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Detecting Recontamination of Pasteurized Milk by Bacteriological Methods^*

New York State Agricultural Experiment Station, Geneva, New York

ABSTRACT

A study of 271 samples of raw and pasteurized milk collected over a one year period at two pasturizing plants by means of standard agar plate counts and plate counts of colon organisms showed that the pasteurized milk from one plant was frequently recontaminated before it was bottled. While recontamination was usually indicated by both standard agar plate counts and counts of colon organisms, there were instances in which one or the other of the two methods indicated recontamination when the other method did not. Therefore, the test for organisms of the colon group should supplement and not supplant the standard agar plate count.

Sources of recontamination as indicated by counts of colon organisms from 92 special samples were (1) failure to steam milk lines leading from outlet valves of batch pasteurizers (2) improper cleaning and sterilization of the milk pump, and (3) leakage of milk and water from a sanitary connection contaminated by human sources into the bowl of the bottler. The latter source was potentially dangerous.

Properly pasteurized milk free from recontamination did not contain colon organisms in 1 cc quantities.

^* Approved by the Director of the New York State Agricultural Experiment Station for publication as Journal Paper No. 118, January 4, 1936.

¹ Student Assistant.

² Associate in Research.

³ Milk Inspector and City Bacteriologist, Geneva, New York.