Slaughter procedures are extremely variable, and common methods in one locality may be unheard of in another. The introduction of equipment, such as a hide puller, may lead to a reorganization of the slaughter line. The main objectives of a slaughter procedure are to get the job done neatly, hygienically and fast. One possible method for beef carcasses 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 (Cagle and Henrickson, 1970). 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 (Dickson and Anderson, 1992). 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 (see Chapter 2) 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 (Field et al., 1990).
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 (van der Wal et al., 1993). Scalding at this temperature for longer than six minutes damages the skin (Mowafy and Cassens, 1975). 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. 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 (Judge et al., 1978). 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 (Figure 1-25) 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 (Cooke, 1987). However, this usually displaces several kilograms of fat from the edible carcass to the inedible hide, with a consequent loss in revenue (Rust, 1974). The kidney and pelvic fat of beef carcasses may be removed before chilling to facilitate lard rendering operations. There is no deleterious effect on the underlying meat (De Felicio et al., 1982).
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 (Barnes and Impey, 1975).
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 (MIRINZ, 1992).
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 (Heagerty, 1928). 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 Salmonella (Anon., 1981a). 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 (Kotula and Stern, 1984).
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 (Peel and Simmons, 1978). 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 (Smeltzer et al., 1979). 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 (Saini and Webert, 1991; Eckersall, 1992).
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 (Mayerson, 1963). 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. A recent study (Lee and Shivers, 1987) shows that the nematode larva may in fact 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 (Kotula et al., 1984). 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 (Ten Horn, 1973). 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 (Thompson, 1979).
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 (Juranek et al., 1976).
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 (Anon., 1981b). 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 (Borchert, 1972), 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 (Anon., 1982). Crenwelge et al. (1984) found that color scores for ham muscles were increased favorably by more rapid chilling.
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 (Hamby et al., 1987). 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 (Cutting, 1974). The temperature of lamb carcasses should not be forced below 10oC within 10 hours of slaughter. These topics are considered in more detail in Chapter 9.
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 (Cuthbertson, 1980). Electrical stimulation may be used to accelerate the conversion of muscles to meat so that hot-boning or accelerated processing can be undertaken. Satisfactory results have been obtained, even with pork carcasses (Neel et al., 1987). 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 (Lilliard, 1982). However, old-fashioned air cooling helps to preserve the flavor of poultry meat and may command a premium price for the product.
RENDERING AND WASTE DISPOSAL
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. The engineering techniques that may be used to achieve this goal are described by Jones (1974), but 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 (Tritt and Schuchardt, 1992).