Milk from 30 grade-A farms was subjected to bacteriological tests including the Standard Plate, total, coliform, psychrophilic, thermoduric, and enterococcus count, resazurin reduction time, and leucocyte count to determine the correlation between these tests and farm production conditions. Farm conditions were evaluated at sample collection time by a farm score, which was based mainly on sanitation.
After collection and immediate transportation to the laboratory, half of each sample was stored at 3.3 C for 72 hr, the remainder was preincubated at 12.8 C for 18 hr after storage at 3.3 C for 54 hr; determinations were then performed. The leucocyte count was determined on the fresh sample.
The psychrophilic count was the only bacterial test that showed significant correlation with the farm score. For samples stored at 3.3 C for 72 hr, all comparisons among bacterial counts showed significant correlation except: psychrophilic count vs. resazurin reduction time; coliform count vs. resazurin reduction time; and, coliform count vs. enterococcus count. For preincubated samples, all comparisons among bacterial counts showed significant correlation except: psychrophilic count vs. resazurin reduction time; coliform count vs. resazurin reduction time; and, coliform count vs. thermoduric count. Higher correlations were obtained on the preincubated samples for all bacterial tests except the thermoduric count. Within this experimental design, preliminary incubation did not improve the ability of the bacterial tests to show statistically significant correlation with the farm score. The leucocyte count showed significant correlation with the farm score, but not with the bacterial test results.
Evaluation of data shows that the bacterial test results are not highly correlated with farm production conditions as measured by farm score. Milking-time inspections are necessary to assure that recommended practices are used in grade-A milk production.
1Journal Paper No. J-6017 of the Iowa Agriculture and Home Economics Experiment Station, Ames, Iowa. Project 1050.
2Present address: American Dry Milk Institute, Inc., Chicago, Ill. 60606.