The effect of cooking on veterinary drug residues in food: 4. Oxytetracycline.

The heat stability of oxytetracycline (OTC) in water and vegetable oil was investigated. Results showed that the drug was unstable in water at 100 degrees C with a half-life of about 2 min, but more stable in oil at 180 degrees C where the half-life was about 8 min. The effect of a range of cooking processes including microwaving, boiling, roasting, grilling, braising and frying on OTC residues in incurred animal tissues was investigated. Substantial net reductions in OTC of 35-94% were observed, with temperature during cooking having the largest impact on the loss. Migration from the tissue into the surrounding liquid or meat juices was observed during the cooking processes. Diode-array analysis of heat-treated OTC standard solutions indicated that no individual closely related compound such as 4-epioxytetracycline, alpha- or beta-apooxytetracycline formed a significant proportion of the breakdown products. OTC was not evenly distributed throughout the tissue, but the effects of this were minimized by selecting adjacent samples for cooking and for the raw control. The findings of this investigation showed that the effect of cooking on residues of OTC should be considered before data obtained from measurements on raw tissue are used for consumer exposure estimates and dietary intake calculations.