want to know: What about PI counts?
Gary Bennett, DVM
“Just tell us what we have to do to keep our PI
count down, and we’ll do it. If we have to milk the cows standing
on our head, we’ll stand on our head!” That dairy farmer sums
up the frustration producers feel dealing with preliminary incubation
To resolve some frustration, I posed questions producers
typically ask me to Steve Murphy with Cornell’s Milk Quality Improvement
Program and Dave Patteson, manager of the Central Testing Laboratory for
Agri-Mark Milk Cooperative. Both have years of experience dealing with
1. What groups of bacteria affect the PIC?
The PIC selects for bacteria that grow best at cooler temperatures. Gram-negative
psychotropic (cold tolerant) rods are more likely to show a significant
increase under these conditions, though
other environmental contaminants may as well. Other bacteria may show
a more moderate increase, perhaps doubling the count in a PIC compared
to a standard plate count (SPC).
Bacteria that grow at cool temperatures are often
associated with dirty equipment. The prime suspect is a milk cooling system,
and possibly dirty cows.
Cold tolerant, gram-negative bacteria are the most common type affecting
2. Do mastitis-causing
bacteria affect the PIC?
Most mastitis bacteria aren’t capable of significant growth under
the conditions of the PIC. There may be exceptions to the rule since many
different strains of bacteria cause mastitis.
to compare the PIC to the SPC. If a PIC is used alone, a high count that
consists primarily of a mastitic organism may be misinterpreted.
The PIC was
designed primarily to detect deficiencies in hygiene, cleaning and sanitation
Rarely will mastitis-causing bacteria affect PIC. Udder contents don’t
tend to affect PIC, but udder surfaces can, especially when predipping
is inadequate or nonexistent.
3. How can
PICs vary so much from day to day?
The bacteria that multiply at PI temperatures are contaminants. Variability
in the PIC on a dairy can result from different levels of contamination
and the types of contaminating bacteria, which can change day-to-day.
that may occur with dirty equipment or cows may have different growth
rates. Since the PIC relies on growth over time as an indicator of farm
practices, the test lends itself to
Udder prep must be consistent and thorough, regardless of who’s
milking. Environmental temperatures can also cause day-to-day PIC changes.
Cold weather can affect wash temperatures; hot weather can push a system
that is cooling marginally over the edge.
Here are some other factors that contribute to an elevated PIC:
bulk tank. Are the first milking and subsequent milkings being properly
cooled? Blind spots in bulk tank cleaning systems can leave milk residues
on agitator paddles, the underside of the
top of the tank and the valve. Valves may have to be cleaned manually.
• Potable water. Rinsing out sediment in a sanitized bulk tank or
chasing milk with water puts this water in the milk. The last thing to
touch milking surfaces prior to milking should be sanitized water or an
• DHI test. It may cause changes in routine or may add improperly
cleaned equipment. Discard milk collected from the meters should not be
added to the tank at the end of the milking.
• Partial pick up. It can leave a milk residue on the tank wall
above the milk line until additional milk is added.
• Dipping household containers into a bulk tank or filling containers
from the valve.
4. How can
the PIC be lower than the SPC/PLC run on milk when “fresh”
In many cases, a PIC slightly lower than a fresh SPC would be within the
error of the method and essentially indicates no change.
In a study of
855 producer samples done at Cornell University, approximately 8% of the
samples had PICs lower than the fresh SPC. Most of these were within the
error of the method, though a few dropped as much as 50% or more.
In these instances,
it’s possible that some bacteria are susceptible to natural inhibitors
in the milk that are active at the PI temperature, or some skin bacteria
or mastitis organisms die off under
If the cold-loving, gram-negative bacteria are not present or crowded
out by other types of bacteria, then the SPC will run higher than the
PIC on the same milk.
5. Is the
PIC an indicator of raw milk’s shelf life integrity?
It's the position of the Cornell Milk Quality Improvement Program that
there is no significant association between raw milk PIC and the shelf
life of pasteurized fluid milk. The use of the PIC test for this purpose
isn't supported by current science.
With regards to a raw milk supply, it’s not a useful test for predicting
shelf life. The PIC measures gram-negative bacteria that don’t survive
pasteurization. If they're found in pasteurized
milk, it’s a sign of contamination after pasteurization.
increasing emphasis on the PIC reflects their concern with gram-positive
bacteria that survived pasteurization and grew at refrigeration temperatures.
The PIC doesn't detect these bacteria.
It’s akin to
shooting around a target in the hopes of hitting the bull’s-eye.
How to reach us...
Bennett, a QMPS veterinarian and senior Extension associate, is based
in the QMPS Northern Lab, Canton, N.Y.
Steve Murphy,607-255-2894. Email:
QMPS is a program
within the Animal Health Diagnostic Center, a partnership between the
New York State Department of Agriculture and Markets and the College of
Veterinary Medicine at Cornell.
The QMPS staff of
veterinarians, technicians and researchers works with New York dairies
to improve milk quality by addressing high somatic cell counts, milking
equipment and procedures, and milker training in English and Spanish.
QMPS also conducts research and teaching
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