The effect of cooking on veterinary drug residues in food .5. Oxfendazole

The heat stability of the anthelmintic oxfendazole in water, cooking oil and as incurred residues in cattle liver was investigated. Some evidence of instability was found in boiling water after 3 h. This degradation was associated with the formation of a product which was identified by mass spectrometry as the amine formed by hydrolysis of the carbamate functional group on the oxfendazole molecule. In hot cooking oil at 150 degrees C and 180 degrees C the half-life of oxfendazole was 15 min and 6 min respectively. The amine war also shown to form in oil as the concentration of oxfendazole depleted. The effects of cooking on incurred residues of oxfendazole in cattle liver were inconclusive from this study. Several variable factors were found to be in place including: an unstable equilibrium between oxfendazole, oxfendazole sulphone and fenbendazole in incurred tissue; the overall instability of these compounds in tissue during frozen storage for the duration of the project; the distribution of the residues within the tissue used for the study and the effect of protein binding on extractability of residues from the tissue. It was nevertheless found that: (i) cooking did not destroy residues although it may affect the point of equilibrium between oxfendazole, oxfendazole sulphone, fenbendazole and some other metabolites in incurred tissue; (ii) the amine derivative was not observed in raw incurred tissue; (iii) residues were not evenly distributed in raw incurred tissue; (iv) storage time affected the measured residue concentration either because of losses or protein binding or a combination of both.