LEC14

14 PSE & DFD

14.1 Introduction

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 pork.

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 been 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 quality.

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.

The DFD 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 customer?

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 the 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 types 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 the 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 other 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 is critical. If the time lapse between slaughter and measurement is not constant, then the results become meaningless.

What is the best method to reduce PSE?

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 being rested.

Alternatively, objective methods for measuring PSE can be used for 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 take 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 factor (preslaughter stress) to alter meat quality. We now know PSS has the same cause as malignant hyperthermia in humans.

The main genetic defect in PSS is the substitution of a single nucleotide causing a single amino acid replacement in the calcium 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 calcium 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,

it may become lethargic,
develop hyperthermia,
it may start to take rapid, shallow breaths with its mouth open (dyspnea),
its skin may appear very pale with a slight blue venous coloration (cyanosis), and
its body muscles may tremble, or become rigid or weak.

Both transport death from PSS and the formation of PSE pork after slaughter are 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 soon 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 enter the abattoir with their muscles almost entirely depleted of glycogen. In this state, lactate production and the post mortem decline in pH is minimal, and leads to a condition opposite to PSE. In commercial crossbred pigs, the incidence of DFD is higher after overnight lairage, particularly if animals from the same farm have been mixed so they fight.

14.4 Dark-cutting beef

Dark-cutting beef is caused by preslaughter stress.
Transport exhaustion, hunger, fear, climatic stress or aggressive behavior cause depletion of muscle glycogen and 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 females.
Young bulls have the worst reputation for dark-cutting and elevated pH values are most severe in longissimus dorsi, semitendinosus, semimembranosus, adductor and gluteus medius 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 dark-cutting in 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 fast‑contracting myofibres with strong ATPase activity. Depletion in slow‑contracting fibers 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 caused by 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 softness.
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.

Further information

Structure and Development of Meat Animals and Poultry. Pages 507-528.

Acknowledgement

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.