Effects of stunning methods on carcass and meat quality
Correct percussion stunning tends to reduce the chances of carcass bruising and reduces the prevalence of ‘blood splash’. Captive bolt stunning in pigs leads to a lot of convulsions and these tend to promote faster muscle acidification. This may result in PSE (pale, soft, exuda- tive) meat. Similarly, very prolonged application of electrical stunning currents can stimulate the musculature and lead to faster acidification.
Electrical stunning can sometimes cause problems of blood splash, haemorrhaging or broken bones, particularly in pigs when very high voltages are used, and if the animals are not suitably restrained and supported. This is because of the violent muscle contractions that sometimes result from passage of the current. In particular, broken backs, with associated haemorrhaging, can occur in pigs stunned using head-to-back electrode placement (Wotton et al., 1992). The meat quality of pigs stunned with carbon dioxide gas is generally considered to be good.
Gas stunning of poultry has certain advantages over conventional electrical systems. These may promote the occurrence of haemorrhag- ing in the meat and bone breakages under certain conditions (Gregory and Wilkins, 1989a). Haemorrhages downgrade the value of the product and bone breakages may lead to bone fragments being left in filleted muscles. Both these problems are reduced in gas-stunned birds.
Additionally, birds killed by anoxia show an increased rate of pH fall in the breast muscle (Mohan Raj, 1994a; Poole and Fletcher, 1995) enabling the carcasses to be held for a shorter time post mortembefore portioning or filleting, without the danger of muscle shortening and so leading to tougher meat.
hoisting the animal after stunning but before bleeding has reduced the dangers of involuntary carcass movements.
When slaughtering sheep the practice has sometimes been to sever the spinal cord where it enters the skull at the occipito-atlantal junction just after the blood vessels in the neck are cut. This procedure has the same effect as pithing cattle in preventing carcass movements, but it does not affect the sensibility of the animal if this has not been previously stunned. It is not a way of causing insensibility.
A very similar procedure still occasionally used for killing cattle, for example in some South American countries, employs a short-bladed, double-edged knife (a puntilla) which is used to sever the spinal cord by stabbing down dorsally through the neck and into the occipito-atlantal space. This process produces instant paralysis but does not cause unconsciousness and therefore cannot be considered humane.
Sticking
It is important to sever both carotid arteries and both jugular veins, or the blood vessels from which they arise nearer the heart. Chest sticking may have advantages in promoting faster exsanguination.
Cutting only one carotid will prolong the time to death. On average, brain death occurs between about 15 and 20 s after correct sticking. By this time around 50% of the total blood which will be lost through exsanguination has been removed (Fig. 4.4). The pattern of blood loss is strikingly similar in the different species implying a common mechanism of loss. Only about 40–60% of the animals’ total blood is lost at exsanguination, a weight of blood equivalent to about 4–5% of their live weight. The remaining blood is probably largely retained in the viscera rather than the carcass. A beating heart is not necessary for effective exsanguination and stunning methods that cause cardiac arrest do not affect the amount of blood retained in the meat. The residual blood content of lean meat is between 2 and 9 ml kg!1 muscle. There is no evidence that this volume is influenced by different slaughter methods, or that large amounts of residual blood affect the keeping quality of meat. However, different stunning methods, and whether the animal is stuck while lying prone or hanging from a shackle, can slightly affect the volume of blood lost.
So-called back bleeding can occur with poor sticking technique in pigs. Pigs are stuck by inserting the knife at an angle of 45° and about 5 cm anterior to the breastbone (sternum) in the midline. This severs the main arteries and veins entering the heart. In back bleeding the knife is pushed too far forwards into the chest cavity and punctures the pleural membrane lining the thorax. This allows blood to bleed back into the thorax. The blood stains the walls of the rib cage and may form clots.
Sometimes pig and cattle blood is collected for human consump- tion (for example to make blood sausage). A hollow knife is used. It is connected to a tank kept under a partial vacuum and an anticoagulant like sodium citrate may be used to prevent the blood clotting.
Alternatively the blood may be stirred to collect the fibrin and maintain the major part liquid. Obviously it is important to prevent contamination of the blood and to ensure that it comes only from healthy animals.
Religious slaughter
The main religious slaughter methods are shechita, used by Jews, the slaughter method used by Muslims to produce halal meat, and jatka used by Sikhs. In shechita and jatka the animal is not stunned prior to killing it by exsanguination. In general, the religious slaughter methods used to produce halal meat do not use prior stunning, although some forms of stunning may in fact be acceptable to some adherents of the Islamic faith. In shechita and halal slaughter the major vessels of the throat are severed by a transverse cut and in jatka the animal is decapitated with a single stroke using a sword. Legislation in many European countries requires that animals are stunned before exsanguination, but there are usually exemptions for those slaughtered by shechita and halal methods. Neither Jews nor Muslims consume pork so the methods can apply to cattle, sheep, goats and poultry.
Fig. 4.4. Rate of loss of blood at exsanguination. Modified from Warriss (1984a) (●
pigs: Warriss and Wotton, 1981; Swatland, 1983; "calves: Cooper and Morris, 1978; #lambs: Blackmore and Newhook, 1976; $broiler chickens).
Small animals can be restrained manually, for example sheep may be placed on their backs on a cradle, but larger cattle must be held in a special pen. The Cincinnati pen designed for Jewish slaughter employs a chin lift device to extend the neck of the animal while in a normal standing position.
In many western cultures preslaughter stunning is considered important for welfare reasons and there is thus some debate about the ethics of religious slaughter. Useful descriptions of the procedures and equipment used in religious slaughter, together with discussions of the ethical questions associated with the practices, have been given by Anil and Sheard (1994) and Grandin and Regenstein (1994). In regard to animal welfare, three important questions were raised. Was the preslaughter handling and restraint unduly stressful, was the incision used to cut the throat painful, and was insensibility or unconscious- ness achieved soon enough after the incision? These are complex questions for which we probably do not yet have universally definitive answers.
The slaughtering industry in the UK
In common with the situation in many developed countries there has been a gradual reduction in the number of red meat slaughter plants in the UK (Table 4.2). This trend is driven by the economies of scale, particularly in regard to the costs of more stringent hygiene require- ments now imposed on slaughter plants. These requirements parti- cularly apply in export plants. In 1995 in the UK, over 70% of animals of all red meat species were killed in plants approved by the European Union for the export of meat although these formed a minority of the total number of abattoirs killing each species (Table 4.3).
Some slaughter plants still process all three red meat species at the same site but this is becoming uncommon. About two-thirds of all pigs are killed at specialist pig abattoirs which process no other species and, traditionally, bacon producers killed all their own pigs for curing.
Table 4.2. The progressive reduction in the number of UK slaughter plants killing the different red meat species.
Cattle Sheep Pigs
1981 965 989 808
1985 904 926 770
1990 698 717 578
1993 583 600 457
1995 436 452 334
An average beef plant in the UK might kill 25,000 cattle, a pig plant 250,000 animals and a sheep plant 150,000–200,000 lambs a year. The largest specialist operations could have throughputs of 100,000 cattle or 500,000 pigs per year but these are still small in comparison with operations in, for example, North America.