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Determination of Improper Pasteurization by Applying the New York City Field Prosphatase Test to Cheddar Cheese^*

Kansas Agricultural Experiment Station

ABSTRACT

Data based upon a series of three independent laboratory trials, five semi-commercial trials and on 120 different lots of cheddar chesse have been obtained to evaluate the possible usefulness of the New York City field phosphatase test in detecting cheddar cheese made from improperly pasteurized milk. The principal results may be summarized as follows:

1. Freshly made cheddar cheese curd and the whey from the cheese were found to give a phosphatase reaction similar to the milk from which they were produced. The addition of as little as 0.25 per cent of raw milk to properly pasteurized milk resulted in positive phosphatase tests in the freshly made cheddar cheese curd produced from such milk.
   
2. No phosphatase activity was observed in many samples of aged cheddar cheese when the cheese was added directly to the buffered substrate in performing the phosphatase test. This was due to a lowering of the pH of the mixture. The pH of the mixture could be maintained at the optimum level for phosphatase activity by first mixing the cheese with buffered water and then adding a portion of the mixture to the buffered substrate.
   
3. In no case did a sample of cheddar cheese with an initial negative phosphatase reaction become positive during a ripening period of six months' duration.
   
4. After a six months' ripening period there was some tendency for phosphatase activity to diminish or become extinct in several of the cheese samples used in this study.
   
5. Control tests were found to be devoid of blue color without exception, thereby indicating that interfering materials which might give rise to false positive tests are not a problem in using the test with cheddar cheese.
   

^* Contribution No. 159, Department of Dairy Husbandry.