Improved microbiological procedure for determining erythromycin in milk and some milk products

An improved microbiological assay for erythromycin in milk and some milk products was developed. Acidification of the samples to pH 4.5 resulted in precipitation of milk solids which were separated by centrifugation. The pH of the aqueous supernatant solution was readjusted to pH 8.0 to obtain maximum sensitivity. Th sample cleanup and the use of a single, thin, seeded agar layer with Microsoccus lutea as the assay organism permitted the mesurement of 0.05 micrograms/mL milk with recoveries averaging 84.0%, 0.50 micrograms/g nonfat dry milk with recoveries averaging 55.4%, 0.10 micrograms/g cream cheese with recoveries averaging 100.2%, and 0.05 micrograms/mL buttermilk with recoveries averaging 44.0%. The improved procedures showed superior recoveries and improved the ability to detect and measure lower levels when compared with commonly recommended assay procedures for erythromycin residues.