Invited review: The application of alkaline phosphatase assays for the validation of milk product pasteurization

Standard practices for indirectly assessing the pasteurization status of milk products are primarily based on the thermal inactivation kinetics of the endogenous milk enzyme, alkaline phosphatase (ALP). This assessment provides an invaluable, if not required, tool for both regulatory and in-house process control and validation. Endogenous milk ALP manifests a slightly higher heat resistance than the pathogenic microflora upon which pasteurization time and temperature requirements are based. Hence, ALP activity is recognized and accepted as the method of choice for the rapid validation of milk product pasteurization. However, ALP assays have notable limitations that must be understood if they are to be administered and interpreted correctly and the results are to be applied judiciously. Issues such as the reactivation of heat-denatured ALP and the presence of both heat-stable and -labile microbial ALP are addressed. A discussion of ALP in the milk of nonbovine species is presented based on the limited literature available. Some discussion of research involving alternative pasteurization indicators also is presented. This article is intended to summarize the pertinent details of the ALP assay for dairy products (noting the basis and limitations of various methods) and the processing, handling, and known compositional factors that influence the assay results.