Fluid raw and pasteurized milk products are routinely sampled under Pasteurized Milk Ordinance (PMO) requirements. The sampling tests for added water, heterotrophic bacteria, coliform bacteria, the presence of indicator enzymes, somatic (body) cell counts, and antibiotic or drug residues. These tests help to ensure that the milk received by retailers and consumed by end-users is of the highest quality.Samples are taken by bulk milk tank haulers, pasteurization plant personnel, and inspectors.
Raw milk from farms for processing into pasteurized milk is sampled to ensure that it meets requirements for Grade A raw milk as specified in §349 of Part VII of Title 51 of the Louisiana Administrative Code. These requirements include ensuring that the raw milk is held at 45°F, has a standard plate count of <100,000 cfu/has a somatic cell count of <750,000 cfu/mL, and has negative results for drug screens. If the milk does not meet these standards, this can be indicative of a less-than-optimally-healthy herd or subpar sanitary practices such as not having clean udders or milking equipment. After two high counts of bacteria or somatic cells, a third high count triggers a "degrade," which means that the milk is classified as Grade C and cannot be sold to pasteurization plants for processing. A positive drug screen would require confirmation, but once it is confirmed, the milk must be destroyed, the dairy must be "degraded," and the department must investigate to determine the root cause of the positive result (this is called an Appendix N investigation after the appendix in the PMO that describes this process). Typically, a positive drug screen indicates that a sick cow was not kept out of the milking rotation long enough for the treatment drugs to no longer show up in her milk.
Raw milk from dairies is tested 11 out of 12 months of the year. In a state like Louisiana that has relatively-mild winters but long and exceptionally hot summers, there is natural fluctuation in test results for items like somatic cell counts. As cows become more stressed (as in high-temperature conditions), they naturally shed more somatic cells in their milk.
Potable water on dairy farms is tested on an annual basis for coliform bacteria. Any positive coliform test requires treatment of the well water until a negative result is obtained.
Milk Pasteurization Plants
Finished milk is Grade A fluid milk for distribution to retailers. This is the milk that you buy in stores. This milk is tested for coliform bacteria, other bacteria, drug residue and phosphatase. The presence of bacteria may be indicative of a failure of the pasteurization process (which would kill both pathogenic and nonpathogenic bacteria in the milk under normal circumstances) or it may signal a breakdown in the cleanliness and sanitization practices at that facility. A third consecutive positive bacterial test would result in the plant being "degraded," meaning that it would be unable to ship milk out-of-state until the problem is corrected.
A positive drug residue screen can be a complicated problem to resolve; if it is confirmed, because milk from different dairies is typically blended prior to processing at the plant, each contributing dairy's raw milk would have to be tested to determine the source of the drug. That dairyman would then be responsible for the cost of that load of milk not only to himself but to every other dairy that contributed to that load, and the department would have to conduct an Appendix N investigation, as described above.
Phosphatases are a category of enzymes that represent an important part of celullar metabolism; our bodies (and cows' bodies) couldn't function properly without them. However, enzymes like phosphatases are typically denatured (broken down into constituent parts) by the high temperatures achieved by pasteurization processes (which can range from 145°F - 161°F). Therefore, the detection of phosphatase in pasteurized milk generally indicates that there may be a problem with the pasteurization process--either the milk is not being heated to the proper temperature or it is not being held at that temperature long enough. If a positive phosphatase test is confirmed, the product would need to be recalled (if any has left the plant) and reworked or destroyed. The plant would then need to take steps to discover the reason for the issue.
Part of the pasteurization process relies of the use of plate heat exchangers. Heat exchangers are devices that rely on the physical principle that heat energy will flow from an area of high concentration to an area of low concentration until equilibrium is achieved. In order to rapidly cool the milk that is leaving the holding tube and entering the cooler, plants employ heat exchangers that are supplied with chilled cooling water. Due to the design of the heat exchangers, there is a chance that a mechanical fault could cause the cooling water to come into contact with the pasteurized product. For this reason, cooling water is also required to be tested for coliform bacteria every six months.
Potable water at pasteurization plants is also tested for coliform bacteria annually.