Stability of tetracycline antibiotics in raw milk under laboratory storage conditions

Raw milk samples collected from bulk milk tankers may be screened for the presence of tetracycline antibiotics using rapid screening tests. If tetracycline residues are detected, the milk may be shipped to a laboratory for high-pressure liquid chromatography (HPLC) analysis. Because the milk may be shipped on ice blocks, it is important to know whether tetracycline residues are stable at that temperature and for how long. Control raw milk samples fortified with 50 ppb each chlortetracycline, demeclocycline, methacycline hydrochloride, minocycline, oxytetracycline, and tetracycline were incubated at 4 degrees C or 25 degrees C, then analyzed using a metal chelate affinity chromatography extraction and HPLC. No loss of tetracycline was observed after 48 h of storage at 4 degrees C or 24 h at 25 degrees C. Losses ranging from 4 to 13% and 0 to 18% were noted after 72 h at 4 degrees C and 48 h at 25 degrees C, respectively.