Interference of non-specific detergents in microbial inhibitor test results for screening antibiotics in goat’s milk

Abstract

Cleaning and disinfection of dairy equipment is essential to ensure the hygienic quality of milk. Occasionally, some farmers use washing-up liquids and disinfectants for home use, especially when cleaning procedures are carried out manually. Residues of detergents and disinfectants in milk may interfere with the response of microbial inhibitor tests used for screening antibiotics in milk. Therefore, the aim of this study was to evaluate the interference of non-specific detergents in screening tests (BRT MRL; Delvotest SP-NT MCS; Eclipse 100) for goat’s milk. Twelve replicates of eight concentrations of five washing-up liquids (0–1%) and one disinfectant (0–1%) were analysed. The results showed that the presence of washing-up liquids at concentrations of ≥1 ml/l leads to positive results in microbial tests. In particular, the product containing sodium laureth sulphate and ethanol produced the largest number of positive outcomes. The presence of disinfectant based on sodium hypochlorite did not affect the test response. The detection capabilities of microbial inhibitor tests for penicillins were also studied in milk with and without cleaning products, calculating the dose–response curve with eight concentrations of amoxicillin, ampicillin, benzylpenicillin and cloxacillin, respectively. The detection limits of the screening tests for penicillins were not modified substantially by the cleaning product based on sodium laureth sulphate and ethanol. Residues of cleaning agents in milk can be avoided when specific detergents and disinfectants for milking equipment are used and good cleaning practices are applied.