Study on optimizing conditions of the thermal stabilization of milk

The purpose of this work was to find the optimal conditions for milk sterilization. Since the activation energy values of microorganism destruction and enzyme inactivation are higher and lower, respectively than the activation energy values of heat damage reactions, it was assumed that the best way to obtain complete milk stabilization with limited heat damage was to carry out a two-step treatment, optimizing the destruction of microorganisms and enzymes separately. Bacillus stearothermophilus spores and a thermoresistant protease were used in order to study stabilizing effects; colour variation (ΔE) was used as an index of heat damage. Both protease inactivation and spore destruction follow first-order kinetics with activation energies of 82.7 and of 288.4 KJ/mol, respectively. ΔE variation follows zero-order kinetics with an activation energy of 101.8 KJ/mol. Data set for optimization was 99.999% reduction of B. stearothermophilus spores and 90% reduction of enzyme activity. It was verified that a treatment at 135 °C for 37 s followed by another one at 95 °C for 51 min fulfils the set stabilizing conditions with a resulting ΔE value of 3.5. Our research demonstrates that a two-step treatment makes it impossible to obtain, with the same stabilizing effect, a higher quality sterilized milk than that obtained by a single-step treatment.