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

Abstract

A detailed kinetic study of alkaline phosphatase, lactoperoxidase and β-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 β-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 (D_{60 °C} = 24·6 min) and 4·3 deg C (D_{71 °C} = 38·6 min) respectively. For the denaturation of β-lactoglobulin we found z values of 7·9 deg C (D_{75 °C} = 49·9 min) in the temperature range 70–80 °C and 24·2 deg C (D_{85 °C} = 3·53 min) in the range 83-95 °C. D_ref and z were evaluated under dynamic temperature conditions. To estimate the statistical accuracy of the parameters, 90% joint confidence regions were constructed.