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The Keeping Quality of Pasteurized Milk in the New York Metropolitan Area during Cool Weather as Determined by Bacterial Counts, Presence of Coliform Bacteria, and Flavor Scores^*

Department of Dairy Industry, Cornell University, Ithaca, N. Y.

ABSTRACT

Pasteurized milk was collected in October from 6 milk plants processing about 800,000 quarts of milk daily for the New York Metropolitan area. The freshly pasteurized milk as collected at the plants had an initial standard arithmetic average plate count of about 12,000 colonies per ml. There was 0 or 1 quart of milk out of 18 samples on each of 3 consecutive weeks showing presence of coliform bacteria and a total of 3 positive samples out of 54 quarts as determined by 2 positive desoxycholate agar plates or by 2 positive brilliant green fermentation tubes out of 3, using 1 ml. of milk per plate or tube. From the 54 bottles of freshly pasteurized milk there were 108 desoxycholate plates poured, of which 9 were positive and there were 6 positive brilliant green bile tubes out of 162. The flavor score of the milk was 40 or "excellent."

Holding the freshly pasteurized cooled milk 6 hours at room temperature to simulate failure to promptly refrigerate delivered milk showed some immediate effects. The standard plate count increased slightly to about 15,000. The number of milks positive for coliform bacteria on 2 plates or 2 tubes was 1 to 3 out of a total of 18 milks on each of 3 consecutive weeks. The flavor score was not changed.

Milk which had been iced and taken on a retail milk delivery route in October was returned to the plant in its original condition.

When the milk was stored at 35°–40° F. the standard plate count was slightly lower after 4 days than when the milk was fresh. The milk samples found positive for coliform bacteria remained constant in count. The flavor score held for 7 days in the "good" class with a score of 37.8 and with no pronounced off-flavors. These results were true for plant samples, plant samples held 6 hours at room temperature, and route return samples.

Milk stored at 45°–50° F. showed a slight decrease in standard plate count after one day. The arithmetic average count began to increase slightly after 3 days and showed a significant increase to 180,000 at the end of 4 days. The bacteria of the coliform group did not decrease and showed a slight increase in the 2-day-old sample. After 4 days the positive samples were 7 to 10 out of 18 total samples on each of 3 consecutive weeks, and several samples gave desoxycholate agar counts of 10,000 to 60,000 colonies. The flavor score held for 7 days as "good" with a numerical average of 38. The pasteurized cooled milk held for 6 hours at room temperature showed slightly higher bacterial counts and more positive coliform tests than shown by the plant samples.

Milk stored at 55°–60° F. showed a slight decrease, in standard plate count during the first day and at the end of the second day it had increased to an average of 134,000, and thereafter exceeded a million. At the end of the first day the milk showed coliform bacteria in 1 to 5 samples out of a total of 18 for each of 3 consecutive weeks. In 4 days the numbers had increased to 41 positives for 54 samples. The flavor was good for 3 days but on the fourth day the milk was unsalable due to high acid or sour flavors. The same results were secured for plant samples, plant samples held 6 hours at room temperature, and route return samples, except that samples held 6 hours at room temperature showed more samples positive to the coliform test.

The presence of coliform bacteria was determined most consistently by considering milk to be positive when at least 2 brilliant green bile tubes out of 3 showed gas with 1 ml. inoculations. There were some counts on desoxycholate agar when the bacteria in the milk produced no gas in tubes.The coliform bacteria began to increase in numbers as soon as the other bacteria at 45°–50° F. and 55°–60° F. The majority of quart bottles of pasteurized milk became positive after 4 days incubation at 55°–60° F. even though tests on the fresh milk were negative. Milk samples taken for coliform tests need to be iced promptly.

Milk collected in February kept as well in flavor as milk collected in October. Oxidized flavor did not develop in the milk as expected.

It was pointed out that refrigerators in the home should not be above 50° F. to best safeguard the healthfulness of milk.

The quality of freshly pasteurized milk and its keeping quality is known to be affected by season, and this study was limited to the cool fall month of October, and a severe winter month of February. The flavor and the bacterial counts should be determined again in the middle of summer to obtain complete information under all seasonal conditions.

^* Acknowledgments: Several of the leading milk companies in the New York—New Jersey Metropolitan area assisted in this study by providing the required samples of milk, by providing necessary laboratory facilities, and through research grants to Cornell University. The author is indebted to Eleanor Brusten and Marion Goldberg for making the bacteriological analyses.