Science in the Small Dairy Industry:
Milk Sediment Testing

According to the United States Department of Agriculture (USDA) National Agricultural Statistics Service (NASS), milk as a commodity was worth $39 billion in 2011.¹ As of 2009, NASS reported that the trend in dairy was toward large scale operations, defined as a milk cow ranch with greater than 500 head of cattle.² However, in 2009, more than 65,000 small dairies in the US were still operating. The small dairies are less likely to use fully automated clean-in-place machinery to prevent contamination. In 2010, the USDA rescinded its requirement that large-scale dairies test their milk for sediment contamination, but smaller dairies collecting milk in cans must continue to follow established guidelines. These small-scale dairies must prove to authorities that their product is clean by allowing milk inspectors to perform official milk sediment testing once per month. Still, the owner of a small dairy may want to perform their own sediment testing more frequently to address contamination issues before the inspector arrives.

Pathogenic Microbes in Human Illness

Before the turn of the century, the role of pathogenic microbes in human illness was discovered by Dr. Robert Koch.³ Dr. Koch and his contemporaries began developing testing methods for the detection of pathogens in water, food, and other products that came in contact with the general public.

With regard to milk, the presence of sediment was highly correlated to bacterial contamination. By 1914, there were twelve widely used methods for detecting contamination in milk alone, falling into two groups: filtration and centrifugation based methods.⁴ Although pasteurization would kill off the bacteria, the byproducts of the contamination remain, causing foul flavors and odors. If milk contains too much sediment, it is rejected by milk inspectors, and steps must be taken to prevent sediment contamination before the supplier’s milk will be deemed suitable for public sale.

In 1952, the Association of Official Agricultural Chemists (AOAC) published a method adopted by the USDA⁵ known as the Milk Sediment Test. It uses a special vacuum filtration apparatus and specific filter papers to collect the sediment. The captured sediment is then graded on a qualitative scale by comparing the sediment disk against the USDA photographic standards.

The Milk Sediment Test requires several parts, including the sediment testing apparatus, sediment filter papers, a vacuum filter flask, a vacuum pump, stopper, and tubing. Some sediment testers have an integrated aspirator pump (a type of vacuum pump). One of the most common variations of the method requires that a one-gallon sample be filtered through a 1.25 inch filter. The milk sediment testing apparatus has been designed to prohibit any milk from bypassing the filter paper. The vacuum pump pulls the milk through the filter at a much faster rate than gravity alone can accomplish, so testing each bulk tank at least three times, as proscribed by the AOAC method, doesn’t take most of a day. Milk sediment filter papers are available in white and black, and it is recommended to use both. The white paper works well for black or colored particulates. The black paper is best for white or clear particulates.

Once the AOAC method becomes part of normal operations, it can be used to identify sources of contamination at every step in the process, from poor cow hygiene to inadequately washed containers to improper cleaning of milking stalls. Milk sediment disk analysis is easily performed by stereo or bright field compound microscopy, because particulates can often be identified by their appearance.

For assistance with milk sediment filtration product selection, please contact a Cole-Parmer Application Specialist.

For more information on the dairy industry, including small dairies and milk standards, go to www.usda.gov.

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¹“National Statistics for Milk,” Retrieved September 28, 2012 from interactive database at http://www.nass.usda.gov/Statistics_by_Subject/ > Sector: Animals & Products > Group: Dairy > Commodity: Milk >Date Option: Milk - Production
measured in $

² “Large Operations Increase Share of Inventory and Milk Production, Retrieved September 28, 2012 from http://usda.mannlib.cornell.edu/usda/current/ USDairyIndus/USDairyIndus-09-22-2010.pdf

³ Robert Koch, Investigations into the Etiology of Traumatic Infectious Diseases, trans. W. Watson Cheyne (London: The New Sydenham Society, 1880).

⁴ M. C. Schroeder, “Dirt Sediment Testing – A Factor in Obtaining Clean Milk,” American Journal of Public Health, January 1914, Pages 50-64.

⁵ “AOAC Official Method 952.21 Sediment in Milk,” Retrieved September 26, 2012 from http://www.aoac.org/omarev1/952_21.pdf