A few years ago, animal slaughtering often was regarded as a low technology operation - certainly not the sort of subject that would be taken seriously by research scientists at a university or industry laboratory. Then the business administrators and bureaucrats re-discovered what butchers and slaughterers had always known, that the events that take place in a few days between the farm gate and the meat counter have a major impact on meat quality and profitability. Months of work and skill in raising an animal can be ruined very quickly on the way to, or in the abattoir. Science in the abattoir assumes even more importance as we steadily improve our humanitarian treatment of meat animals.

Preparation for slaughter

The optimum amount of rest required by meat animals before they are slaughtered depends on the climate, the distance they have travelled, their method of transport and their general health. In some countries, where animals are auctioned at stock yards before they are taken to an abattoir, the rest periods are sometimes inadequate. This creates a commercial problem that is difficult to evaluate. On one hand, animals lose weight during transport and in holding pens, and it is undesirable to use pens and labour to prolong a rest period that confers no immediately obvious commercial advantage. On the other hand, stressed or weary animals sometimes produce meat with an unacceptable appearance or water holding capacity, and this may create economic losses later on. Animals lose about 0.2% per hour of their live weight once feeding has ceased, but this is very variable. For beef cattle, losses in 48 hours of fasting may range from less than 1% to 8%. About half the live weight loss shows up as a loss in carcass weight. However, improvements may be gained by electrolyte therapy, allowing animals free access to drink electrolytes during lairage.

In some situations, a rest period of one day for cattle and two or three days for pigs is considered to be optimum. However, such rest periods may be counter productive if the animals fight among themselves. Animals are not fed in the 24 hour period prior to slaughter.

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.

Meat animals may be stunned by passing an alternating electric current through the brain. The method is widely used for stunning pigs, as shown above, as well for poultry, and more recently, for cattle. 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. While this may be acceptable for pigs, neck to brisket stunning may not be for cattle. Even for pork, 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 stunning of pigs also 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 may also be used for turkeys.

Meat animals are usually stunned, shackled and exsanguinated, in that order. However, poultry may be shackled or hooked by their feet as soon as they are unloaded from the crate. Getting poultry into crates prior to transport is a major commercial problem. Live birds are easily bruised or more seriously damaged: this causes suffering to the birds and creates carcasses with an unattractive appearance. Poultry are sometimes exsanguinated without first being stunned. However, electrical stunning is very effective, and it facilitates the subsequent removal of the feathers. Concussion from a hammer-blow is commonly used to stun ducks.


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.

They had him thrown out of a club in Bombay
For, apart from his Mess bills exceeding his pay,
He took to pig-sticking in quite the wrong way.
I wonder what happened to him!

Noel Coward

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 carcasses is described below. It is important to have some idea of the relative weights of the components removed during slaughter. 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.

In lambs and sheep, the forelimb metacarpal cannon bone is removed at its distal extremity at the break or spool joint. Force is applied to this joint and it is loosened with a knife. In relatively young animals, the epiphyseal growth plate fractures to give a "break joint". In older animals, the end of the bone is fused to the shaft of the bone so that the joint breaks to reveal a "spool joint". When a spool joint is revealed, the animal is classified as yearling mutton or mutton. The time at which a spool joint is apparent is quite variable, ranging from 9 to 21 months depending on the animal.

Hairs and bristles must be removed from pork carcasses when the skin is to be left on the carcass. After exsanguination, otherwise intact pork carcasses may be immersed in a scalding tank that contains water at about 60oC. Under normal conditions, with normal pigs, there is little or no heat penetration into the underlying musculature so that meat quality is unaffected. Scalding at this temperature for longer than six minutes damages the skin. Lime salts or a depilator such as sodium borohydride are added to the water to facilitate loosening of the hair. After five or six minutes, the carcass is lifted out of the tank and is placed in a dehairing machine, where it is repeatedly slapped by strong rubber paddles with metal edges (as shown in the picture near here). Loosening of the hair by hot water also may be accomplished by the action of steam on carcasses hanging vertically from an overhead rail. Microbial contamination is minimized but costs due to energy and water are increased. Other possibilities include scraping loose hairs from the skin with a jet of fast-moving ice particles, as currently being tested in Denmark.

After removal of the hoof from each toe, the pork carcass is re-suspended from the overhead rail. Then the carcass is quickly singed with a gas flame that burns all the fine hairs which have escaped the dehairing machine. The carcass is shaved with a sharp knife until it is clean. However, this often damages the skin and it may spoil the skin for leather production. Sometimes it is almost impossible to remove the stumps of strong bristles from the skin, particularly in the early months of the winter.

In some abattoirs, pork carcasses are skinned like beef carcasses. This enables better quality leather to be made from the skin and, in the long run, is less expensive than scalding. In this procedure, the skin is manually detached in the ventral region of the head and body, and on the medial faces of the limbs. Then the skin is removed from the dorsal part of the carcass with an air-knife or with a hide-puller. The hide-puller is driven by a powerful motor or hydraulic piston and it simply rips the skin off the carcass. Ideally, the vertebral axis of the animal should be temporarily strengthened by brief electrical stimulation to tighten the muscles, otherwise some hide puller may cause a separation of the vertebrae, particularly in younger cattle. However, this usually displaces several kilograms of fat from the edible carcass to the inedible hide, with a consequent loss in revenue. The kidney and pelvic fat of beef carcasses may be removed before chilling to facilitate lard rendering operations.

Usually poultry are scalded to facilitate the removal of their feathers. The ease with which feathers may be removed is related to the temperature and duration of scalding. However, high temperatures (> 58oC) cause the skin to become dark, sticky and easily invaded by bacteria. Consequently, hard scalding (at 70 to 80oC) is only used for low grade poultry destined for immediate use in processed products. For broilers, the appearance of the skin is unharmed by about thirty seconds of semi-scalding in water at 50 to 54oC. Both temperature and duration are precisely controlled, depending on the age and condition of the birds. After the feathers have been loosened, they are removed by machines that have thousands of rubber fingers mounted on rotating drums. However, many of the strong pin feathers on the tail and wings may survive this treatment and must be removed manually.

The feathers on the carcasses of ducks and geese are difficult to remove. Following scalding and the mechanical removal of as many feathers as possible, ducks and geese may be quickly dipped in hot wax. After the birds have been removed and cooled, the wax sets hard and can be pulled off together with large numbers of feathers. The wax is melted and recycled, and the birds are picked bare manually.

Methods for the evisceration of poultry are even more variable than those for meat animals and many of the operations for poultry evisceration have been successfully automated. Poultry usually are suspended on some type of moving overhead rail. Sometimes they are suspended by their feet, sometimes by their heads, and sometimes by both, so that the vent or cloaca bulges downwards. One possible method for the evisceration of poultry is as follows:

(1) after stunning and exsanguination, the bird is suspended from its head, and the oil gland at the base of the tail is removed,

(2) an incision is made through the skin along the back of the neck, from the head to the shoulders,

(3) the crop and the trachea are removed,

, (4) the bird is re-suspended by its feet, and an incision is made through the skin, around the cloaca and towards the sternum,

(5) the viscera and the intact cloaca are pulled out and inspected for signs of disease,

(6) the liver is removed and the green gall bladder is discarded, without contaminating the carcass with bile,

(7) the muscular wall of the gizzard is slit open so that the inner lining and the contents can be discarded,

(8) the heart is removed from the hanging viscera and trimmed,

(9) the remaining viscera are removed and discarded, and the lungs, kidneys and ovary or testes are removed from under the vertebral column with a suction tube,

(10) the head, neck and feet are removed,

(11) the carcass is chilled in a mixture of ice and water,

(12) after chilling, the giblets (neck, gizzard wall, liver and heart) are packed into the carcass.

Although mass produced poultry are now almost all eviscerated prior to distribution to retail outlets, intact poultry carcasses keep quite well if their viscera are left in place. Growth of intestinal bacteria is minimal below 7oC and, at temperatures below 4oC, uneviscerated carcasses may be stored for at least as long as eviscerated carcasses.

Although automated evisceration of poultry has been around for years, it is only recently that the major engineering problems associated with eviscerating larger animals have been solved. Both New Zealand and Australia have developed large-scale systems, for lamb and beef, respectively. If successful they are destined to have a dramatic impact on the meat industry where labor costs in slaughtering have always been a major factor in locating abattoirs in relation to meat producing regions. Recent developments include automated equipment for removal of the brain (either as an edible byproduct or for extraction of pharmaceuticals), brisket cutting, evisceration, removal of muscles from the neck and between the ribs, removal of shoulder and chine bones, and inspection stamping.

Meat Inspection

Meat inspection involves the examination of live animals (ante mortem inspection), carcasses and viscera (post mortem inspection), and finished products. The buildings and equipment of the abattoir must conform to a prescribed standard of hygiene, and abattoir workers must be properly trained. The main objectives of meat inspection are (1) to ensure that consumers receive only wholesome products for consumption, (2) to ensure that by-products are properly treated so as to cause no direct hazard to health, and (3) to provide a warning of the presence of serious contagious diseases among farm livestock. The purpose of ante mortem inspection is to identify injured animals that must be slaughtered before the others, and to identify sick animals that must be slaughtered separately or subjected to special post mortem examination.

Man has exhibited a fear and dislike of contaminated meat throughout recorded history. The Mosaic food laws with a religious basis have survived to the present day, while Greek and Roman civil laws have slowly evolved into modern civil legislation. Many European countries have a long history of legislation relating to meat hygiene and, by 1707, these laws had reached Canada. However, it was not until the early years of the present century that modern meat inspection started to develop with the science of veterinary microbiology as its basis (1906 in the USA, and 1907 in Canada). The general principle of commercial responsibility in meat inspection should be that the party responsible for a condemnation must bear the financial loss. In many cases, only a relatively small part of the carcass is condemned. Diseases and conditions that may be attributed to the producer include items such as abscesses, antiobiotic residues, parasitic infections, hernias, and a range of bacterial and viral diseases. Conditions such as bruises, bone fractures, frostbite and pneumonia may be attributed to the producer or to the packer, depending on when they are found. In Canada, they are the responsibility of the producer if they are found within 24 hours of leaving the farm. After that, they are attributed to the packer. Contamination of the carcass, loss of identity and recent parturition are attributed to the packer.

One of the major factors in the design of a slaughter line is the minimization of the spread of Salmonellae. There are well over a thousand species of this bacterium, and they are frequently found in the feces of meat animals and poultry. When the bacteria are transmitted to human food, they may infect the human digestive system and cause a food-borne illness. Although Salmonellae on meat are killed by the heat of thorough cooking, they can cause illness by contaminating other foods which are eaten raw. For example, they may contaminate a salad that has been prepared on a cutting board previously used for contaminated poultry. Another microorganism that causes gastroenteritis, Campylobacter jejuni, also may be transmitted on contaminated meat, particularly poultry. This bacterium is killed by cooking procedures that reach 60oC or more.

Most of the hygienic precautions in the abattoir are quite straightforward. For example, the knives used to remove animal hides often become severely contaminated. Thus, they must not be used for later operations when the carcass meat has been exposed, and they must be decontaminated by a method such as dipping in hot water at 82oC for 10 seconds. Contamination is not limited to knives, and relatively large numbers of Salmonellae can be found on steel-mesh safety gloves, cutting boards and stainless steel tables. Salmonellae also may contaminate the mixtures of ice and water used to chill poultry carcasses after evisceration.

Meat inspection at present is a labor-intensive procedure with a high degree of subjectivity and is poorly suited to industries where volume and speed are essential for economic survival. Many disease conditions are subtle and difficult to detect by inspection alone, while there are other factors such as drug residues that can only be detected by laboratory tests. Thus, it seems likely that meat inspection will move progressively towards blood testing. Acute phase reactants are a group of plasma proteins produced during the acute phase of tissue inflamation and injury, and may provide a useful indicator for meat inspection purposes

Lymphatic system

Blood is brought to the body tissues in arteries and is removed by veins. However, interstitial fluid from between the cells of a tissue also is removed by the lymphatic system. Lymph vessels have extremely thin walls, and the lymph fluid they contain is wafted along by body movements that massage the lymph vessels. A system of flap-like valves prevents backward movement of the lymph. As well as fluids, the lymphatic system also recycles proteins that leak from the vascular system. This is an important factor in the determination of the osmotic balance between the interstitial fluid and the blood. In starved animals, the scarcity of blood proteins unbalances the system and leads to the accumulation of interstitial fluid (edema).

If body tissues are invaded by disease-forming bacteria, some of the bacteria drift into the lymphatic system. The lymphatic system is arranged like a system of rivers leading to an estuary. The final opening of the system is called the right thoracic duct, and this returns the lymph to the vascular system at a point where the main veins of the body enter the heart. Lymph nodes with a gland-like appearance are located at regular intervals throughout the lymphatic system (Figure 1-27). Their function is to filter and destroy invading bacteria. When lymph nodes are successful, they prevent the spread of disease from the region of tissue that has been invaded. The activated lymphocytes of the lymphatic system may play a major role in attacking and destroying invading bacteria.

The lymph nodes that guard healthy tissues are compact in structure and pale brown in color.They become swollen and discolored when they are activated by invading bacteria. The meat inspector systematically examines the lymph nodes of the viscera and the dressed carcass. Lymph nodes that appear to be abnormal are sliced open for inspection. Knives must be re-sterilized once they have been used to open an infected lymph node. Once alerted to the presence of diseased tissue, the inspector determines the type and severity of the disease. The whole of the carcass or just the diseased parts may be condemned. It is essential, therefore, that any offals that have already been removed from the carcass can all be traced back to the carcass from which they originated. This also includes any blood which may have been collected as an ingredient for processed meat products. Blood for human consumption is usually collected with a hollow knife in order to minimize contamination from the surface of the carcass.

Tuberculosis is a bacterial disease transmitted from cattle to humans by the ingestion of milk or meat. Diseases also may be transmitted on by-products such as hides or fleeces. The bacteria that cause anthrax require free oxygen in order to form spores. Workers who handle infected hides or wool may be infected by skin contact or by inhalation. Fortunately, these two serious diseases are rare in the industrialized countries, and the every-day work of the meat inspector is really part of the overall system for the quality control of meat products. Most industrialized nations have a complex system of legislation relating to the disposal of condemned meat. In many cases, condemned meat can be rendered safe for consumption by cooking or prolonged freezing prior to sale.

There are many parasites that attack farm animals and retard their growth. In temperate climates, however, only a few types of parasite occur in the muscles of a dressed carcass. Trichinella spiralis is a small nematode worm that sometimes appears within bundles of muscle fibers in pork carcasses, particularly in wild boar meat produced as a specialty. Nematode larva may be located intracellularly within a nurse cell derived from a parasitized muscle fiber. If the worms are not destroyed as the meat is cooked (at about 60oC), they will reproduce in the human intestine. The larvae burrow through the wall of the intestine and through the body tissues. This causes a disease known as trichinosis. Although mild cases are not serious, heavy infections may be fatal. Pigs may become infected when they eat uncooked garbage or the flesh of rodents that carry encysted worms in their own muscles. Once a pork carcass is infected, the encysted worms are most likely to be found in the muscles of the tongue, diaphragm, larynx, abdomen or under the vertebral column. The great problem for the meat inspector is that the encysted worms in pork are too small to be seen without a microscope. In Germany, pork carcasses are examined by a simplified microscope technique, but the number of infected carcasses that are detected is very small. Thus, under typical commercial conditions, although no pork carcass can be guaranteed to be free from Trichinella, it is not a serious problem provided that the incidence of the parasite is kept low. For this reason, pork producers have the responsibility to cook any waste food or garbage that is fed to pigs, and consumers have the responsibility to make sure that all pork products are thoroughly cooked.

Echinococcus granulosus is a tapeworm, a cestode parasite. The adult tapeworm is quite small (8 mm long) and lives in the intestine of a dog or fox. The eggs of the parasite leave the body of the host in its feces. If a sheep eats grass contaminated by these eggs, the eggs hatch in the intestine of the sheep and the larvae migrate into the blood stream. The parasite then becomes lodged in the body tissues and grows to form a large (10 cm) hydatid cyst that contains inactive worms. The life cycle of the parasite is completed if hydatid cysts from the flesh of a dead sheep or from abattoir waste are consumed by another carnivore. Any parts of a lamb or mutton carcass that contain hydatid cysts are condemned by the meat inspector following post mortem examination. The hazzard to human health is in the possible contamination of human food by fecal material from dogs. A hydatid cyst may then develop in the human body. In order to prevent the completion of the parasite's life cycle through sheep, it is essential that dead sheep are properly disposed of, and that dogs are prevented from gaining access to abattoirs or to abattoir waste. In areas where there is a high incidence of this parasite, dogs should regularly be dewormed. There are various subspecies of E. granulosus that involve other herbivores and carnivores.

Taenai saginata and T. solium are tapeworms that live in the human intestine. Contamination of feed for cattle and pigs by eggs from human feces completes a life cycle that leads to the presence of tapeworm larvae in meat. Cysticercus bovis is the larval form of T. saginata in beef, and Cysticercus cellulosae is the larval form of T. solium in pork. Cysticercae in meat appear as oval vesicles, almost a centimetre in length and with a white, gray or translucent appearance. Cysticercae are most commonly found in the heart and masticatory muscles. Cysticercae are detected during post mortem examination, after cuts have been made through these muscles. Once a cyst has been found, carcasses should be cooked or made safe by prolonged freezing.

Carcass Refrigeration

Carcasses are chilled to reduce the microbial spoilage of their meat. The rate of chilling is determined by the temperature, the relative humidity and the velocity of the air in the meat cooler. In addition to direct heat losses by conduction, convection and radiation, heat is lost from a carcass when water evaporates on its surface. Carcasses cool rapidly if they have a large surface area relative to their mass, and if they have only a thin covering of subcutaneous fat for insulation.

The heat exchange units in meat coolers resemble automobile radiators filled with a refrigerant. The refrigerant is a gas that has been compressed to a liquid by a powerful compressor. Compression liberates heat, and the hot liquid is now pumped to another unit, usually outside on the roof of the meat cooler. The hot liquid, still under pressure in a pipe, is cooled as it passes its heat to the atmosphere or to a water fountain. The cold liquid then is pumped through a small orifice that resembles, in principle, the carburator of an automobile. The conversion of a liquid to gas absorbs heat, so that the resulting gas is very cold. This cold refrigerant passes through the heat exchange units inside the meat cooler where it cools the air inside the meat cooler.

The air inside the meat cooler is kept moving by powerful fans. The arrangement of heat exchangers and their fans in the meat cooler is carefully planned to produce an even distribution of cold air. However, there is always a problem in overcrowded coolers. When hot carcasses are first placed in a cooler, a high air speed is maintained to accelerate initial cooling. Later on, the air speed is reduced so that the surface of the carcass does not become desiccated. To minimize surface dehydration, the air should not blow directly on the carcasses.

Evaporation losses from pork carcasses may be reduced by rapid cooling after slaughter. Pork carcasses can be briefly pre-chilled by very cold air or by immersion in liquid nitrogen for about thirty seconds, sufficient to precool the skin without cracking it. In bad conditions, evaporation losses from pork carcasses within 24 hours of slaughter may exceed 3% of the initial hot carcass weight. With rapid chilling, evaporation losses may be held below 1% in the first 24 hours after slaughter. In laboratory conditions, rapid chilling may enhance the quality of the pork by reducing the incidence of paleness and softness, but there is also some risk causing a deterioration in meat quality under commercial conditions. Evaporation losses from pork carcasses may also be minimized by the use of a spray-on cellulose film.

Large carcasses take a long time to cool to 0oC in an ordinary meat cooler, and it is not usually until the day after slaughter that the deep parts of a beef carcass reach the temperature of the surrounding air. Shrinkage due to water loss by evaporation causes economic losses in beef carcasses. The acceleration of carcass cooling by cold water sprays reduces shrink losses, but there may be problems with microbial spoilage even if the water is chlorinated. Intermittent spraying with dilute (1%) acetic or lactic acid solutions, however, largely prevents surface microbial spoilage. Once carcasses have been chilled after slaughter, they may be stored just above 0oC at a relative humidity of 90% with some slight air movement (0.3 m/sec). Higher relative humidities reduce evaporation losses but also encourage surface spoilage by micro-organisms.

Meat must never be frozen before rigor mortis has occurred and the meat has become stiff and inextensible. When meat that has been frozen before the onset of rigor mortis is thawed prior to consumption, it undergoes thaw shortening and becomes very tough. Even if the meat is not frozen, cooling that is too rapid may also make the meat very tough. This is called cold shortening. Beef carcasses should not be subjected to air less than about 5oC with a velocity over 1 meter per second within 24 hours after slaughter. The temperature of lamb carcasses should not be forced below 10oC within 10 hours of slaughter.

There is usually no reason why meat cannot be removed from the carcass while it is still warm (hot-boning or hot processing). It is more difficult to handle floppy warm meat, but requirements for refrigerated storage space are reduced and there are favorable changes in the water holding capacity of meat so that drip losses are reduced. The temperature, the hygiene and the shape of isolated pieces of hot meat must be carefully controlled. Electrical stimulation may be used to accelerate the conversion of muscles to meat so that hot-boning or accelerated processing can be undertaken. Presumably, in this situation, the tendency for electrical stimulation to cause PSE (pale, soft, exudative) pork is offset by the accelerated rate of meat cooling.

At present, the least expensive method of chilling poultry is by immersion in a mixture of water and ice. Carcasses have a water uptake of 8 to 12% and microbial contamination is controlled by chlorination. Spray cooling wastes water while air cooling may cause dehydration of the carcass. However, old-fashioned air cooling helps to preserve the flavor of poultry meat and may command a premium price for the product.


The overall objective of rendering is to produce clarified homogeneous substances such as lard and tallow from a heterogeneous mixture of animal trimmings and scraps. Because of the vast volume of such items produced by a large abattoir, rendering and waste product utilization are major factors in the economics of meat processing. Visceral adipose tissue that has been maintained in a wholesome condition is rendered to produce lard and tallow for cooking or for further processing in the food industry. Intrinsically dirty parts of the carcass such as the head and feet are subjected to inedible rendering as the first step in the manufacture of soap and grease.

In the now out-dated method of wet rendering, steam was injected into a pressurized tank full of trimmings with a high fat content. Eventually, the molten fat floated freely on top of an aqueous solution with a high protein content. At the bottom was a slurry of solids. The partial recovery of proteins from the aqueous layer was achieved by a subsequent evaporation process. In dry rendering, the steam is confined to a jacket around the tank, and the contents of the tank are held at a negative pressure. This enables a far greater recovery of proteins which would otherwise greatly elevate the BOD of the abattoir effluent. Thus, a general principle of modern abattoir operations is to minimize the amount of water that is added to animal wastes as they are cleaned from the premises. Not only is water expensive, but much of it has to be removed later by evaporation, and this uses a considerable amount of energy. Many inedible waste products such as clotted blood, bone dust and manure from the rumen and from stockpens are best maintained for recovery operations in as dry a condition as is possible. Further progress towards reducing water and waste is still required. Anaerobic processing and methane production might provide the best treatment for abattoir effluent with a relatively high content of plant material from rumen and stomach contents.