Preliminary Incubation (PI) test commonly applied to raw milk is not an appropriate test for predicting fluid milk shelf-life
In an effort to improve pasteurized milk quality and extend shelf-life, fluid milk processors have begun to emphasize raw milk quality. A test that has gained popularity for evaluation of raw milk is the Preliminary Incubation Count (PIC). This test pre-incubates raw milk at 55°F (13°C) for 18 hours prior to performing a Standard Plate Count (SPC). Results from this test are then compared to those from an SPC conducted on the same raw milk sample. The PIC is based on the theory that the natural flora of the cow will not increase in number under PI conditions, whereas other bacterial contaminants (e.g., from dirty equipment, cows, etc.) may increase to significant numbers. The PIC has been used to test for the possibility of farm production methods in need of improvement that are not detected by the SPC method. Cooperatives are then basing premium payments to farmers based on their PIC data.
Currently, the industry is hampered by a lack of practical tests that will reliably predict the influence of raw milk microbiological quality on pasteurized milk shelf-life. Recently however, the Preliminary Incubation Count (PIC) has been applied as a means to mark raw milk supplies that may have a negative impact on the quality and shelf-life of pasteurized fluid milk. A justification that has been used in some cases is that high PIC milk may result in poor product shelf-life.
The Milk Quality Improvement Program (MQIP) engaged in a multidairy study to see if the PIC was indicative of finished product quality at the end of shelf life. This study used four dairies in New York state and was conducted over a 12-month period. Further, we did an exhaustive literature review, which failed to find any scientific study that determined an association of the PIC with pasteurized fluid milk shelf-life. Results of our MQIP study comparing PIC counts of plant raw milk supplies to shelf-life of pasteurized milk made from the same raw milk show no correlation. Further, most research that investigated the types of bacteria that cause high PICs have found that gram-negative organisms are predominant. The scientific literature indicates that gram-negative bacteria do not survive pasteurization, unless they are present in very high numbers that are beyond Grade A limits. A study of 855 dairy farms conducted by MQIP found no correlation of the PIC with bacteria that survive pasteurization (i.e., Laboratory Pasteurization Count) or with heat resistant psychrotrophic spore-formers (Boor et al. 1998. J. Dairy Sci. 81:1743-1748). Because the PIC is based on incubating raw milk, the results will vary depending on a number of factors including types and numbers of contaminants, the age and handling conditions of the sample and variability in the testing parameters. This variability negatively influences repeatability of the results and makes the selection of appropriate cut-off values and proper interpretation of results difficult.
Our study and results have increased pressure on milk cooperatives to stop using PIC as a marker when paying premiums to farmers for quality milk. It is our position that there is no significant association of raw milk PIC with the shelf-life of pasteurized fluid milk. The use of the PIC test for this purpose is not supported by current science.
NYS Milk Promotion Board
- Boor, Kathryn Jean
- Carey, Nancy R.
- Laue, Shelley
- Murphy, Steven C
- Pierik, Tanya M
- Ralyea, Robert David
- Woodcock, Nicole H
academic priority area
Land Grant Mission
domestic geographic focus
All NY Counties
All U.S. states
USDA topic area
Enhance Economic Opportunities for Agricultural Producer
Support Increased Economic Opportunities and Improved Quality of Life in Rural America