An average beef plant in the UK might kill 25,000 cattle, a pig plant 250,000 animals and a sheep plant 150,000–200,000 lambs a year. The largest specialist operations could have throughputs of 100,000 cattle or 500,000 pigs per year but these are still small in comparison with operations in, for example, North America.
organisms originally present on the hide or fleece of the animal. This has been elegantly demonstrated by the employment of ‘marker’
organisms, which are used deliberately to contaminate a site on the animal before the start of dressing. Subsequently the organism is looked for at other sites to define the degree of cross contamination. An example of the use of the technique was described by Hudson et al.
(1998) who used a non-pathogenic strain of Escherichia coliK12 that was resistant to nalidixic acid. By incorporating nalidixic acid into the growth medium used to culture the swab samples, the naturally occurring strains of E. coliwere eliminated allowing accurate detection of the marker strain.
The dressing of the carcass differs in detail for the different species and only the briefest general outline can be given here. Further information can be found in textbooks of meat inspection, for example Wilson (1998). In cattle and sheep the oesophagus is sometimes tied or closed with a clip to prevent backflow of the rumen contents (in cattle this is referred to as weasand tying). In these species, the head and feet are removed and the carcass is skinned. The head is normally removed at the occipito-atlantal joint and the feet below the carpals (knee bones) in the foreleg and tarsals in the hind leg. The feet thus include the cannon bones. Because of the dangers of contamination of the surface of the carcass with soil and dirt from the hide or pelt, skinning requires considerable care and skill. After the initial cuts with a knife, mechanical hide-pullers may be used to pull the hide or pelt from the carcass. As well as the benefits of ease and efficiency these are likely to improve hygiene by requiring less handling of the carcass.
Pigs are generally not skinned (except for making pigskin leather) but the hair is removed by scraping. This is facilitated by immersing the pig, after completion of bleeding, in a tank of water at 60°C for about 5 min (scalding). The hot water loosens the hairs and the outer layer of skin, which can then be scraped off, nowadays by passing the carcass between revolving metal-tipped paddles. The carcasses are washed and cleaned with revolving brushes and the toenails removed.
Before washing the carcass may be singed. In this process the carcass passes through a gas flame which burns off any remaining hair and tightens up the skin. Both scalding and singeing to some degree kill bacteria on the surface of the body but this is not sterilized. The scalding tank tends to accumulate large quantities of dirt and bacteria.
Vertical scalding, in which condensed steam is passed over the hanging carcasses, is sometimes used and may produce bacterio- logically cleaner carcasses. The feet are left on pig carcasses and the head is only removed at a much later stage.
Before evisceration, the skinned or dehaired carcass is removed from the shackles that have up to now suspended it from the overhead rail if this has not been done previously. It is resuspended from the
achilles tendons of the hind legs using a gambrel. To eviscerate the animal a cut is made around the anus allowing the rectum and the rest of the gut to be removed from the abdominal cavity through a ventral incision along the whole length of the carcass. This cut continues through the breastbone (sternum), allowing access to the thoracic cavity, to the throat. The sternum in mature cattle often needs sawing through (brisket sawing). The diaphragm is cut round and the trachea (windpipe), lungs, oesophagus (gullet), heart and liver are removed together as the ‘pluck’.
The carcass is often then split into the two sides by sawing down the backbone. After trimming, and sometimes removal of kidneys and other parts, the carcass is inspected, further trimmed and placed in a chiller to cool. Immediately before chilling the carcass may be washed, weighed and graded. In North America, beef carcasses can be ‘shrouded’
prior to chilling. The sides are covered in a tightly applied cloth shroud wetted with salt solution. This improves the surface appear- ance of the carcass, especially that of the fat, and reduces evaporative weight loss. The shrouds are removed after the initial period in chill.
The trend in modern slaughtering practice is for more operations to be carried out by machine. This especially applies to hide and pelt removal but it seems likely that mechanical evisceration will be developed for red meat species as it has already for poultry.
Particularly in countries such as Australia and New Zealand, there are increasing degrees of mechanization being developed in the dressing and automated boning of sheep and beef carcasses (Longdell, 1996). As well as reducing manpower requirements mechanization can improve carcass hygiene through reducing contamination, produce better hide and pelt quality, and give higher yields of retail joints when compared with manual methods.
Poultry processing
After stunning, poultry have the blood vessels in the neck severed to exsanguinate them. Usually this is carried out automatically with a knife with a rotating blade. A ventral neck cut should be used. In practice, many processors may use a dorsal cut to prevent severance of the oesophagus and trachea. This facilitates their removal from the neck flap when the carcass is eviscerated. Using a dorsal cut may only sever the blood vessels of one side and lead to slower bleedout.
Because birds’ brains are rather resistant to anoxia this will prolong the time to brain death unless, as often happens, the birds are killed by the stunning current passing through their heart.
To remove the feathers the birds are scalded, usually at a fairly low temperature (less than 55°C for broilers) for about 30 s. The hot water
softens the skin so that the feathers are easily pulled out. Use of higher temperatures damages the skin and downgrades the appearance of the carcasses. Plucking is by rubber-fingered rotating cylinders. Geese and ducks are often dipped in molten wax, which is then cooled to harden it. The solid wax, together with the feathers, is then pulled off. The valuable soft down on the underside of the breast is removed before waxing to use as insulation for quilts.
Evisceration is physically separated from the defeathering process to reduce carcass contamination. The skin along the back of the neck is cut and the trachea and crop freed. The abdominal cavity is opened up by an incision around the vent (cloaca) to enable the viscera to be pulled out. These include the intestines, gizzard, liver and heart and reproductive system. The lungs lie tight under the vertebral column and need careful removal. The head, neck and feet are cut off and discarded, and the carcass is chilled either in cold air or in iced water.
Water chilling tends to be used in North America and air chilling in Europe. The giblets, comprising the neck, liver, heart and gizzard, are sold with the carcass, usually packed inside it. The term ‘New York- dressed’ poultry refers to carcasses which are not eviscerated directly after plucking but are held refrigerated in an entire state until retailed.
Although traditionally popular, this is unusual nowadays.
Poultry processing is now highly mechanized with most, if not all, operations carried out by machine. This includes evisceration. A potential problem is that any microbial contamination of the carcass, for example by accidental breakage of the gut, is likely to be spread to large numbers of other carcasses by the processing machinery.
Carcass washing
After the completion of dressing, carcasses are washed with a fine spray of water to remove surface blood, bone dust and any visible soiling. Unless prolonged, washing with cold water has little effect on bacterial numbers recovered from the carcass surface. Effectiveness may be increased by using hot (80°C) water or by including low concentrations of organic acids, chlorine or other agents in the water (see Chapter 9). The traditional practice of wiping carcasses with cloths is now considered unhygienic and is banned in some countries.
Post-mortem inspection
The carcass and viscera are inspected as soon as possible after slaughter.
As in ante-mortem inspection, this is to identify abnormalities or disease that would make the meat and edible offal unfit for human
consumption. To this end it is important that the carcass retains its identity with the parts and viscera removed from it. In modern systems this is often achieved by synchronized parallel line systems. Inspection is normally carried out by specially licensed veterinarians or meat inspectors. As well as inspection, they will often have other roles, including overseeing animal welfare and hygiene standards.
Tissues and organs are examined by visual inspection, palpation and incision. Considerable attention is paid to routine incision of lymph nodes to detect disease states. In infection, bacteria collect in the lymphatic system and are concentrated and destroyed in the lymph nodes. These occur at intervals throughout the system. In disease they become swollen and abnormal in colour, so alerting the inspector to look for the disease. Conditions such as pneumonia and tuberculosis have characteristic lesions. Parasitic diseases such as those caused by tapeworms (cestodes) are often manifested in cysts of the larval forms in the heart, liver and muscles. For example, Cysticercus bovis is the encysted intermediate stage of the beef tapeworm Taenia saginatathat lives in the small intestine of man (see Chapter 9). ‘Measly’ beef and
‘measly’ pork were previously referred to in cases where there had been extensive infection with C. bovis and the pork tapeworm Cysticercus cellulosae respectively. Mature liver flukes (Fasciola hepatica), which are trematodes, can occur in the bile ducts of the liver of infected cattle, sheep, goats and pigs. The larval stage of the round- worm (Nematoda) Trichinella spiralis forms cysts in pork muscle, which, if eaten by man, can cause trichinosis (trichiniasis) if the meat has been undercooked. The larvae of the nematode parasite of pigs, Ascaris suum, may lodge in the liver and result in small foci of connective tissue giving rise to a condition known as milk spot liver.
The adult worms live in the pig’s small intestine.
Inflammation of the lining of the thoracic cavity (the pleura) or the lining of the abdominal cavity (the peritoneum) may indicate infection.
Swine erysipelas, a bacterial disease caused by Erysipelothrix rhusiopathiae, produces characteristic, red, diamond-shaped marks on the skin of infected pigs. Abscesses, arthritic joint capsules and bruises may make meat unfit for human consumption. In the UK, The Fresh Meat (Hygiene and Inspection) Regulations 1995 list 46 specific conditions which, if found, render the whole carcass, offal and blood unfit for human consumption. Various findings may require condemnation as unfit of parts of the carcass and, if necessary, trimming of tissue. Details of meat inspection procedures can be found in standard textbooks (Gracey and Collins, 1992; Bremner and Johnston, 1996). In modern production systems the emphasis is on presenting only healthy animals for slaughter, controlling the levels of parasites and of infectious diseases largely by measures taken at the farm.
Carcass butchery
Carcasses are sold whole, or as sides, or may be cut into smaller ‘primal’
or ‘retail’ joints. The exact way a carcass is butchered into joints varies between countries, within countries and depending on the particular use that may be made of the meat. Three ways of dividing up a side of beef into primal joints are shown in Fig. 4.5. These illustrate the variation between countries. In particular, in the French system, the carcass is cut into many more joints. This reflects the way retail joints in France are often based on individual, or small groups of adjacent muscles, dissected from others along the natural lines of separation between muscles. This allows inherent differences in the quality characteristics of different muscles, particularly tenderness, to be best exploited. In the systems used in the UK and USA, the differences in inherent quality are still recognized. For example the high value cuts of the loin and hindquarter (sirloin, rump, topside and silverside) are distinguished from lower value cuts such as those of the shoulder, brisket and flank, but the differentiation is less detailed. In the UK and USA, retail joints tend to be cut with less regard for differences between individual muscles, the cutting lines being across muscles rather than between them.
The smaller carcasses from pigs and lambs are cut into fewer primal joints. In the pig, after removal of distal ends of the legs, the carcass is cut into four main primal joints, the loin, belly, hind leg or ham, and the foreleg/shoulder. In North America the foreleg/shoulder is cut into the ‘butt’ and the ‘picnic’. An excellent account of the different cutting systems is given in Swatland (1994).