Thermodynamic analysis of the thermal stability of sulphonamides in milk using liquid chromatography tandem mass spectrometry detection

The present study investigates the kinetics of the degradation of eight sulphonamides in skimmed milk when heated at 60, 70, 80, 90 and 100 °C using an LC–MS/MS methodology. To determine the thermal stability of these compounds, the first-order kinetic model was applied and the activation energies, half-lives and degradation percentages were calculated. Application of kinetic equations to the different heat treatments used in dairy processing indicates that sulphonamides are very stable during pasteurisation (63 °C; 30 min and 72 °C; 15 s) as well as UHT sterilisation (140 °C; 4 s). In contrast, the calculations performed with the kinetic model estimated losses in concentrations between 6.5% (sulfadimethoxine) and 85.1% (sulfamethazine) for the sterilisation at 120 °C for 20 min. The existence of thermodynamic compensation was also tested for sulphonamide degradation. Results show that enthalpy and entropy values displayed a good linear relationship, and thermodynamically we can establish that the thermal degradation of sulphonamides in skimmed milk exhibits enthalpy–entropy compensation.