Bulk Tank Milk Testing: What Every Producer Should Know About Microbial Measures

Bulk Tank Milk Testing: What Every Producer Should Know About Microbial Measures

By Nicole Martin, PhD

Bulk tank milk samples are collected at every milk pickup or direct load. These samples serve several purposes, including determining producer payments, ensuring regulatory compliance, and calculating quality premiums.

Every sample must undergo a Standard Plate Count (SPC) for regulatory purposes. Most samples are also tested for additional microbial measures, including the Preliminary Incubation (PI) Count, Laboratory Pasteurization Count (LPC), and Coliform Count.

Each test measures a different group of bacteria and provides different information about milk quality and farm management.

Standard Plate Count (SPC)

The Standard Plate Count (SPC) estimates the total bacterial population in bulk tank milk. It serves as an overall indicator of how cleanly the milk was produced and handled on the farm.

SPC can be measured using traditional agar plate counts or flow cytometry methods, such as BactoScan. Regardless of the method, the test measures a broad range of bacteria introduced from several sources, including:

Cows with intramammary infections.
Environmental contamination, such as manure or soil.
Improperly cleaned milking equipment.

Bulk tank SPC is commonly below 10,000 cfu/mL, well under the regulatory limit of 100,000 cfu/mL.

A recent study from my laboratory found that the average SPC across 100 New York dairy farms sampled six times over one year was approximately 3,300 cfu/mL (Figure 1). This supports current recommendations that SPC should consistently remain below 5,000 cfu/mL.

Preliminary Incubation (PI) Count

The Preliminary Incubation (PI) Count has been used since the 1950s as an indicator of poor sanitation.

The test begins by incubating milk at 55°F for 18 hours, followed by a Standard Plate Count. A large difference between the SPC and PI count suggests inadequate equipment sanitation.

The incubation period encourages the growth of psychrophilic, or cold-loving, bacteria. Pseudomonas is the most common psychrophilic organism found in raw milk, although many other bacteria also grow under these conditions.

Because the PI test promotes the growth of cold-loving bacteria, it is especially sensitive to sample handling after collection. Results can be affected if samples:

Are not maintained at or below 45°F.
Are held longer than 24 hours before testing, even when properly refrigerated.

Laboratory Pasteurization Count (LPC)

The Laboratory Pasteurization Count (LPC) measures thermoduric, or heat-resistant, bacteria.

Developed in the early 1900s, the test originally helped identify poor equipment sanitation and estimate bacterial populations capable of surviving vat pasteurization, the primary pasteurization method used at that time.

To perform the test, milk is heated to 145°F for 30 minutes, the same conditions used for vat pasteurization, before conducting a Standard Plate Count.

Today, dairy processors pasteurize fluid milk using high-temperature, short-time (HTST) pasteurization at 161°F for at least 15 seconds.

Most thermoduric bacteria that survive vat pasteurization do not survive HTST pasteurization. Exceptions include spore-forming bacteria such as Bacillus and highly heat-resistant non-sporeforming bacteria such as Microbacterium.

For this reason, the LPC should not be used to predict which bacteria will survive HTST pasteurization or to estimate the shelf life of pasteurized dairy products.

Coliform Count

Coliforms are a group of bacteria commonly found in the intestinal tract of warm-blooded animals. For more than a century, they have been used as indicators of fecal contamination in food and water.

However, most coliforms found in bulk tank milk originate from environmental sources and thrive at lower temperatures. As a result, the coliform count cannot reliably indicate manure contamination in milk.

Testing methods include traditional microbiological techniques, which require confirmation steps, as well as rapid methods such as Coliform Petrifilm.

Interpreting Test Results

Microbial testing provides valuable information about milk quality and on-farm sanitation practices. However, accurate interpretation requires understanding both what each test measures and what it does not.

Proper sample collection and handling are equally important. Delays in testing or improper refrigeration can significantly affect results, particularly for the PI count.

By understanding the purpose and limitations of each microbial test, producers and their advisors can more accurately identify the causes of elevated bacterial counts and implement effective corrective actions.

Table 1. Overview of microbiological tests that are commonly conducted on bulk tank milk

Test	Groups of Microbes Selected For	Purpose	Methods Used	Limitations and other Considerations
Standard Plate Count (SPC)	Estimate of overall bacterial populations	An overall measure of microbial milk quality and hygienic production and handling conditions	Flow cytometry (e.g., Bactoscan), or traditional microbial testing including use of standard methods agar	Traditional tests only detect bacteria that can grow in the presence of oxygen and at moderate temperatures, possibly missing other bacterial populations of relevance Flow cytometry methods are most accurate at bacteria counts exceeding 10,000 cfu/mL, which is above the level for most producers Flow cytometry may count dead cells, which is why this method should not be used for heat treated milk
Preliminary Incubation (PI) Count	Cold-loving, or psychrophilic, bacteria	Indicator of poor sanitary conditions or poor milk handling (e.g., poor cooling)	Incubation of the milk at 55°F for 18 hours followed by conducting a SPC	Very sensitive to sample handling conditions prior to testing, possibly resulting in unexplained spikes in results Does not reflect the level of bacteria in the bulk tank sample at the time of collection, rather, the potential for bacterial growth under temperature abuse conditions
Laboratory Pasteurization Count (LPC)	Bacteria capable of surviving vat pasteurization heat treatment	Identify insufficient cleaning and sanitation practices on farm, or determine bacterial populations that survive pasteurization	Heat treatment of the bulk tank milk at 145°F for 30 minutes prior to conducting a traditional SPC	Should not be used to determine bacterial populations likely to survive HTST pasteurization (e.g., sporeforming bacteria) Petrifilm aerobic plate count media should not be used due to limited recovery of thermoduric bacteria after heat treatment leading to seemingly lower LPC results Split samples may be taken and frozen for short periods of time (e.g., up to 1 week) without affecting thermoduric bacteria populations or count
Coliform Count	Coliform bacteria (i.e., those that ferment lactose to produce gas and acid within 48h at 90-95°F)	Identify poor milking hygiene or cross contamination with manure	Traditional microbiological methods or more rapid methods like Coliform Petrifilm	Cannot be used strictly to identify fecal contamination Some coliforms are psychrotolerant, possibly resulting in higher results if samples are mishandled after collection

Representative Sampling for Better Bulk Tank Milk Insights

Microbial testing provides valuable insight into milk quality, sanitation, and handling practices, but results are only as reliable as the samples being tested. Because measures like SPC, PI count, LPC, and coliform count can be influenced by where and how a sample is collected, accurate and representative sampling is essential for meaningful test results. QualiTru sampling systems help dairy producers collect aseptic samples at strategic points throughout the milking process, from individual strings and inline locations to the bulk tank. By accurately isolating variables, identifying contamination sources, and supporting data-driven decisions, QualiTru helps producers protect milk quality and maintain confidence in their results. Learn more at www.qualitru.com.

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