Evaluation of Phosphatase Inactivation Kinetics in Milk Under Continuous Flow Microwave and Conventional Heating Conditions

Raw milk was subjected to conventional isothermal water bath heating, continuous flow microwave heating and continuous flow thermal holding in the pasteurization temperature range (60-75 degrees C), and then immediately cooled in an ice-water bath. The associated alkaline phosphatase (ALP) residual activities were evaluated. Based on the gathered time-temperature profiles, the come-up time (CUT) and come-down time (CDT) contributions to enzyme inactivation were assessed and adjusted prior to first order rate kinetic data handling. The time-corrected D-values of ALP varied from 1250 s at 60 degrees C to 1.7 s at 75 degrees C with a z-value of 5.2 degrees C under conventional batch heating conditions, 128 s at 65 degrees C to 13.5 s at 70 degrees C, with a z-value of 5.2 degrees C under continuous-flow thermal holding condition, 17.6 s at 65 degrees C to 1.7 s at 70 degrees C with a z-value of 4.9 degrees C under continuous-flow microwave heating condition. D values associated ALP inactivation under microwave heating were therefore an order of magnitude lower than under conventional thermal heating. The results thus emphasize that ALP inactivation occurred much faster under microwave heating condition than under conventional heating thereby confirming the existence of enhanced thermal effects from microwave. Because of the enhanced effects, MW pasteurization would reduce the severity of the treatment and hence potentially offer pasteurized milk of higher quality.