A positive phosphatase test on ice cream could indicate which of the following?

• A. Improper pasteurization
• B. High bacterial count
• C. Reactivated phosphatase
• D. False-positive test caused by ingredients

Answer: (A)

This test checks whether pasteurization was thorough by measuring residual alkaline phosphatase activity. Alkaline phosphatase is present in raw milk and is inactivated by proper heat treatment, so a positive result after processing indicates the heat step did not reach the required time–temperature, meaning improper pasteurization. This is a direct signal about the effectiveness of the pasteurization step, not just overall bacterial load, because the enzyme is more heat-stable than many hazards and should be inactivated if pasteurization is done correctly.

High bacterial counts would require a different test to quantify microbes, not the phosphatase test. Reactivated phosphatase isn’t a typical or reliable concept after pasteurization because inactivation of the enzyme is generally irreversible under processing conditions. Ingredients causing false positives are not the intended interpretation of this test, though some interference can occur in practice, it’s not the primary meaning of a positive result.