Formation of anhydrotetracycline during a high-temperature treatment of animal-derived feed contaminated with tetracycline

Residues of tetracyclines and their epimers were frequently found in bones of slaughtered animals, and also in meat and bone meals for animal feed that had been heat treated according to EU legislation. The aim of this study was to examine the formation of anhydrotetracycline and 4-epianhydrotetracycline, as toxic degradation products of tetracycline, during a heat treatment. Meat and bone meals containing bound tetracycline residues were heat treated at 100 and 133 °C for up to 45 min, extracted with hydrochloric acid and subsequently analyzed for tetracycline, 4-epitetracycline, anhydrotetracycline and 4-epianhydrotetracycline, using HPLC and LC–MS–MS. Small amounts of anhydrotetracycline and 4-epianhydrotetracycline were even found in the pre-treatment control samples. The heat treatments led to a significant increase of the amounts of these degradation products. During the most rigorous heat treatment, at 133 °C for 45 min, the mean percentage decrease of the concentrations of tetracycline and 4-epitetracycline was 50%, while the mean percentage increase of the concentrations of anhydrotetracycline and 4-epianhydrotetracycline was 533%.