Hygiene of Slaughter – Pigs - Hygiene of Abattoir Operations

Hygiene of Abattoir Operations

5.4 Hygiene of Slaughter – Pigs

Hygiene of abattoir operations

Pre-slaughter phase

On arrival at the abattoir, it is important to ensure that pigs do not become stressed. Pigs have a very sensitive nervous system, and become stressed easily. Also, they have very fragile blood vessels, and therefore can bleed and bruise easily. These stress-related aspects can have a deleterious effect on the quality of meat, more so than in case of ruminants. Therefore, the unloading, lairaging and ante-mortem inspection of pigs must be conducted with great care. During summer months, pigs may be sprayed with water. This calms them down, and can assist the process of electrical stunning by facilitating conduction of electricity. Lairage areas contain a lot of faecal material, which will easily contaminate the animals’ skin if they lie down. Therefore, the lairage areas must be regularly cleaned. Passage of pigs from the lairage to the stunning area is mechanically aided in industrial, high-throughput abattoirs.

Stunning

Pig stunning can be conducted using electrical methods or with 70% CO₂ (see Chapter 5.1). With the former method, a conveyor can be used, which brings pigs individually to the stunning point. With the latter method, some animal welfare concerns have been expressed, due to a potential for pigs feeling pain when breathing carbon dioxide: CO₂ produces acid when dissolved in the blood. Mechanical stunning methods are not used for pigs.

Sticking/bleeding

The animal should be bled as quickly as possible after stunning. Sticking of pigs is not normally conducted using two knives, in contrast to the process for sticking ruminants. If blood is to be collected for human consumption, it must be collected in a closed system, using a special hollow knife with tubing connected to a dedicated container. These knives are sterilized between animals, as blood is very sensitive to spoilage, and can support growth of bacterial pathogens. Bleeding takes approximately 6 minutes, and sufﬁcient bleeding time must be enforced to ensure live pigs are not sent into the scalding tank.

Scalding

Scalding is treatment of the skin with hot water, approximately 60–62°C.

The water temperature is important, as too low a temperature will not loosen hair to facilitate its removal; and too high a temperature causes immediate denaturation of proteins at the hair root and skin, making the hair difﬁcult to remove. Therefore, the temperature of the scalding water

must be maintained and monitored. Scalding can be conducted using different techniques: (i) in a tank with submerged pigs moving horizontally through the water; (ii) by spraying with hot water in vertical position; or (iii) by treating the skin with steam. For practical reasons, the most common method in the UK is the horizontal scalding tank, although the latter two methods are considered as being more hygienic.

The hygiene of the scalding water is very relevant to the microbiological status of the carcass. Scalding dirty pigs in a scalding tank results in higher microbial contamination of the water. The water temperature (60–62°C) can kill a signiﬁcant proportion of bacteria, but when dirty carcasses move through the tanks quickly, the level of incoming new bacterial contamination can be higher than the die-off. Consequently, the scalding tank water must be changed when it becomes visibly contaminated. This can be costly, in terms of heating fresh water, and because the production line must be stopped, so cleanliness of pigs is an important issue.

Contamination of the sticking wound – and even internal tissues – can occur during scalding. For this reason, the sticking wound should be as small as possible, and is always removed at a later stage during dressing.

Also, ‘aspiration’ of dirty tank water into the lungs can occur during submersion scalding, rendering them inedible. Naturally, this could occur if an animal is stunned but still alive (breathing) when placed into scalding water, which would represent a gross breach of animal welfare. However, this lung contamination can occur even in some dead animals, as the muscles of the elastic thorax relax, enabling water to enter the lungs. This lung contamination may be prevented by placing plugs in the animals’

throat, but this has varying success. From a hygiene perspective, vertical scalding (spray) methods are more desirable, as lung contamination with scald water does not occur. However, these methods are more expensive, due to increased consumption of water and energy, greater equipment costs and more demanding maintenance.

Dehairing

Dehairing is usually done by placing scalded pigs into a dehairing machine, with horizontal rotating cylinders with rubber (or metal) ﬁngers rubbing across the skin, which removes the bristles. The dehairing machine must be very well designed and maintained; if not, the heavy force and pressure can cause damage to loss of the skin integrity, leading to meat contamination. The dehairing machine can become very heavily contaminated as it is used on every animal, so its continuous rinsing/cleaning is desirable. Bristles removed by dehairing machine must be regularly removed and disposed of in a suitable manner.

The scalding and dehairing areas in the slaughterhouse are considered as dirty. Therefore, movement of all personnel, equipment and air from these areas to the dressing line must be restricted.

Singeing

Dehairing machines cannot remove all hair from the pig, so some hair remains, particularly at less accessible sites of the carcass. Therefore, these remaining bristles are removed by singeing. Essentially, singeing is done by placing carcasses for a short time (seconds) in a gas-ﬂame oven with high temperatures (around 1000°C), which burns off the remaining hair.

The surface colour of the skin changes during singeing to golden-brown due to the high temperature, which may be desirable for carcass quality reasons, e.g. with bacon pigs. Furthermore, singeing is an effective antimicrobial treatment for pig carcasses, and leaves the skin in an excellent hygienic status.

Scraping/polishing

Singeing leaves burnt hair on the carcass skin, which must be removed.

This can be done in various ways, but usually carcasses pass through a machine with hard rotating brushes which scrub the burnt hair off. The brushes can easily become microbiologically contaminated as they are constantly in contact with a large number of carcasses. Unfortunately, this scraping/polishing process usually results in extensive microbial re- contamination of the carcasses. Therefore, the microbiological status of the post-polish skin is normally signiﬁcantly worse than that of the same carcass before polishing, i.e. after singeing.

Ear and eye removal occur next, and the carcasses are normally washed between the completion of dehairing/polishing and the beginning of evisceration.

Skinning

Skinning of pigs, instead of scalding and dehairing, can be applied in some cases. The skin of older pigs may be required by the leather industry, and in this case the skin cannot be heat treated by singeing. Also, very large pigs – special breeds or older animals – may not ﬁt into the normal slaughter line. The principles of hygiene for skinning of pigs are the same as for cattle. Again, the outside surface of the skin must not touch the underlying carcass, hands should not alternate and spear cutting must be used. Skin from the back is usually removed mechanically; some designs are more hygienic than others.

Evisceration

The cut in the middle line of the abdomen is made with a special round- tipped knife, which is much less likely to puncture the digestive tract as compared to an ordinary knife. The abdominal cavity is then opened, and the bladder and genital organs are removed, taking care not to allow urine to contaminate the meat.

All organs within the abdominal cavity are then removed in one step.

Then, the sternum is cut to eviscerate the thoracic cavity, using sterilized equipment. Thoracic organs are removed and placed in trays or racked for inspection; usually, the liver stays attached to the lungs and heart via the diaphragm.

Different types of equipment are used to free fatty tissue from the abdominal cavity; understandably, these must be sterilized between carcasses.

Carcass splitting

Carcass splitting through the vertebral column is often done with automatic equipment which, again, must be cleaned and sterilized between animals. Mobile parts of equipment are always in contact with edible tissue, so sterilization must be assured to prevent between-carcass cross- contamination.

Washing

Carcasses are normally washed after polishing and before evisceration, as well as after splitting.

Carcass classiﬁcation

Pigs are normally classiﬁed on-line, using a combination of several measurements. The pH is measured immediately to determine the existence of pale, soft, exudative (PSE) meat. The thickness of back fat, as well as of muscles, is also measured using electronic measuring and recording equipment. Also, after carcasses are classiﬁed, they must be marked accordingly, so the value of each pig carcass is known by the end of the slaughter line. Different carcasses are then normally diverted for different purposes, according to their value, e.g. for table meat, sausage production, bacon or ham production, etc.

Chilling, cutting and dispatch

Principles and hygiene of these processes are similar to those previously described for cattle.

Hygiene of Slaughter – Pigs

Hygiene of Abattoir Operations

5.4 Hygiene of Slaughter – Pigs

Further Reading