Is My On-farm Milk Pasteurizer Working? – Pat Hoffman, Vita Plus

By Pat Hoffman, Vita Plus dairy technical specialist

Answer: Alkaline phosphatase activity

That answer may come as a surprise to most dairy farmers and calf growers who own and operate an on-farm waste milk pasteurizer.  Most commonly, we think of using standard plate count to test pasteurization because it is used in milk grading and raw milk sales.  But, like most things, there is more to the story.

Yes, standard plate count is used to monitor milk pasteurization, but alkaline phosphatase (ALP) activity is also used to evaluate milk pasteurization because the ALP test is specific to temperature, low cost, rapid, accurate and abundantly available.

ALP is an enzyme naturally present in raw milk and its activity is used as an indicator of proper milk pasteurization.  When milk is properly pasteurized, temperature and temperature duration will inactivate the ALP enzyme. Because the heat stability of the ALP enzyme is greater than survivability of most milk pathogens, the enzyme serves as a marker that milk was heated to a proper temperature.

Sample outcomes are typically not numerical, but are simply presented as “negative ALP activity.”  A negative ALP activity test means the milk was heated to a high enough temperature and the temperature was maintained long enough to inactivate the ALP enzyme.  As such, the temperature was also high enough and maintained long enough to kill milk pathogens.  While ALP activity is seldom expressed numerically, a value of greater than 1 microgram of ALP activity is indicative of improper pasteurization. The ALP assay is very sensitive and can find 0.1 percent raw milk contamination in pasteurized milk.

Testing for ALP in milk is commonly done at laboratories that support our milk processing industries.  For example, AgSource Laboratories routinely conducts a multitude of milk tests, including the ALP test, and offers a full testing package for on-farm waste milk pasteurization efficacy for a very reasonable cost.  The testing package includes standard plate count, alkaline phosphatase activity, fat, protein, lactose, solids, and seven common individual bacterial species.  It is important to recognize that combining an ALP test with a standard plate count is especially informative because the ALP defines if the milk was heated properly and the standard plate count is reflective of the total bacterial load either pre- or post-pasteurization.

Is your on-farm milk pasteurizer as good as the milk plant’s?  If so, an on-farm milk pasteurizer would have to pass standard plate count, coliform and ALP criteria tests to make a proper comparison.  For more information on ALP, standard plate count and other pasteurized milk tests, contact your Vita Plus consultant or AgSource Laboratories (715.898.1408).