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M. Chase Scott
Extension Agent, Southwest Virginia
(276) 223-6040; miscott1@vt.edu
Preliminary incubation (PI) count is becoming an increasingly popular measure of milk quality. The PI test is especially sensitive to psychrotrophic bacteria. These particular bacteria are capable of growing at colder temperatures, which is why dairy processors are encouraging farmers to maintain a low PI count.
Cold-tolerant bacteria produce enzymes during chilled storage that can damage both the milk protein and fat, leading to decreased milk quality. Furthermore these enzymes can survive pasteurization and potentially decrease the milk’s shelf life.
The name of the test is indicative of the method used to determine the count. PI count of milk is determined by incubating a sample at 55º F for 18 hours. A sample of the “preincubated” milk is then cultured for the standard plate count or SPC.
Interpretation of a high PI count is difficult without a corresponding SPC. For instance, a high PI count and a low SPC could indicate a pipeline/ tank cleaning problem, refrigeration problem or poor udder cleaning. A high PI and high SPC could indicate a mastitis problem—a determination which could be reinforced by examining the somatic cell counts of both the herd and individual cows.
The PI count of a given sample will generally be higher than the SPC. Following are guidelines for the goal and levels for PI Counts.
