CtLaneaatar Farming, Saturday, Novambar 13,1993
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PORCINE STRESS
SYNDROME
The PSS/PSE can of worms has
been open since the 19605, when
our industry began turning the pro
duction crank for meat-type hogs.
This admirable response to con
sumer needs. unfortunately, turned
out to be a nightmare sudden
death in the pig. and pale, soft, and
exudative (PSE) pork. So we dis
covered, the hard way, that we
couldn’t have our cake and eat it
too.
Our next major discovery was
that the problem is not all genetic.
Environmental influences, such as
feeding, handling, and weather,
play an important role as well.
And now, another breakthrough
has been unleashed, this one so
prevailing that the final solution to
the PSS problem is within reach.
This tool, the DNA blood test, does
nothing more than identify stress
positive pigs and stress carriers.
But with that knowledge, we’re
finally in a position to control our
own destiny. If it were only that
simple. A part of the industry sees
the test as a means to eliminate the
gene. Others are using to test to
keep the gene for its lean-meat
benefits, with the hope that careful
control of breedings will minimize
the PSE problem.
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Pork Prose
by
Kenneth B. Kephart
Pann State Extension Swina Specialist
An extensive survey of pork
quality was sponsored by the
National Pork Producers Council
(NPPC) during the summer of
1991. Led by Bob Kauffman at the
University of Wisconsin, a team of
four scientists evaluated mote than
10,000 hams for quality character
istics. Sixteen percent of the hams
were PSE.
What Happens
In The Pig
Pigs known to be stress positive
are, in genetic terms, homozygous
recessive (nn) for the stress gene.
Because of their unusual metabol
ism, stress (castration, loading,
hauling, mating) can trigger a five
fold increase in calcium concentra
tions in the muscle cell.
The high level of calcium then
activates a prolonged muscle con
traction, which rapidly uses glyco-
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gen, a type of carbohyrate stored in
the muscle.
Because of the prolonged con
traction, oxygen can’t get to the
muscle fast enough, so the glyco
gen is metabolized anaerobically
(without oxygen). Two by
products of the event are heat and
lactic acid. The heat quickly raises
the body temperature, while the
lactic acid is involved in the pale,
soft character of pork from PSS
pigs.
- About 89 percent of stress posi
tive pigs will have PSE pork.
Related to this scenario is strong
evidence that PSS hogs have more
or larger “alpha-white” muscle fib
ers than normal hogs. The alpha
white fiber, which becomes predo
minant after genetic selection for
heavily muscled hogs, has the
biochemical machinery for rapid
break-down of glycogen.
What you see in the pig is a rapid
increase in body temperature
often as much as 10 degrees in a
matter of minutes. Red and purple
blotches on the skin often accom
pany this rise in temperature. The
lactic acid increases the acidity of
the blood (blood pH drops) and
leads to an increase in respiration
rate an automatic mechanism
used by all animals for correcting
low blood pH. This is one reason
pigs suffering from PSS will
breath very rapidly.
Normal animals (NN) may also
have PSE meat under some condi-
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tions, but the incidence is appa
rently very low (less than 3 per
cent). Carrier animals (Nn) evi
dently show no signs of PSS dur
ing stress, but they often have PSE
qualities in the pork.
Research at Lacombe and Den
mark suggests the carriers produce
60 percent PSE pork. Dr. Austin
Murray, meat biochemist for Agri
culture Canada, estimates a lower
incidence, around 30 percent, of
PSE pork in carrier pigs. But the
point is, even though the carrier
pig may not show outward signs of
PSS, the pork quality will be less
than ideal.
This is further evidenced, by a
study at the University of Minne
sota:
Table 1: Effect of PSS Gene on Carcass Composition and Quality
Backfat, in
Loin eye area, sq in
Color
Firmness
Marbling
Adapted from Shurson (1992).
Color: l=palc, pinkish gray;
s=dark purplish red. Firmness:
l=very soft; s=very firm. Mar
bling: l=devoid; s=moderately
abundant
Although the pigs in this study
are a bit fat, two points are evident.
First, pigs positive for the PSS
gene are leaner and have more
muscle. Second, they have muscle
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that is paler, softer and without
marbling.
PSE Effects
On Pork Processing
Because of the rapid drop in pH,
PSE muscles lose their ability to
retain juices. Muscles take on a
mushy, pale appearance. The prob
lem is most apparent in the loin and
outer ham muscles, and often leads
to a two-toned color in the ham
cuts.
Fresh cuts lose more water and
nutrients, are less attractive to con
sumers, and have a shorter shelf
life. Processed products shrink
excessively (about 3 percent above
normal). Frozen cuts lose more
moisture during thawing. Obvi
ously PSS can be a problem for
producers. But PSE, a direct result
of the stress syndrome, is a prob
lem for both packers and
consumers.
Causes Of
PSS And PSE
There is little question that
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