No abstract

A reverse-phase HPLC method is described that permits detection of quaternary ammonium sanitizers in milk. The commercial mixture of quaternary ammonium sanitizer used, n-alkyl (50% C14, 40% C12, and 10% C16) dimethyl benzyl ammonium chloride, was extracted from raw and homogenized milks by the Mojonnier milk fat extraction protocol, selectively eluted from the reverse-phase column in the presence of organic solvent and ion-pairing agent, and then analyzed by diode array detection at 217 and 280 nm. Quaternary ammonium sanitizer concentrations in augmented milk samples in the range of 1.0 to 60 micrograms/ml were analyzed with high precision as indicated by a small variance around the mean. Starter culture strains currently in use for manufacture of various fermented milk products available in the Department of Microbiology Culture Collection were examined for their sensitivity to quaternary ammonium sanitizer. All strains were greatly inhibited by as little as 20 micrograms/ml in milk; some were inhibited by only 10 micrograms/ml.

Antibiotic resistance of 44 lactic starter culture strains currently used by the dairy industry has been examined in Mueller- Hinton medium, supplemented with glucose (5 g/L) and yeast extract (5 g/L), by the agar disk diffusion assay. Results with various classes of antibiotics used to treat cattle mastitis indicated that lactococcal cultures should be diluted 20% and incubated at 30°C for 16 to 18 h in this medium for reliable results. The degree of antibiotic susceptibility was compared to standards established by the National Committee for Clinical Laboratory Standards. The procedure outlined in the present study is recommended as a routine method for testing antibiotic resistance of lactococci. This represents a more logical drug sensitivity approach than the use of noncheese starter bacteria to test for antibiotics in milk.