Milk produced on 30 grade-A farms was analyzed bacteriologically after the following storage treatments: less than 2 hr; 3.3 and 7.2 C for 1, 2, and 3 days, and 3.3 C for 54 hr followed by preliminary incubation at 12.8 C for 18 hr. The effects of these storage treatments were determined with the following bacterial tests: Standard Plate, thermoduric, coliform, total, psychrophilic, and enterococcus counts.
The Standard Plate and total counts showed essentially the same response to sample storage. Preincubation and storage at 7.2 C for 3 days were the only storage treatments that caused a marked change in the counts; with these treatments, the numbers more than doubled. The coliform count response to storage was similar to that of the Standard Plate and total counts, except coliforms decreased when the sample was stored at 3.3 C. The psychrophilic count showed the most marked increase, of any of the tests, to sample treatments. After storage for 1 day, the psychrophiles increased, especially at 7.2 C; there was more than a 10-fold increase during storage at 7.2 C for 3 days and during preliminary incubation. The thermoduric and enterococcus counts did not change a statistically significant amount during sample storage.
These results emphasize the importance of maintaining milk at temperatures of 3.3 C or below and not attempting to hold milk too long. The potential spoilage problem that psychrophiles may present is shown.
1Journal Paper No. J-6021 of the Iowa Agriculture and Home Economics Experiment Station, Ames, Iowa. Project 1050.
2Present address: American Dry Milk Institute, Inc., 130 N. Franklin St., Chicago,-Ill. 60606.