Laboratory Test Procedures for Raw and Retail Milk

Standard Plate Count

Milk and dairy products are generally very rich in nutrients which provide an ideal growth environment for many microorganisms. Therefore, all milk samples are carefully maintained with a temperature of 0.0º to 4.5ºC from collection until the samples arrive in our laboratory within 60 hrs from the time of collection. The standard plate count (SPC) method is the first test performed on a milk sample after it arrives in our laboratory. This test is used to evaluate the general sanitation of the dairyman’s equipment and the overall health of his herd. This method is one of the procedures used throughout the dairy and regulatory enforcement agencies for the determination of quality and the detection of microbial contamination for raw and retail (processed) milk. For a raw milk sample the total bacterial count must be less than or equal to 100,000/ml. For a retail product the total count must be less than or equal to 20,000/ml or gram. For frozen desserts the total bacterial count must be 50,000/gram or less.

Detection of Antibiotic/Drug Residues for Raw and Retail Milk

Common practice for management of livestock, including dairy animals includes antibiotic drug therapy. Residues of these drugs may enter the milk supply if the cow is continued to be milked. Regulations require that milk from the treated animal must be withdrawn for a prescribed time. When this is not followed, the contaminated milk containing the drug residues may be sent to the plants. Milk containing such antibiotics cannot be used for human consumptions, due to possibly triggering hypersensitivity reactions in susceptible individuals. Regulatory standards and customer specifications are met by our laboratory to ensure that milk from these antibiotic treated cows does not make it to market.

Raw Milk-Standard Plate Count Method

Somatic Cell Count – used to detect abnormal raw milk

Regulations require that raw milk for commercial sale be derived from one or more healthy cows. Abnormal milk includes milk from cows with mastitis. Screening and confirmatory tests estimate the number of somatic cells in milk. These cells, which include white blood cells and tissue cells are associated with the inflammation of the mammary glands, but are not intended for the diagnosis of mastitis. High counts can help the dairyman determine if there is a problem with the herd health, therefore affecting total milk production and his income. Somatic cell counts greater than 750,000/ml are reported to the State and will result in a warning letter or off-grade category, which will put a restriction on the sale of that particular dairy’s milk.

Freezing Point for Raw and Retail Milk

Pure water freezes at 0.0ºC. The average freezing point for normal raw milk has been accepted to be -0.540ºH. Our laboratory uses a cryoscope to determine the freezing point of the sample. The cryoscope analysis is used to determine whether or not water has been added. Regulations have tolerances for added water, and penalties are imposed on producer’s cryoscope reading under -0.525ºH.

Coliform Test for Retail Milk

This procedure is used by our laboratory to measure the quality of the practices used during the plant processing of retail dairy products. Coliforms detected from pasteurized products can often suggest improper pasteurization or post pasteurization contamination. Coliform tests are conducted following pasteurization primarily to detect bacterial recontamination of the milk. Retail milk products must have a coliform count less than 10/ml or gram. Frozen desserts must have a coliform count less than 40/gram to pass State regulation.

Phosphatase for Retail Milk

This test is used to determine that all retail products have been pasteurized properly by examining the products for alkaline phosphatase enzyme, which is normally destroyed during the pasteurization process. This flurometric procedure used must obtain a reading of less than 350mU/L, (3 micrograms) per liter (0.075% raw milk equivalent). This test procedure is capable of measuring down to 0.003% raw milk.

Aflatoxin for Raw Milk

Our laboratory also tests individual dairies for aflatoxin. Aflatoxin is a naturally occurring mycotoxin produced by certain strains of the molds Aspergillus which are often found in feed grains consumed by dairy cows. When this mold is consumed by dairy cattle, it can be metabolized and resides primarily in the milk. Aflatoxins are known carcinogens. FDA has set the following limits for Aflatoxin: Food and feed products-20ppb; Milk- 0.5ppb. Regulatory labs routinely screen raw producer samples for the presence of Aflatoxin to help the State monitor actionable levels. Dairy milk supplies with above-actionable levels are officially detained and removed from the market until subsequent testing indicates that the milk is either free of Aflatoxin or at below-actionable levels and the milk can be release for human consumption.


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