LAB 2.3 Stunning, Exsanguination & Evisceration


Stunning methods

   Animals may be stunned by passing an alternating electric current through the brain.  The method is widely used for stunning pigs and poultry. Unconsciousness is induced by a wide range of voltages, from about seventy volts to several hundred volts. The length of time that the current is passed through the brain may be reduced to only one or two seconds if abattoir workers are waiting to shackle the pig's hindlimb with a chain, and then to exsanguinate the animal immediately. With a simple, hand-held electric stunner, the current is applied to the pig's head with two electrodes that protrude from an insulated handle. The electrodes must be cleaned at frequent intervals to ensure good electrical contact with the pig. The transformer which supplies the current is usually mounted on a nearby wall. However, large automated stunning systems are used in most commercial abattoirs and these may have one of a variety of different patterns of electrodes. Some designs have a current flow through the chest to stop the heart (although the heart starts to beat again when the pig is hoisted up on a shackling chain).  High voltage head to back stunning intended to stop the heart may cause vertebral fractures and blood splashes in the meat if the system is not carefully operated and monitored.  Essentially, if the rear electrode is moved forward there is less damage to the carcass, but also a reduced probability of stopping the heart.  High frequency electrical stunning of pig may be used to reduce carcass damage.

   Pigs may be stunned by placing them in an atmosphere which contains 65% carbon dioxide. Carbon dioxide is heavier than air and is trapped in a pit or deep tunnel into which the pigs are conveyed. After about one minute, the pigs are withdrawn in a cage or on a conveyer belt, and are then exsanguinated as rapidly as possible.  Carbon dioxide stunning requires some major features in the architecture of the abattoir which make it difficult to retro-fit  whereas automated electrical stunning apparatus is simple to  incorporate because it does not require pits or tunnels.  Modern carbon dioxide stunning apparatus is very effective because carbon dioxide concentration can be accurately controlled by solid-state gas sensors. In older system the gas levels were not accurately controlled.


pig sticking

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. 

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. 

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

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.

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.

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.


The viscera are removed from posterior to anterior - first the viscera from the abdominal cavity, then removal of the diaphragm, then the plucks (trachea, heart + lungs).