Transfer of antibiotics from goat's milk to cheese and whey

Résumé

The presence of antibiotic residues in milk and dairy products poses a risk for consumer health, mainly the development of antimicrobial resistance. Maximum Residue Limits (MRLs) for veterinary drugs have not been established for dairy products. Furthermore, the transfer of antibiotics from milk to cheese and whey fractions during cheese-making has been scarcely studied and, therefore, the impact of the use of whey containing antibiotics for the manufacture of foodstuffs for human and animal consumption is unknown. The aim of this thesis was to evaluate the transfer of antibiotics from milk to cheese and whey fractions, as well as the validation of the performance of several methods to screen antibiotics in whey samples. In the first study, a multiresidue UHPLC-HRMS method using the Orbitrap ExactiveTM analyser for the quantitative screening of antibiotics in milk, fresh cheese, and whey samples was validated according to Commission Decision 2002/657/EC, using samples from three different dairy matrices (milk, fresh cheese and whey) from cows, sheep and goats. The partitioning study by UHPLC-HRMS method indicated that most antibiotics were mainly transferred from milk to whey fraction (up to 85.9%) during cheese-making. Thus, retention rates in the rennet curd fraction were lower than 50%, except for ceftiofur (59.7%) and dicloxacillin (52.8%), and very variable between drugs. In most cases, drug distribution was unaffected by the antibiotic concentration present in milk for cheese production and was poorly related to the drug lipophilicity. In the second study, the performance of different methods for screening antibiotics in whey samples was evaluated in accordance with Commission Decision 2002/657/EC, by conducting three experiments focused on commercially available screening tests, microtiter plate bioassays, and a semi-quantitative multi-plate system, respectively. Specificity (false-positive rate) and Detection Capability (CCß) of a microbial inhibitor test (Eclipse Farm coupled to e-Reader device) and receptor-binding assays (3Aminosensor, Quinosensor, Twinsensor, and Tylosensor) were evaluated in whey samples from goats, having in general, similar results than those obtained when they are applied for milk analysis. Three microtiter plate bioassays with dichotomous response containing Bacillus subtilis, Geobacillus thermocatenulatus and Geobacillus thermoleovorans, respectively, were evaluated simultaneously with commercially available tests using Geobacillus stearothermophilus var calidolactis. High specificity values (>=98%) were obtained when whey samples were heat treated (85ºC, 10 min) prior to analysis. Bacillus subtilis, having lower CCß values for quinolones and macrolides, was the most interesting option to improve the detection profile of the Eclipse 100. Regarding the multiplate system Screening Test for Antibiotic Residues (STAR) using Geobacillus stearothermophilus for ß-lactams and sulfonamides, Bacillus subtilis for aminoglycosides, Kocuria varians for macrolides, Escherichia coli for quinolones and Bacillus cereus for tetracyclines, high specificity values (>=98%) were obtained in most cases. The CCß values obtained using the STAR protocol in whey samples exceed the MRL established in milk for most of the substances considered. However, this method could become an adequate tool in post-screening and reduce the number of samples destined for the quantitative analysis by LC-MS/MS, which is a more complex and expensive method. The production of cheese using milk containing antibiotics generates drug residues in whey. Thus, the suitable performance of screening methods for the detection of veterinary drug residues in this cheese-making by-product will allow the establishment of an adequate control strategy to prevent the presence of antibiotic residues in whey and to avoid the hazards associated to human and animal health and environment.