News & Industry Articles

Stress can affect meat quality

Posted by: SAPPO

By Dr Jim Robinson, veterinarian consultant, Western Cape

It is a cliché to say that meat is for eating and, like most clichés, it is absolutely true but not given enough attention. We can apply all the tools of excellence in breeding, housing, feeding, disease control and the like to produce the sort of pig carcasses that win competitions, but it all means next to nothing if we mess it up at the end and produce an unappetising article that is merely food. The secret of a successful enterprise is repeat business — in a competitive market it is essential to have our meat-eating customers coming back for more pork because the first experience was so good.Perception of quality

For the consumer, wide-ranging taste panel tests over the years show consistently that the most important characteristics of meat quality are:

•  tenderness,
•  juiciness, and
•  flavour

in that order, although colour is very influential for the initial attractiveness of the cut in the display cabinet of the butcher.
The inherent physiology and handling experience of the meat animal have a distinct influence on the meat that ends up on the consumer’s plate.

Important factors are:

• Genetics, such as species differences — there are distinct differences between pork, lamb and beef, with chicken having the least inherent flavour. Genetic make-up also influences the proportions of fat to lean in a carcass and the modern aversion to fat in a meat dish may have slight benefits in restricting energy intake in a human diet, but this is at the expense of flavour and juiciness.
• Age also has a bearing, which is why the meat of a trek ox will have more flavour than veal but will be a lot tougher, and broiler chickens tend to be tender but bland, and old sows end up in the boerewors.
• Poor handling of animals prior to slaughter can have a serious deleterious effect on the eating quality of a cut of meat.  Pigs are a good example of how this can happen, being somewhat more sensitive to handling errors than most other food animals, with detectable and measurable changes. Pigs which are carrying the malignant hyperthermia (MH) gene, also called the porcine stress syndrome (PSS) gene are particularly susceptible to any excessive exertion or handling stress, but even MH negative animals are badly affected by bad handling during selection, loading, transport, off-loading, lairaging and stunning technique.

It works like this:

How muscles work and what happens as they become meat

There are a number of complicated biochemical changes that occur as muscles work but, put simply, muscle contractions, which are happening all the time — breathing, standing, walking, balancing, digesting, maintaining body temperature, all require muscle movement and this requires energy.

Energy is supplied for contraction by a carbohydrate in the muscle tissues called muscle glycogen. This is broken down in the presence of oxygen by very rapid enzyme action when required, to produce the energy needed by muscle fibres to contract. This contraction is used for the essential and unconscious life-maintaining functions mentioned, and also for deliberate muscle activity for physical exertion.
The net result in the muscle fibre of this breakdown of muscle glycogen using  oxygen form the bloodstream is called aerobic glycolysis, and produces:

• energy for contraction plus some by products, mainly —
• water, carbon dioxide and some heat.

All of these by-products can be processed and dealt with quite easily, even when the demand is reasonably high as with vigorous exercise, by increased respiration, raised heart rate and blood pressure, sweating and resting, provided the muscle glycogen and oxygen are replaced in time for the next effort. (The pig has a particular problem with heat-dispersal, being unable to sweat).

If there is insufficient oxygen present, as with extreme exertion or fear or both, the supply of energy can be prolonged as an emergency measure:

•  this is the so-called “fight or flight reflex” to enable an athlete to make an effort that is beyond expectation, whether or not enough oxygen is present for aerobic glycolysis.
•  This process is called “anaerobic glycolysis” and produces the energy required but the main by-products are lactic acid and heat.
Lactic acid is not so easily dealt with and accumulates in the muscles, causing a drop in pH; this is self-limiting and the muscle will be unable to contract properly, eventually seizing up, often in a contracted state as with cramp.

Post-mortem anaerobic glycolysis

When a well-oxygenated and physiologically normal animal dies, its muscle fibres continue to contract, mostly at an invisibly small level, and without control from the brain. There is no oxygen being delivered to the muscles so that the anaerobic pathway is followed, resulting in meat with a low pH usually around 5.8 to 5.6 after 24 hours of chilling.

The low pH has a number of important advantages in meat:

•  It inhibits bacterial action, so that the meat has a longer shelf life;
•  It promotes “setting” of the carcass , making it easier and less wasteful for the butcher to cut up;
•  It enhances the release of flavour in the cooked product;
•  It helps in the pickling and curing process, which results in good colour and the desirable flavours of ham, bacon and related products.

However, if the pigs are roughly or badly handled in the hours before slaughter, serious and irreversible effects on meat quality will result.

PSE pork

The undue stress caused by bad handling produces a state of high adrenaline release, excessive muscular effort and virtual panic.
An exaggerated fight or flight reflex takes over, the pigs’ muscles go into anaerobic glycolysis, there is an abnormal collection of lactic acid and a very low pH before the animal is killed and immediately after slaughter while the carcass is still hot, resulting in what is known as pale, soft and exudative (PSE) pork.

This is very undesirable because:

•  The pH in the hot carcass reaches a level at which its water-holding capacity is at a minimum, resulting in fluid loss from the tissues – a loss of saleable meat to the consumer which may be eight percent or more;
•  The proteins in the meat become changed and soft and, together with the fluid loss, this reduces the juiciness and flavour of the product;
•  The pickle is not well absorbed which affects the quality of the processed products.
•  The very low pH (less than 5,4) at warm temperature changes the optical reflectivity of the denatured proteins, producing a pale appearance.
•  Hence – Pale, Soft and Exudative pork – a poor product after all the trouble taken to grow it!

The message is: producing an optimum meat product does not end on the farm: the whole process continues up to the final product in the display cabinet.

If we want our South African pork products, fresh or processed, to match up to the excellent genetic potential in our herds, take these messages home:

•  Do not pre-mix stranger pigs that are going to market – select, load quietly and go; once going do not stop except for traffic reasons;
•  Make sure loading platforms are pig-proof and at the right level;
•  Separate sizes and sexes where practical, load so all pigs have just enough space to stand but not fight; use movable partitions; give some non-slip surfacing on the floor;
•  Avoid the sun; load early or give shade without closing off ventilation;
•  At the abattoir, unload within passageways; never use electric prodders or whips or heavy sticks to drive pigs;
•  Allow time for pigs in lairages to cool down, drink and rest; do not hold beyond a couple of hours: it mainly just prolongs the stress situation, aggravated by hunger and apprehension in strange and menacing surroundings.
•  Go with your consulting vet to the abattoir and look not only at lungs, livers and other important things regarding parasites and diseases which can occur, but also at what mistakes are made in those last few hours of the pig’s life.

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