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The Development of a Positive Phosphatase Test in Refrigerated, Pasteurized Cream

Department of Agricultural Bacteriology, University of Wisconsin

ABSTRACT

The results of these studies on the development of a positive phosphatase reaction in refrigerated, pasteurized cream may be summarized as follows:

1. Commercially pasteurized creams, regardless of their butter fat content, changed in their reaction to the phosphatase test from negative to positive after storage for three to four days at 4° or 10° C. and while still in a salable condition.
   
2. Cream from different sources varied in the length of time of heat treatment at 62.2° C. required to obtain a negative phosphatase test.
   
3. When the temperature of the heat treatment for 30 minutes was increased, or the time of heat treatment at 62.2° C. was increased, the period of storage at 10° C. required for the development of a positive phosphatase reaction in the cream was increased.
   
4. Pasteurized cream samples which developed a positive phosphatase reaction after storage at 10° C. or 4° C. always contained Gram-positive, spore-forming bacilli that produced phosphatase and were identified as strains of Bacillus cereus and Bacillus mesentericus. These organisms were not detected in samples negative to the phosphatase test.
   
5. The positive phosphatase test on pasteurized, refrigerated cream apparently was not caused by bound or protected phosphatase adsorbed to the milk fat.
   
6. When mercuric chloride was used to inhibit bacterial growth, no positive phosphatase reactions developed in samples of pasteurized cream or in steamed or pasteurized cream inoculated with phosphatase-producing organisms.
   
7. When pure cultures of these strains of Bacillus cereus and Bacillus mesentericus were inoculated into cream, the rapidity with which a positive phosphatase reaction developed depended upon the number of organisms and the amount of bacterial phosphatase added by the inoculum.
   
8. The strains of Bacillus cereus and Bacillus mesentericus were able to grow and produce phosphatase at 4° and 10° C. in pasteurized cream.
   
9. A positive reaction was obtained on pasteurized cream when the microscopic count of these phosphatase-producing organisms was between 1.48 and 7.8 millions per milliliter.
   
10. The production of bacterial phosphatase in cream increased in rapidity and less time and fewer organisms were required for the development of a positive phosphatase test as the severity of heat treatment of the cream decreased from steaming to 62.2° C. for 30 minutes.
    
11. Bacterial phosphatase was more active at pH 9.7 than was milk phosphatase; both kinds of phosphatase showed activity in the same general range of pH values.
    
12. In cream, bacterial phosphatase withstood heat treatments as high as 76.7° C. for 30 minutes, while milk phosphatase was inactivated in 30 minutes at 62.8° C.
    
13. Bacterial phosphatase could be distinguished from milk phosphatase by reheating a sample of cream for 30 minutes at temperatures above those normally used for pasteurization but below 76.7° C, and then testing for phosphatase activity by means of the New York Rapid Field Test.
    
14. The change of phosphatase reaction is of importance since it takes place within two or three days after pasteurization even though the cream is properly refrigerated and is still in a salable condition. Hence, erroneous conclusions may be drawn as to the previous heat treatment of the cream.