Inactivation kinetics of alkaline phosphatase and lactoperoxidase, and denaturation kinetics of beta-lactoglobulin in raw milk under isothermal and dynamic temperature conditions.

A detailed kinetic study of alkaline phosphatase, lactoperoxidase and beta-lactoglobulin was carried out in the context of identifying intrinsic time-temperature indicators for controlling the heat processing of milk. The heat inactivation or denaturation of alkaline phosphatase, lactoperoxidase and beta-lactoglobulin under isothermal conditions was found to follow first order kinetics. Experimental results were analysed using both a two step linear regression and a one step non-linear regression method. Results obtained using the two statistical techniques were comparable, but the 95% confidence interval for the predicted values was smaller when the one step non-linear regression method was used, indicating its superiority for estimating kinetic parameters. Thermal inactivation of alkaline phosphatase and lactoperoxidase was characterized by z values of 5.3 deg C (D60 degrees C = 24.6 min) and 4.3 deg C (D71 degrees C = 38.6 min) respectively. For the denaturation of beta-lactoglobulin we found z values of 7.9 deg C (D7.5 degrees C = 49.9 min) in the temperature range 70-80 degrees C and 24.2 deg C (D85 degrees C = 3.53 min) in the range 83-95 degrees C. Dref and z were evaluated under dynamic temperature conditions. To estimate the statistical accuracy of the parameters, 90% joint confidence regions were constructed.