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 each milk pickup or on each load of milk for farms using direct load. These samples serve multiple purposes, from determining producer payments, ensuring regulatory compliance, and calculating premium payments.

While Standard Plate Count (SPC) is required on all bulk tank milk samples for regulatory purposes, most bulk tank milk samples are also tested for a variety of microbial measures. These measures include Preliminary Incubation (PI) count, Laboratory Pasteurization Count (LPC), and Coliform Count. Here I break down each of these tests: what groups of bacteria are targeted, what the purpose is, along with the methods and limitations.

Standard Plate Count (SPC):

The SPC is a test that provides an estimate of the bacterial populations in bulk tank milk, It is considered an overall hygiene indicator for how the milk was produced and handled at the farm. Multiple methods may be used to evaluate the bulk tank SPC, including traditional agar plate counts and flow cytometry methods (e.g., BactoScan). Regardless of the method, this test is meant to capture a broad range of different bacteria, including those that are introduced from the cow (e.g., because of bacterial infection of the udder), from the environment (e.g., because of manure or soil contamination), or from insufficiently cleaned equipment. It is common for bulk tank SPC to be below 10,000 cfu/mL, far below the 100,000 cfu/mL regulatory limit. A recent study from my research laboratory found that the average SPC on 100 NY dairy farms sampled six times over a year long period was ~3,300 cfu/mL (Figure 1). Thisaligns with more contemporary recommendations that SPC should consistently remain below 5,000 cfu/mL.

Preliminary Incubation (PI) count:

The PI test has been utilized as an indicator of poor sanitary conditions since the 1950’s. The premise of the method is to conduct a preliminary incubation of the milk at an elevated temperature of 55°F for 18 hours followed by conducting a SPC. When a large difference is detected between the bulk tank SPC and the PI test, it is thought to indicate inadequate sanitation of equipment. The groups of bacteria that grow during the preliminary incubation period are often those that grow well at low temperatures (psychrophilic or cold–loving). Pseudomonas is the most common psychrophilic bacteria found in raw milk, but there are many others that can also grow at low temperatures. Because the PI test relies on encouraging the growth of cold-loving bacteria, this test is especially sensitive to handling conditions after sample collection and prior to testing. For example, if a sample is collected and not kept consistently at or below 45°F or is held for more than 24 hours prior to testing, even at 45°F, the results can be affected.

Laboratory Pasteurization Count (LPC):

The LPC is used to determine the populations of thermoduric, or heat resistant, bacteria present in milk. The test was originally developed in the early 1900’s as a method to detect insufficient equipment sanitation. It was also used to assess bacterial populations likely to survive vat pasteurization, the most common method of pasteurization used at that time. Indeed, the test itself is a heat treatment of the bulk tank milk at 145°F for 30 minutes prior to conducting an SPC. This is the same heat treatment as vat pasteurization. Contemporary fluid milk is pasteurized using high-temperature, short-time (HTST) pasteurization at 161°F for a minimum of 15 seconds. Most thermoduric bacteria that can survive vat pasteurization do not survive HTST pasteurization, except for sporeforming bacteria like Bacillus, or very heat resistant non-sporeforming bacteria like Microbacterium. It’s important to note that the LPC should not be considered a measure of bacteria that survive HTST pasteurization, and therefore not used to indicate how pasteurized products will perform during storage.

Coliform Count.

Coliforms are a group of bacteria that can originate from the intestinal tract of warm-blooded animals, which is why they have been used for well over a century to indicate fecal contamination in food and water. The types of coliforms found in bulk tank milk, however, often originate from environmental sources and thrive at lower temperatures, meaning that this test cannot reliably be used to indicate milk contaminated with manure. The methods used to test coliform count include traditional microbiological methods, which require confirmation steps, and faster methods like Coliform Petrifilm.

Microbial testing of bulk tank milk provides valuable information about milk quality and on-farm sanitation practices. Interpreting results accurately requires understanding what each test measures — and what it does not. Equally important is recognizing that sample collection and handling between collection and the laboratory can significantly affect results, particularly for tests like the PI count. By understanding both the purpose and the limitations of each test, producers and their advisors are better equipped to identify the root causes of elevated counts and take meaningful corrective action.

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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