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Dairy Pipeline: March 2005
Alan G. Grove
Extension Area Dairy Agent,
Valley of Virginia
(540) 564-3080
email: agrove@vt.edu
The preliminary incubation or P.I. test was developed in the 1930's as an indicator of unsanitary conditions on dairy farms. The P.I. test indicates the presence of psychrotrophic bacteria, which are primarily made up of gram negative bacteria (pseudomonas, some coliforms, flavobacterium, and some alcaligenes), and are capable of growing at temperatures below 15 degrees C. (59degrees F.). The P.I. test is conducted by incubating the sample of raw milk at 13 degrees C. (55 degrees F.) for 18 hours and then a Standard Plate Count (SPC) is run. The preincubation allows psychrotrophic bacteria to grow until numbers are high enough to be detected by the SPC. A good (low) P.I. count is less than 10,000 cfu/ml; an acceptable level is less than 50,000 cfu/ml; and a count over 50,000 cfu/ml should be a concern. To prevent a high P.I. count dairymen should maintain a good sanitation program for cows and milking equipment. This includes removal of udder hair and use of a sanitizing teat dip to prep cows for milking. All equipment should be properly washed after each milking, and sanitized prior to use including the bulk tank. Inflations and milk hoses should be replaced at regular intervals, and the milk cooled to less than 40 degrees F. within two hours of milking. A final place to check for high counts is your farms water supply. By implementing a rigid sanitation program on your farm you can improve your milk quality.