Bulk tank milk testing as a method for measuring herd milk quality has been around for decades. It is included in the Pasteurized Milk Ordinance (PMO), so every producer knows that samples are being taken daily to assess how he or she is doing with udder health and milk quality. Despite the common knowledge, there is often confusion around what the different values on a processor milk quality report actually mean and how they are impacted, either positively or negatively, by events happening on the dairy.
I frequently receive questions about the different test name acronyms and how to interpret what those results tell them. More importantly, I am asked how producers and industry service providers troubleshoot high counts. A basic understanding of the different ways in which bulk tank milk may be tested and what those tests measure is extremely helpful in understanding what impacts milk quality on the farm.
Quantitative milk testing
Quantitative milk testing, at either the individual cow or bulk tank level, is intended to give an accurate measurement of bacteria or cells per milliliter of milk but does not provide bacterial identification. These tests are mostly used by milk processing plants for assessing milk quality, to ensure compliance with the PMO and with any individual state or processor requirements. All quantitative milk testing protocols come from the Standard Methods for the Examination of Dairy Products and are conducted in regulatory laboratories certified by the Food and Drug Administration and Interstate Milk Shippers (FDA-IMS) across the country.
These protocols and the certification process are aimed at ensuring these tests are being performed identically between laboratories to produce consistent results. Every dairy producer shipping milk under the PMO must meet standards for somatic cell count (SCC) and standard plate count (SPC) to be included in the Grade A market, and as such, it is these tests which are most familiar.
SCC
All dairy producers have some appreciation for somatic cell counts, at least in the sense that low numbers are “good” and high numbers are “bad.” Bulk tank SCC is driven by the number of leukocytes (white blood cells) present in the milk and is indicative of infected quarters, so this number gives us an estimate of the level of mastitis in the herd. While we expect infected cows to be the primary drivers of SCC and much less likely to be equipment- or environment-related, these other factors may play critical roles in terms of spreading or creating infections and must be taken into consideration. Somatic cell count is the most common metric used to assess udder health, but it is only one metric. Understanding the different tests and types of bacteria counts is where uncertainty may enter the picture.
SPC
The definition of the SPC is a quantitative measurement of the number of bacteria (cells) per milliliter of milk and is expressed as cells per milliliter. However, this test has many names and may also be referred to as a plate count (PC), plate loop count (PLC), aerobic plate count (APC), plate aerobic count (PAC), total bacteria count (TBC) or just simply the “raw count.” Over the years, I have seen all these acronyms used on reports and it is important to be clear that all of these tests are a measurement of the number of bacteria per milliliter of milk, under standard growth conditions.
Diagnosing the problem
Factors impacting the raw bacteria count may be infected quarters, equipment issues, environmental issues or, most commonly, combinations of all the above. Troubleshooting bacteria counts is often complex and multifactorial.
When considering sources of high bacteria counts, we often default to assuming cleanliness issues, dirty cows, dirty milking equipment, refrigeration problems and/or inadequate milking preparation are the culprits. While this can absolutely be the case, infected quarters are also more than capable of skyrocketing bacterial counts.
For example, streptococcal infections are known as “high shedding” infections, which means the infected quarters shed a very high number of bacterial colonies into the milk. A single infected quarter is capable of shedding bacteria in the tens of thousands or even millions, and in smaller herds, a single cow may be capable of pushing the raw count above the legal limit. These high-shedding phases of the infection are not indefinite, and the high counts will eventually drop as the cow’s immune system pushes back on the infection, creating “intermittent” shedding patterns as this cycle repeats.
This pattern of seemingly random spikes in bulk tank SPC is common and a source of frustration to producers when the source of the bacteria is unknown. If high bacterial counts are occurring because of mastitis organisms from infected quarters, no amount of cleaning the pipeline will alleviate the issue.
Some like it hot, some like it cold
Processors may also require other quality tests, such as lab pasteurized count (LPC), preliminary incubation count (PIC) or total coliforms (TC), which all fall under the umbrella of quantitative milk testing. Unlike the raw bacteria count, which quantitates the bacteria count under normal growth conditions, these tests are measuring growth under temperature-specific conditions.
Additionally, these three tests fall outside of the PMO. While it is common for processors to require them, they may not be required in all milk markets. The PIC and LPC tests are quantitative measurements of the number of psychrotrophic and thermophilic bacteria, respectively, per milliliter of milk. These two categories of bacteria can be problematic for milk processing plants as spoilage organisms in finished product.
Psychrotrophic
Psychrotrophic bacteria are “cold-loving” bacteria and capable of growth at temperatures equal to or lower than 44.6ºF (7ºC), which allows them to continue to multiply at refrigeration temperatures. PI counts are considered significant if found to be three times the raw bacteria count. High PI counts present risk of spoilage or shortened shelf-life, especially if fluid milk products are contaminated after the pasteurization process. These bacteria do not survive pasteurization and are less likely to be a result of infected quarters. Typically, high PI counts are a result of dirty milking equipment, inadequate machine wash cycle, water quality problems or inadequate cooling.
Thermophiles
In contrast, thermophiles are a heat-resistant category of bacteria known as spore-formers and are capable of surviving pasteurization. High LPC results are frequently a product of dirty equipment and/or dirty cows milked into the tank but very rarely an animal health concern. While it is very common to find these spore-forming bacteria in the dairy environment, they present a risk to the quality of finished products if present in elevated numbers in the bulk tank milk.
Thermophiles present a serious problem for processing plants as both a spoilage concern and a human food-borne pathogen, due to potential survival of the pasteurization process. This is especially true with milk powder products, so milk processing plants in those markets may have strict LPC requirements.
Finally, the total coliform (TC) test is a quantitative measurement of total coliforms per milliliter of milk. This test is the least likely to be required by conventional processing plants but is more common in organic and raw milk markets. Coliform infections in cows may contribute to overall TC counts, but these results are most likely to be driven by inadequate milking routine preparation and milking dirty cows into the bulk tank.
Utilizing the information
Diagnosing the cause of high counts from these types of quality tests often requires a complex investigation of environmental sources. This includes water quality, evaluation of milking routine practices and equipment wash-up analysis. So how can producers and professionals alike troubleshoot these different counts and make better management decisions? The key is to identify the causative organism driving the high counts.
Qualitative testing
Qualitative bulk tank culture delivers bacterial identification, along with a relative differential count of the organism classes present in the sample. A comprehensive bulk tank culture can ideally detect and give names to all the important bacteria present in a bulk tank milk sample, with particular emphasis on sensitive detection of contagious mastitis pathogens. This complex level of detection requires use of multiple selective agars, enhancement methods and molecular confirmation of some bacterial species, which is beyond the scope of many laboratories.
Bulk tank culture test methods are not a regulated procedure and test results will likely vary to some degree between laboratories. The sophistication and detection capability of the protocol used will significantly impact results, which can create confusion and risk for producers if important mastitis pathogens are misidentified or remain undetected in the herd. To help avoid this, it is recommended to use laboratories following National Mastitis Council (NMC) standards and protocols.
Application
Bulk tank testing is both a requirement of producing milk for the Grade A market and an immensely powerful monitoring tool. The quantitative tests give real-time measurements of milk quality on the farm, but should be paired with qualitative bulk tank culture to identify the agents of disease or high counts on the dairy. Identifying the agent directs you to the solution and allows for a plan of action.
