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CHAPTER 2: Effects of stress and injury on meat and by-product quality

A. Meat quality
B. Hides and skins quality

A. Meat quality

Pale Soft Exudative (PSE) meat (Fig. 1)
Dark Firm and Dry (DFD) meat (Fig. 1)
Spoilage of meat
Bruising and injury (Fig. 2 and Fig. 3)

The energy required for muscle activity in the live animal is obtained from sugars (glycogen) in the muscle. In the healthy and well-rested animal, the glycogen content of the muscle is high. After the animal has been slaughtered, the glycogen in the muscle is converted into lactic acid, and the muscle and carcass becomes firm (rigor mortis). This lactic acid is necessary to produce meat, which is tasteful and tender, of good keeping quality and good colour. If the animal is stressed before and during slaughter, the glycogen is used up, and the lactic acid level that develops in the meat after slaughter is reduced. This will have serious adverse effects on meat quality.

Pale Soft Exudative (PSE) meat (Fig. 1)

PSE in pigs is caused by severe, short-term stress just prior to slaughter, for example during off-loading, handling, holding in pens and stunning. Here the animal is subjected to severe anxiety and fright caused by manhandling, fighting in the pens and bad stunning techniques. All this may result in biochemical processes in the muscle in particular in rapid breakdown of muscle glycogen and the meat becoming very pale with pronounced acidity (pH values of 5.4-5.6 immediately after slaughter) and poor flavour. This type of meat is difficult to use or cannot be used at all by butchers or meat processors and is wasted in extreme cases. Allowing pigs to rest for one hour prior to slaughter and quiet handling will considerably reduce the risk of PSE.

Dark Firm and Dry (DFD) meat (Fig. 1)

This condition can be found in carcasses of cattle or sheep and sometimes pigs and turkeys soon after slaughter. The carcass meat is darker and drier than normal and has a much firmer texture. The muscle glycogen has been used up during the period of handling, transport and pre-slaughter and as a result, after slaughter, there is little lactic acid production, which results in DFD meat. This meat is of inferior quality as the less pronounced taste and the dark colour is less acceptable to the consumer and has a shorter shelf life due to the abnormally high pH-value of the meat (6.4-6.8). DFD meat means that the carcass was from an animal that was stressed, injured or diseased before being slaughtered.

Fig. 1: A. Pale Soft and Exudative (PSE) meat

Fig. 1: B. Normal meat

Fig. 1: C. Dark Firm and Dry (DFD) meat

Spoilage of meat

It is necessary for animals to be stress and injury free during operations prior to slaughter, so as not to unnecessarily deplete muscle glycogen reserves. It is also important for animals to be well rested during the 24-hour period before slaughter. This is in order to allow for muscle glycogen to be replaced by the body as much as possible (the exception being pigs, which should travel and be slaughtered as stress free as possible but not rested for a prolonged period prior to slaughter). It is important that the glycogen levels in the muscles of the slaughtered carcass are as high as possible, to develop the maximum level of lactic acid in the meat. This acid gives meat an ideal pH level, measured after 24 hours after slaughter, of 6.2 or lower. The 24h (or ultimate) pH higher than 6.2 indicates that the animal was stressed, injured or diseased prior to slaughter.

Lactic acid in the muscle has the effect of retarding the growth of bacteria that have contaminated the carcass during slaughter and dressing. These bacteria cause spoilage of the meat during storage, particularly in warmer environments, and the meat develops off-smells, colour changes, rancidity and slime. This is spoilage, and these processes decrease the shelf life of meat, thus causing wastage of valuable food. If the contaminating bacteria are those of the food poisoning type, the consumers of the meat become sick, resulting in costly treatment and loss of manpower hours to the national economies. Thus, meat from animals, which have suffered from stress or injuries during handling, transport and slaughter, is likely to have a shorter shelf life due to spoilage. This is perhaps the biggest cause for meat wastage during the production processes.

Bruising and injury (Fig. 2 and Fig. 3)

Bruising is the escape of blood from damaged blood vessels into the surrounding muscle tissue. This is caused by a physical blow by a stick or stone, animal horn, metal projection or animal fall and can happen anytime during handling, transport, penning or stunning. Bruises can vary in size from mild (approx. 10-cm diameter) and superficial, to large and severe involving whole limbs, carcass portions or even whole carcasses. Meat that is bruised is wasted as it is not suitable for use as food because:

· It is not acceptable to the consumer;

· It cannot be used for processing or manufacture;

· It decomposes and spoils rapidly, as the bloody meat is an ideal medium for growth of contaminating bacteria;

· It must be, for the above reasons, condemned at meat inspection.

Fig. 2: Severe bruising - Cattle carcass

Bruising is a common cause of meat wastage and can be significantly reduced by following the recommended correct techniques of handling, transport and slaughter.

Injuries (Fig. 4) such as torn and haemorrhagic muscles and broken bones, caused during handling, transport and penning, considerably reduce the carcass value because the injured parts or in extreme cases the whole carcass cannot be used for food and are condemned. If secondary bacterial infection occurs in those wounds, this causes abscess formation and septicaemia and the entire carcass may have to be condemned.

Fig. 3: Severe bruising - Cattle head

Fig. 4: Transport injury

B. Hides and skins quality

Hides and skins should have the highest value of any product of slaughter animals, other than the carcass. This is particularly so of cattle hides and small ruminants and ostrich skins. In the case of pigs and poultry, the skin forms part of the edible meat.

Useful leather can be made only from undamaged and properly treated skins. Proper handling of these items is important to produce a valuable commodity. Careless damage to hides and skins will cost the industry much loss.

Hides and skins of slaughter livestock (Fig. 5) can be damaged by thoughtless handling and treatment of these animals in the following ways:

1. Before slaughter:-
· Indiscriminate branding;
· Injuries from thorns, whips, sticks, barbed wire and horns;
· Unsuitable handling facilities;
· Badly designed and constructed transport vehicles.
2. During slaughter:-
· Causing the animals to become excited and injuring themselves;
· Hitting or forcefully throwing the animal;
· Dragging the carcass along the ground, alive or dead.
Fig. 5: Hide damage - Brands and injury

Consideration for animal welfare during transport and handling will improve the value of these by-products.

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