From the NMC Newsletter "Udder Topics", February 1997

Bulk tank milk analysis is an important tool for troubleshooting high bacteria counts in farm milk. These tests and interpretation methods provide an indication of whether high bacterial counts are due to mastitis, pre-milking hygiene, equipment cleaning and sanitation, or incubation of bacteria in the milk handling system during milking. This is invaluable information to the dairy producer and processor. The recommended tests include:

Standard Plate Count (SPC): The Standard Plate Count is the number of colony forming units in one ml of milk when incubated for 48 hours at 32° C (90° F). The SPC should be less than 5,000 if cow and equipment sanitation is good and cooling is adequate. A SPC of less than 1,000 indicates excellence in all of these areas. Most industry standards require a SPC of less than 50,000.

High bacteria counts may result from Streptococcus agalactiae mastitis infection in the herd. If the SCC and SPC are both high, a thorough bulk tank culture should be performed to determine the type of mastitis organisms present in the milk. This information is useful to manage mastitis in the herd. Other types of bacteria represent contamination from the environment. These organisms are transported during milking from the skin of the udder into the milk and onto milk handling equipment. These bacteria multiply during the milking process and may continue to multiply between milkings if they are not removed or killed.

Lab Pasteurized Count (LPC): The Lab Pasteurized Count is the number of bacteria per ml of milk which survive laboratory pasteurization at 62.8° C (143° F) for 30 minutes. This procedure kills the usual mastitis-causing bacteria leaving only those organisms from the environment which can survive elevated temperatures. These types of organisms will grow and multiply in the milk handling equipment if cleaning and sanitation procedures are inadequate. The LPC should be below 100 to 200 if equipment cleaning and sanitation are good. A LPC below 10 indicates excellent equipment hygiene.

Coliform (Coli): The major source of coliform bacteria in bulk tank milk is transportation on the udders of cows from the environment. The Coli count thus provides an indication of both the effectiveness of cow preparation procedures during milking and the cleanliness of the cows’ environment. Coliform counts between 100 and 1,000 are generally an indication of poor milking hygiene. Coliforms will also incubate in residual films left on milk contact surfaces. Coliform counts in excess of 1,000 suggest incubation in milk handling equipment. A Coli count less than 100 per ml of milk is considered acceptable for raw milk for pasteurization. In states where raw milk may be sold to consumers, Coliform count must be less than 10/ml. Coli counts less than 10 indicate excellence in both pre-milking hygiene and equipment sanitation.

Another test which indicates the cleanliness of cows when they are being milked is the sediment in the bulk tank milk. A sediment level less than 1.50 mg per gallon is considered acceptable.

Source: 1997 National Mastitis Council Annual Meeting Proceedings, pg. 65.