THE EFFECT OF COOKING ON VETERINARY DRUG RESIDUES IN FOOD .3. SULFAMETHAZINE (SULFADIMIDINE)

The heat stability of sulphamethazine was investigated. The drug was shown to be stable in boiling water at 100 degrees C. In cooking oil at 260 degrees C, losses were observed, indicating a half-life of about 5 min. At 180 degrees C in cooking oil, sulphamethazine was unstable with a half-life of about 2 h. The effect of a range of cooking processes (boiling, roasting, grilling, frying, pressure cooking and microwaving) on sulphamethazine residues in incurred animal tissue was studied. Once allowance for weight loss during cooking had been made no net unaccountable change in concentration of sulphamethazine was observed in any of the cooking processes investigated. During frozen storage, sulphamethazine residues were found to be stable over a period of 3 months. It was found during this investigation that the method used for analysis which involved acid extraction converted the N-4-metabolites to parent sulphamethazine, and hence only sulphamethazine was measured. Sulphamethazine was found to be evenly distributed in the raw incurred tissue used for analysis. Migration from the tissue into the surrounding liquid or meat juices was observed during the cooking process. The findings of this investigation show that surveillance data obtained from measurements on raw tissue are applicable for use in consumer exposure and dietary intake calculations, but only if an acidic extraction method which converts the N-4-metabolites to parent sulphamethazine is used for the surveillance. This may not however conform to current Maximum Residue Limit legislation which refers to total parent sulphonamides. Different methods of analysis for sulphonamides are likely to give rise to inter-laboratory variation.