Bacteria numbers in milk are routinely monitored. In order to be sold for Class I (beverage milk) purposes, the number of bacteria must not exceed 100,000 colony-forming units per milliliter.
Many fluid milk processors have far stricter standards than that.
Typically, milk that comes from a healthy, infection-free udder will contain less than 1,000 cfu/ml, and many dairies will consistently achieve bulk tank bacteria levels in this range. Certainly, bacteria numbers less than 10,000 cfu/ml should be achievable.
Processing plants conduct a number of tests to measure bacterial levels. Each of these tests tells us something different about the nature of the bacteria involved.
The standard plate count (often referred to as the SPC or raw count) is the most common test, measuring the number of bacteria in the bulk tank when milk is picked up at the farm.
Many labs will run preliminary incubation (PI) counts, lab pasteurized counts, and coliform counts. These are useful in identifying the specific types of bacteria present in the bulk tank count. Knowing something about the actual type of "bug" involved can be very helpful in focusing on the problem.
A PI test, measuring the number of bacteria that grow at cool temperatures (55 degrees F), provides a good indication of on-farm sanitary conditions.
Lab pasteurized counts are used to estimate the number of bacteria that survive pasteurization. Organisms that grow on this test tend to be associated with poor cleaning equipment and solids buildup, or rubber components that are old and cracked.
Coliform counts are typically associated with manure contamination. Poor parlor hygiene or pre-milking udder prep can raise these levels.
Remember, if you see elevations in coliform or PI counts, don't forget to check the water supply. Con-taminated water sources can affect these tests.
These tests allow us to focus more quickly on possible sources of the problem, such as equipment, cows, environment or management practices. Certain conditions, such as slow cooling of milk, growth of bacteria in liners and milk filters, as well as residual milk films, may contribute to the problem as well.
Mark Wustenberg is a technical services veterinarian with Monsanto Dairy Business.