Stability of 10 β-lactam antibiotics in raw milk under different storage conditions

Abstract Text: B-lactam antibiotics are used to cure mastitis which inflicts severe economic losses in dairy farming. However, improper use of antibiotics may lead to residues in milk, which could be toxic and dangerous for human health, and may cause allergic reactions and antimicrobial resistance. At present, several methods for detecting the residue of β-lactam require milk samples to be sent back to a testing laboratory and stored until further analysis. In this study, we attempted to evaluate the stability of 10 β-lactam antibiotics, include amoxicillin (AMOX), cloxacillin (CLOX), oxacillin (OXAC), penicillin G (PENG), nafcillin (NAFC), cefoperazone (PER), cephapirin (PIR), ceftiofur (TIO), cefazolin (ZOL) and cefalonium (LON) in raw milk under different storage conditions, such as storage temperature and time, thawing temperature, freeze-thaw cycle times, and the addition of preservatives. Raw milk samples were collected from a local farm in Beijing and transported to our lab as soon as possible. Milk samples were spiked with the Maximum Residue Limits (MRL) levels in China and the antibiotic residues were determined by UPLC-MS/MS. Results showed that most of these antibiotics were quite stable (recovery = 90 ~ 120%) under different storage conditions, and their degradation rate increased with the increasing storage temperature and time, thawing temperature and freeze-thaw cycle times. Among the variables, preservatives played a critical role in the stability of β-lactam. The storage of PIR at room temperature for 1 day in raw milk containing preservatives resulted in a degradation rate up to 41.7%. Above all, the milk samples should be stored at -80^oC less than 7 days so that the β-lactam residues can be analyzed accurately.

Keywords: raw milk; β-lactam; storage conditions