milk quality

Preliminary Incubation Count (PI count): The PI count is an estimate of the number of pyschrotrophic, or cold-loving, bacteria in milk. The PI count is not a regulatory test, and results of this test are interpreted as a general reflection of milk production practices on the farm and used as a tool to identify inadequate on-farm sanitation practices and holding temperature of milk in the bulk tank. The PI count is conducted by holding milk at 55°F for 18 hours. Bacteria that grow under refrigerated conditions are enumerated using the SPC method described above. PI counts are generally higher than SPCs. Selection of a PI count cut-off and interpretation of PI count results are difficult because variability in PI counts negatively influences repeatability. Some milk plants use a specific cut-off number while others use PI counts in relation to SPC. PI counts less than 10,000 cfu/mL are considered low, while PI counts more than 20,000 cfu/mL are considered high. A PI count three to four times higher than the SPC is suggestive of potential problems related to cleaning and sanitation of the milking system or poor udder preparation before milking (Figure 1). Failure to cool milk rapidly, marginal cooling, prolonged storage times, milking cows with wet teats, and/or extremely wet and humid weather conditions may also result in high PI counts. A PI count equal or slightly higher than a high SPC (more than 50,000 cfu /mL) suggests that the high SPC is possibly due to mastitis (Figure 1). The PI count has been used by some as an indicator of the shelf-life of processed dairy products. However, research conducted at Cornell University and Penn State University has shown that the PI count alone cannot be directly correlated with the flavor quality of raw milk or quality or shelf-life of processed dairy products. PI counts are most useful with data from other tests and additional information such as farm observations and inspections.
Laboratory Pasteurization Count (LPC): The LPC, also known as the thermoduric count, is an estimate of the number of bacteria that can survive laboratory pasteurization at 62.8°C (143°F) for 30 minutes. This process destroys most of the mastitis causing pathogens, selecting for those bacteria that can survive pasteurization temperatures (thermoduric bacteria). This is not a regulatory test required by state or federal agencies; however, some milk processors perform this test to ensure quality of the final product. Bacteria not killed by pasteurization are enumerated using the SPC method. LPCs are generally much lower than. An LPC of more than 200 cfu/ml is considered high (Table 1). A high LPC is most often seen with persistent cleaning problems; faulty milking machine or worn out parts such as leaky pumps, old pipe line gaskets, inflations and other rubber parts; and milkstone deposits. Significant contamination from soiled cows can also contribute to high LPCs.
Coliform Count (CC): The CC is a test that estimates the number of bacteria that originate from manure or a contaminated environment. Milk samples are plated on Violet Red Bile agar or MacConkey’s agar and incubated for 48 hours at 32°C (90°F), after which typical coliform colonies are counted. Coliform counts reflect hygiene and sanitation practices followed on the farm. Coliforms enter the milk supply as a consequence of milking dirty cows or dropping the milking claw into manure during milking. Coliform counts >100 cfu/mL suggest poor milking practices, dirty equipment, contaminated water, dirty milking facilities, and/or cows with subclinical or clinical coliform mastitis.

Table 1. Interpretive Criteria for Bulk Tank Milk Monitoring

Parameter	Low	Medium	High
Bulk tank SCC	<200,000	200,000-400,000	>400,000
Standard Plate Count (SPC)	<5,000	5,000-10,000	>10,000
Preliminary Incubation Count (PIC)	<10,000	10,000-20,000	>20,000
Lab Paseurized Count (LPC)	<100	100-200	>200
Coliform Count	<50	50-100	>100