LAB 1.3 Stunning, Exsanguination & Evisceration


Stunning methods

There are several criteria for a good slaughter method:
(1) animals must not be treated cruelly,
(2) animals must not be unnecessarily stressed,
(3) exsanguination must be as rapid and as complete as possible,
(4) damage to the carcass must be minimal, and the method of slaughter must be
(5) hygienic,
(6) economical and
(7) safe for abattoir workers.

To avoid the risk of cruelty, animals must be stunned or rendered unconscious before they are exsanguinated. When religious reasons do not allow stunning, extra care is needed to ensure that exsanguination causes the minimum of distress to the animal. In the Kosher method of killing, conscious cattle are suspended with the head stretched back, and then the throat and its major blood vessels are severed. Drugs cannot be used in the meat industry to induce unconsciousness in animals for slaughter since unacceptable residues would remain in the meat.

Animals can be effectively stunned by concussion. Concussion may be induced by a bullet or a bolt that penetrates the cranium, or by the impact of a fast-moving knocker on the surface of the cranium. In modern abattoirs, the primitive pole-axe has been replaced by devices which use expanding gas, either from an air-compressor or from a blank ammunition cartridge. First, the animal is restrained in a narrow pen or knocking box in order to minimize its head movements. The concussion instrument is then accurately located at a point on the midline of the skull, above the level of the brow ridges of the eye sockets. Concussion stunning should not be applied on the neck or posterior part of the skull.

The knocker is a heavy instrument held with both hands. There is a safety catch on the handle, but the actual trigger protrudes from the head of the knocker and is activated as the knocker is tapped against the animal's head. The captive bolt pistol resembles a heavy hand gun, but a blank cartridge is used to propel a cylindrical bolt rather than a bullet into the skull. After penetration, the bolt is withdrawn into the barrel of the pistol and the pistol is reloaded. Steers, heifers and cows are normally stunned with a knocker or a heavy captive bolt pistol, but bulls and boars which have massive skulls are sometimes shot with a rifle bullet. Pigs and lambs may be stunned with a light-weight captive bolt pistol.


Cattle and pigs are usually exsanguinated by a puncture wound which opens the major blood vessels at the base of the neck, not far from the heart. The trade name for this process is sticking. In sheep, lambs and small calves, the major blood vessels may be severed by a transverse cut across the throat, near to the head. Poultry can be exsanguinated with a diagonal cut from the corner of the jaw towards the ear on the other side, or by a knife thrust through the roof of the mouth to severe the brain and its major blood vessels. For poultry, the cut may be made on the side of the head if the head is later to be removed automatically by machine.

If the sticking wound is inaccurately placed, exsanguination may be too slow, and it may be almost halted by the formation of large blood clots. The formation of blood clots is accelerated when large areas of tissue are damaged by repeated inaccurate punctures. If the trachea is severed by the sticking wound, blood may be drawn into the lungs as the animal breathes. Later in the slaughter procedure, this may necessitate the trimming of blood clots from the pleural membranes after they have been inspected. If the oesophagus is severed, the vascular system may be contaminated by the entry of food particles into the venous system. If the connective tissues of the shoulder are opened, blood may seep into the shoulder region to form blood clots between the muscles.

Incomplete exsanguination increases the amount of residual blood in the carcass. The lean meat may then appear unduly dark and the fat may become streaked with blood. On the surface of incompletely exsanguinated poultry, the skin may appear dark and bloody over the breast, neck, shoulders and wings. The microscopic tissue damage that may later be caused by the freezing and thawing of poultry enables residual blood to leak from skin capillaries. Thus, the results of incomplete exsanguination are often more noticeable to the consumer than to the producer.

The exsanguination or sticking of meat animals in an abattoir is usually performed by severing the carotid arteries and the jugular vein at the base of the neck. In poultry, these vessels may be cut only on one side of the neck. The sticking knife must be kept clean otherwise bacteria might be introduced into the venous system and spread through the otherwise relatively sterile muscles of the carcass. Once exsanguination has started, the pulse and mean blood pressure rapidly decline because of the reduced stroke volume of the heart. Blood pressure changes are monitored physiologically by baroreceptors in the carotid sinuses. During exsanguination, respiratory movements of the thorax may be stimulated, and neurogenic and hormonal mechanisms attempt to restore the blood pressure by increasing the peripheral resistance by vasoconstriction. The heart keeps beating for some time after the major blood vessels are emptied, but rapidly stops if exposed and cooled. Electrical stunning of pigs may terminate cardiac activity so that, at the start of exsanguination, the blood escapes by gravity rather than being pumped out. In pigs, cardiac arrest does not affect the rate and extent of exsanguination. After exsanguination has started, the heart usually re-starts and attempts to pump, until it runs out of energy. Thus, in many cases, there is no reason why animals such as pigs and sheep cannot be killed by electrocution rather than being merely electrically stunned. In cattle stunned by concussion, more or less complete exsanguination may be obtained without ventricular pumping. Similarly, normal exsanguination is obtained in poultry that have been killed by electrocution rather than by being electrically stunned. In meat animals, "head to back" stunning may be used to stop the heart.

Blood loss as a percentage of body weight differs between species: cows, 4.2 to 5.7%; calves, 4.4 to 6.7%; sheep, 4.4 to 7.6%; and pigs, 1.5 to 5.8%. Blood content as a percentage of live weight may decrease in heavier animals since the growth of blood volume does not keep pace with growth of live weight. Approximately 60% of blood is lost at sticking, 20-25% remains in the viscera, while a maximum of 10% may remain in carcass muscles. Different stunning methods may modify the physiological conditions at the start of exsanguination and, also, the neural responses to exsanguination. Electrically stunned sheep lose more blood than those stunned with a captive bolt, but they also have more blood splashes in their carcasses.

Reduction of blood flow to the kidneys causes the release of a proteolytic enzyme, renin, which acts on a plasma protein to produce a polypeptide, angiotensin I. This polypeptide is converted enzymatically to angiotensin II which then causes widespread vasoconstriction. Vasoconstriction is important because it decreases the retention of blood in meat. Angiotensin II vasoconstriction is operative in both conscious and anaesthetized animals. Catecholamines and antidiuretic hormone (ADH) may also enhance vasoconstriction during exsanguination. Speed of exsanguination may modify the balance between neural and hormonal vasoconstrictive mechanisms, with hormonal vasoconstriction predominating in rapid exsanguination. However, asphyxia prior to exsanguination may result in vasoconstriction due to the activity of the sympathetic nervous system.

It is traditionally maintained that poor bleeding leads to dark meat with poor keeping qualities due to microbial spoilage and rancidity. However, there is little scientific evidence in support of this view, and it may be false, even in animals which retain massive amounts of blood in their carcasses. Delayed exsanguination of cattle may lead to a slight reduction in the amount of blood removed so that the carcass and spleen are slightly heavier. The effects on meat quality, however, are negligible. I am certainly not proposing that poor exsanguination is a good thing, but should it occur, it is not such a disaster as some meat inspectors suppose.

Factors that regulate the balance between extracellular and intracellular fluid compartments in meat are poorly understood. Fluid is delivered to living muscles by arteries, but it may return to the heart by either of two routes, in the venous system or in the lymphatic system. The route taken by intercellular fluid depends primarily on the extent to which fluid is taken up by capillaries and then passed to the venous system. In living animals, the venous return is far greater than the lymphatic return. The lymphatic capillaries which drain skeletal muscles are mostly located in the connective tissue around bundles of muscle fibres. The small amount of lymph that drains from muscles is increased after neural stimulation, and its lactate dehydrogenase content (LDH - an enzyme from within the muscle fibre) increases dramatically following muscle damage. In sheep, the flow of lymph from lymph nodes increases within 15 minutes of stress due to pain. Haemorrhage may or may not cause absorption of intercellular fluid into the blood stream, depending on the degree of vasoconstriction and consequent hydrostatic pressure in the vasculature.

Blood to the brain

Arterial blood to the brain is evenly distributed by a circular pattern of arteries called the circle of Willis. The circle of Willis receives blood from the intracranial carotid rete (a rete is a meshwork of blood vessels). In sheep, the external carotid arteries supply the intracranial carotid rete, via the internal maxillary arteries since the internal carotid arteries are absent in adults. However, blood may also reach the intracranial carotid rete from vertebral arteries via the occipito-vertebral anastomosis (an anastomosis is a communicating link between two vessels). The situation in cattle is similar, but with an additional supply to the intracranial carotid rete from vertebral and occipital arteries. The extent to which intact vertebral arteries might prolong a supply of oxygenated blood to the brain once an animal's throat has been cut is difficult to assess. In sheep, consciousness may persist for 65 to 85 seconds. In pigs, the delay between exsanguination and termination of electroencephalographic (EEG) activity is approximately 20 seconds following proper stunning. However, anoxia causes the dilation of cerebral blood vessels so that their storage capacity may be increased. An important point to bear in mind in considering studies on this topic is the difference between severing the carotid arteries (as in the Jewish Shechita method) and in ligation of the carotids (as in experiments attempting to simulate Shechita conditions). In the former case there is a rapid loss of blood supply to the brain whereas, in the latter case, the blood supply may be maintained.

Utilization of blood

The recovery of animal blood for utilization in food products for human consumption should be attempted. The main problems are to prevent the contamination of collected blood by bacteria from the skin, and to keep the blood of different animals separate until their carcasses have passed veterinary inspection for human consumption. Blood may be collected hygienically with a hollow knife. Coagulation of the blood can be prevented by the addition of anticoagulants such as citric acid or sodium citrate. Alternatively, the fibrin which binds blood clots together can be removed by stirring with a paddle. When utilized for human food or pet food, blood contains easily assimilated iron. Blood proteins have a high nutritional value and a high water binding capacity in processed products. The red blood cells burst if water is added to blood. If they are kept intact, red blood cells can be removed by centrifugation in order to prepare plasma. Plasma is a yellow liquid, rather like egg-white, and it may be dried to a powder for use in human food. If blood is discharged into the abattoir effluent instead of being utilized, it increases the biological oxygen demand (BOD) of the effluent. Chemical oxygen demand is another index of the pollution load of the abattoir effluent: it can be measured in several hours rather than in the several days required for BOD determinations.


A method for eviscerating, cleaning and trimming a beef animal to produce a dressed carcass is described below. In this case, the carcass has already been suspended on an overhead rail in a manner that enables the removal of the distal parts of the hindlimbs.

(1) skin the head and remove the skull and lower jaw, leaving the whole of the neck and the skin of the head hanging on the carcass,

(2) remove each foot and the distal part of each limb by cutting through the joint immediately proximal to the long cannon bone,

(3) make a long incision through the hide in the midline of the chest and abdomen, and continue the incision along the medial face of each of the limbs,

(4) remove the hide altogether if suitable equipment is available, or just remove it from the ventral part of the body and leave it temporarily hanging from the animal's back,

(5) open the thoracic cavity with a midventral saw-cut through the breast bone or sternum,

(6) open the abdomen with a long mid-ventral incision, and remove the penis or udder tissue, and any loose fat in the abdominal cavity,

(7) split the pelvic girdle with a mid-ventral knife-cut or saw-cut through the cartilage that separates the pelvic bones in the midline,

(8) cut around the anus and close it off with a plastic bag,

(9) skin out the tail (if this was not done earlier),

(10) separate the esophagus (which takes food to the stomach) from the trachea (which takes air to the lungs), by pulling the esophagus through a metal ring; close off the esophagus by knotting it,

(11) eviscerate the carcass by pulling out the bladder (and uterus if present),intestines and mesenteries,rumen and other parts of the stomach,liver; after cutting through the diaphragm, remove the plucks (heart, lungs and trachea),

(12) separate the left and right sides of the carcass by sawing down the midline of the carcass, through the vertebral column,

(13) trim and weigh the carcass to obtain its HOT WEIGHT,

(14) wash the carcass and pin a shroud over it to smooth the subcutaneous fat.

The other species of meat animals are treated in a corresponding manner, except for the head, feet and hide. With calves, the skin may be left on until the eviscerated carcass has been chilled. Beef carcasses are first shackled with a chain around the foot, but before the feet are removed, the carcasses are re-suspended from a hook under the Achilles tendon at each hock. However, the feet are usually left on pork carcasses. After being shackled during exsanguination, usually by one hindlimb, pork carcasses are re-suspended from a hooked bar or gambrel. This is inserted beneath tendons that have been freed underneath the hind-feet. When pigs are shackled by one hindlimb during exsanguination, differences in meat tenderness may be created between left and right hams. In some abattoirs, carcasses are skinned while they are on a metal cradle which holds them off the floor.

Washing of the dressed carcass is more complex than it might first appear. Apart from considerations relating to water purity and waste treatment, consideration must be given to sanitizing factors such as chlorine, organic acids and high temperature. Sanitizing agents may greatly reduce the levels of surface bacteria when the carcass is washed, but at the risk of hiding poor sanitation at earlier stages of processing. There is much to commend the philosophy of preventing initial contamination rather then removing it once it is present.

Composition of 550 kg steer

60.0% carcass

10.0% fill (gut contents, etc)

7.3% hide

7.2% inedible organs

6.0% head,tail & feet

2.4% diaphragm and internal (visceral) fat