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Thermal Inactivation Characteristics of Alkaline Phosphatase in Ultrafiltered Milk¹

Dairy Science Department, South Dakota State University, Brookings 57007

ABSTRACT

The objectives of this study were to determine the heat inactivation kinetics of alkaline phosphatase in UF milk and to study the effect of concentration of UF milk on heat inactivation of alkaline phosphatase. Equal quantities of alkaline phosphatase were added as raw milk to pasteurized skim milk and UF skim milk retentates (10.01 and 16.61% protein). Residual alkaline phosphatase was measured in all milk samples heated to 60°C for 20, 30, and 40 min and to 63°C for 10, 20, and 30 min. Heat inactivation of phosphatase was more rapid in the 16.61% protein retentate than in non UF skim milk and 10.01% protein retentate at both 60 and 63°C and for all holding periods. The inactivation constant for the 16.61% protein retentate was higher than for skim milk and 10.01% protein retentate. Addition of lactose to 16.61% protein retentate increased the heat resistance of alkaline phosphatase. Results suggest that the removal of lactose during UF alters the heat inactivation characteristics of alkaline phosphatase; thus, the use of this enzyme to measure the efficiency of pasteurization of highly concentrated UF milk is questionable.

¹ Published with the approval of the Director of the South Dakota Agricultural Experiment Station as Publication Number 2350 in the Journal Series.