14 PSE & DFD
PSE stands for pale, soft and exudative. It is a
description of meat - not live animals. Pork can be PSE, but there is
no such thing as a PSE pig. Confusion originates from the fact there is
a condition of live pigs called porcine
stress syndrome, abbreviated to PSS.
Thus a pig can be suffering from PSS, but there is no such thing as PSS
PSE pork was once quite common in Ontario supermarkets, but now it is
rare. There has been a reduction in numbers of pigs with PSS
which often (but not always) produced PSE pork. Also, much of the pork
in our supermarkets is injected with phosphate (enhanced or spiced
pork) which reverses the PSE condition. PSE is still important
because the mechanism involved still affects fluid losses from pork -
and fluid losses from pork amount to millions of dollars annually.
Traces of PSE can sometimes be found in chicken and turkey, where the
fluid losses are the major commercial factor. Beef and lamb do
not develop obvious PSE, but the mechanism involved can still affect
fluid losses - which are always commercially important.
Two pork chops are shown below, the one on the left is PSE.
PSE pork avoided in the fresh
state by a consumer because
of its unattractive appearance is just as likely to be rejected by a
meat processor. One of the great secrets of making good meat products
is to get a good structure or texture. If pork has been minced or cut
to make a processed product, the pieces must be made to adhere to each
other in the finished product - whether it is a sausage or a canned
ham. Products made from PSE easily fall apart. Likewise, water is
needed to create succulence, but the water must be within
the meat fragments not between them. Products made from PSE pork
are surrounded by an excess of fluid, which is unattractive and
wasteful. A good use for PSE pork in processed products has not yet
found. Even in dry sausage, where the use of PSE pork enables shorter
drying times, there is a deterioration to a soft, crumbly texture in
the finished product. When PSE pork is used in processing, it must be
mixed with quantities of normal pork sufficient to maintain product
Porcine stress syndrome,
may kill pigs before they reach the abattoir, as shown below. If PSS
pigs remain alive long enough to be slaughtered in an abattoir, they
are highly likely to produce PSE pork. But sometimes they produce
normal pork. And sometimes they produce DFD pork.
DFD pork (shown below) is dark, firm and dry. It is the
opposite condition to PSE pork.
condition is a major concern in beef. It is caused by factors such as
shivering and aggressive behaviour causing glycogen depletion. Glycogen
resynthesis is slow in cattle. If they are slaughtered without muscle
glycogen, the pH stays high and the result is DFD.
14.2 Practical questions on PSE & PSS
Is PSE pork harmful for the
Absolutely not. In fact, PSE pork usually has a longer shelf life
and lower spoilage rates than normal pork.
Does all PSE pork originate
from PSS pigs?
No. PSS pigs can be detected reliably by various tests
(particularly the DNA test) and pigs that pass these tests may still
produce PSE pork. Some pig-producing areas have a low (less than 5%)
incidence of PSS pigs but, in the summer months, the incidence of PSE
carcasses may be much higher (around 30%).
Eliminating hidden carriers of PSS, using new methods such as
DNA test developed by Dr. Peter O'Brien while at the University of
Guelph may be used to reduce the incidence of PSS, leading to a
reduction of PSE, but careful attention is required for the transport
and slaughtering of non-PSS pigs which still are liable to produce PSE
if poorly handled.
Is PSE pork from normal pigs
the same as PSE pork from PSS pigs?
Probably not. PSS pigs produce the most severe PSE, characterized
by extreme paleness from protein denaturation (like the colour changes
produced by mild cooking). On the other hand, PSE pork from normal pigs
may not become as pale, but the fluid released from between the
myofilaments of the myofibrils may be worse. Scientifically, different
of PSE can be identified by differences in their reflectance of violet
and red light.
What causes normal pigs to
produce PSE pork?
Probably the way they are transported and slaughtered, but there
are no simple relationships. One abattoir may have a low incidence of
PSE pork while a nearby plant supplied from the same population of pigs
may have a high incidence. Thus, there is an effect due to handling
that contributes to PSE produced from normal pigs.
PSE is almost always worse at the height of the summer or when
climate is variable. Thus, there is probably a seasonal effect that
contributes to PSE produced from normal pigs.
In some abattoirs, where pigs are shackled by one ham and the
is free to kick, the incidence of PSE is worse on the free, kicking
side. Thus, there must be an effect from struggling that contributes to
PSE from normal pigs.
What are the effects of
transportation and handling?
Unfortunately, these factors are too complex to enable any
universally true predictions, but here are a few:
- high heat and humidity, exhaustion, fear, or fighting
continuously from the farm gate to the kill floor usually causes DFD.
- stress for a short period (fear or fighting) followed
by a rest period which may or may not be adequate for recovery usually
produces an unpredictable range from PSE, through normal to DFD pork.
- a cold-water shower before slaughter may cause PSE.
- severe rough handling or electrical prodding may cause
PSE, but a reasonable amount of mild coercion at just the right time
can produce very mild DFD which is desirable.
What is the best way to stun
pigs to cause the least PSE?
The best method is the one causing the least amount of
suffering and reflex struggling by the pig. There are no foolproof
methods, because even inferior equipment used skillfully may cause less
PSE than expensive equipment used incorrectly. However,
assuming equipment is properly adjusted and skillfully used,
stunning methods damaging the skull (such as captive bolt pistols
and knocking hammers) generally produce more struggling and more PSE
than methods using electrical and carbon dioxide stunning. In
comparing different methods, look at the amount of struggling,
including the muscle contractions during and after
sticking and bleeding. Another point to look for is to check the
stunning method does not cause the rupture of blood
capillaries by elevating the blood pressure at the start of slaughter.
Blood speckles in pork, while they pose no health risk, are visually
unacceptable to most customers.
What about electrical
stunning killing the pig?
If it reduces the overall amount of struggling it will probably
reduce the amount of PSE. As yet, there have been no major problems
with getting a normal bleed-out, but this requires constant vigilance.
Is it worthwhile sorting
normal from PSE carcasses?
Sometimes. Sorting costs money, and you need to gain more than you
spend. The benefits may be a higher payment for shipments containing
few PSE cuts or greater yields in further processing.
Can PSE carcasses be
detected before cutting?
Yes, but not very reliably immediately after slaughter. Severe PSE
from PSS pigs develops very rapidly and can be detected fairly reliably
45 minutes after the start of slaughter, but the milder PSE from normal
pigs is less predictable. Sometimes it is already apparent at 45
minutes, but more often it develops after 45 minutes. Once cutting of
the carcass has started, it may be difficult to retrieve all the cuts
from a PSE carcass, so sorting should be done before cutting starts.
Sorting around 24 hours after slaughter can be quite precise.
What methods are available
for detecting PSE?
Several, but only three that are fast enough and robust enough to
be of any practical value.
- 1. Fibre-optic probes. These use optical fibres to illuminate
inside muscles and to measure the
amount of light reflected.
- 2. Electronic probes. These measure the electrical
properties of the pork and detect the amount of exudate ready to be
released once the muscle is cut.
- 3. pH probes specially designed for carcass use. These detect
the accumulation of acid after slaughter.
In all three methods, the time at which the measurement is made
critical. If the time lapse between slaughter and measurement is not
constant, then the results become meaningless.
What is the best method to
Unfortunately, there is no universally valid answer. First you must
implement one of the objective methods for measuring PSE and determine
how the incidence of PSE changes during the day and during the week.
From this information you can sometimes find a factor contributing
to the formation of PSE pork. For example, the PSE carcasses may come
from animals shipped, unloaded and slaughtered without
Alternatively, objective methods for measuring PSE can be used
in-house experiments. For example, you might wish to change an
operating procedure and see the result (if any). Every abattoir seems
to be unique and there are few solutions that work for everyone.
If some of the PSE is originating from PSS pigs, it does not
too much detective work to identify the suppliers once you have an
objective method for measuring PSE.
A key point to remember is if the incidence of PSE pork is
severe enough to be of concern, then there are probably a number of
contributory factors to be attacked one-by-one. PSE pork is
unlikely to disappear completely after making a single improvement.
14.3 Causes of PSS & PSE pork
The porcine stress syndrome (PSS) first named by Topel in 1968,
is the best known example of a biological factor
(hereditary susceptibility to stress) interacting with an environmental
(preslaughter stress) to alter meat quality. We now know PSS has
the same cause as malignant
The main genetic defect in PSS is
the substitution of a
single nucleotide causing a single amino acid replacement in the
release channel protein of the sarcoplasmic reticulum. This was
proved by O'Brien at the University of Guelph in 1986. The calcium ion
release channel is identified by its binding of ryanodine, a
plant alkaloid. The ryanodine receptor protein is inactivated by
phosphorylation and has a negative feed-back control by the
released. Thus, in PSS pigs, calcium release from the
sarcoplasmic reticulum in response to activation by transverse tubules
causes a greater than normal release of calcium ions.
If a PSS pig is unable to
cope with the
stress of being moved to an abattoir,
transport death from PSS and the formation of PSE pork after slaughter
associated with a very rapid
production of lactate, either causing an unusually
low blood pH while animals are alive
or an unusually low pH in the meat
after slaughter. The early post
mortem rate of decline in pH is particularly
important since early acidity strikes at proteins still at body
temperature and more readily denatured. Paradoxically, however,
some PSS pigs
produce DFD instead of PSE. Pigs
producing DFD may have been stressed, but they have survived to
abattoir with their muscles almost entirely depleted of glycogen. In
state, lactate production and the post mortem decline in pH is minimal,
leads to a condition opposite to PSE. In commercial crossbred pigs, the
incidence of DFD is higher after overnight lairage, particularly if
from the same farm have been mixed so they fight.
- it may become lethargic,
- it may start to take rapid, shallow breaths with
- its skin may appear very pale with a slight blue
coloration (cyanosis), and
- its body muscles may tremble, or become rigid or
14.4 Dark-cutting beef
- Dark-cutting beef is caused by
- Transport exhaustion, hunger, fear,
climatic stress or aggressive behavior cause depletion of muscle
this limits the amount of lactate formed post mortem.
- DFD has a high ultimate pH (typically above pH 5.9).
- Climatic stress often creates a seasonal rhythm in the
incidence of dark
cutting, and aggressive behavior is more common between males than
- Young bulls have the worst
reputation for dark-cutting and elevated pH values are most severe in
longissimus dorsi, semitendinosus, semimembranosus, adductor and
whereas, in other muscles, pH values may be near to normal.
- Thus, the overall changes produced in beef carcasses by
preslaughter stress are quite variable, and the heritability of
beef is low.
- In naturally occurring dark-cutting beef,
the sporadic pattern of elevated pH between muscles differs from the
generalized elevation of pH throughout the whole musculature
induced experimentally by the injection of epinephrine.
- Glycogen depletion from
aggressive behavior between young bulls is most severe in
myofibres with strong ATPase activity. Depletion in slow‑contracting
with weak ATPase activity is far slower, but lasts for longer.
- Glycogen levels
take three days to return to normal after a stress period of a few
hours and a similar recovery time for glycogen repletion
is required after cattle have been deprived of food for a few days.
- The slowness of glycogen repletion in ruminants may be
their typical low blood glucose and insulin
14.5 Mechanisms of PSE & DFD
- Paleness and darkness.
Pale meat scatters more light than dark meat. In severe PSE pork,
scattering is increased by the acid-precipitation of sarcoplasmic
proteins. In all meat, scattering is increased by a low pH. Low
pH decreases the negative electrostatic repulsion between
Myofilaments. This increases their refractive index. Organelles
with a high refractive index scatter light.
- Softness and
firmness. This is determined by water distribution.
If myofilaments have strong negative electrostatic repulsion between
them, then they maintain a large space for water. Myofibrils are
turgid and meat is firm. If the water is released to the spaces between
myofibrils, between myofibres and between myofasciculi, then the meat
becomes soft. Detachment of myosin molecule heads may contribute to
- Exudation and dryness.
If water is lost from the myofibrils, eventually it is free to exude
from the meat. The paradox is DFD meat seems
dry because no water is released when the meat is chewed - ALTHOUGH IT
CONTAINS MORE WATER THAN PSE MEAT.
Structure and Development of Meat
Animals and Poultry. Pages 507-528.
The illustrations used here came from a symposium at the University of
Wisconsin around 1970. I forgot who took the pictures but I will
never forget the two Norwegian Blue pigs who fell off their perches.