: 4, 8, and 16 microg/liter; cloxacillin: 60, 120, and 240 microg/liter; cefoperazone: 55, 110, and 220 microg/liter; cefquinome: 100, 200, and 400 microg/liter; cefuroxime: 65, 130, and 260 microg/liter; cephalexin: 80, 160, and 220 microg/ liter; and cephalonium: 15, 30, and 60 microg/liter). The method used was a bioassay based on the inhibition of Geobacillus stearothermophilus var. calidolactis. The results showed that heating milk samples at 40 degrees C for 10 min hardly produced any heat inactivation at all, while the treatment at 83 degrees C for 10 min caused a 20% loss in penicillin G, 27% in cephalexin, and 35% in cefuroxime. Of the three dairy industry heat treatments studied in this work, low pasteurization (60 degrees C for 30 min) and treatment at 140 degrees C for 10 s only caused a small loss of antimicrobial activity, whereas classic sterilization (120 degrees C for 20 min) showed a high level of heat inactivation of over 65% for penicillins and 90% for cephalosporins.