5 Slaughter and Dressing When any animal is slaughtered for food, it is important for ethical reasons that the methodology employed does not inflict pain. Researchers have demonstrated a relationship between the degree of pre-slaughter stress and carcass and meat quality, with increasing levels of stress resulting in decreasing levels of quality. Stunning prior to slaughter by exsanguination was introduced for mammals in the UK in the 1920s. The concept of ‘stunning’ in relation to an animal means any process that causes immediate loss of consciousness, which lasts until death (The Welfare of Animals (Slaughter or Killing) Regulations – DEFRA, 1995). It is essential that once an animal is stunned it is slaughtered as quickly as possible to prevent recovery before or during the bleeding process. A number of systems are specifically designed to kill the animal at the point of stun and following these methods exsanguination simply voids the carcass of blood. Mechanical stunning employs a percussive blow to produce brain dysfunction through the induction of a concussed state. The stun can be recoverable, e.g. as in a boxer’s ‘knock-out blow’, or irrecoverable if extensive trauma to brain tissue is produced. The importance of the role of the skull in the induction of concussion is recognized in UK legislation (DEFRA, 1995) where the stun must be produced without fracture to the skull when using a non-penetrative blow. With mechanical stunning it is difficult to calculate exactly the forces acting on the head. However, the energy of the mechanical stunning system can be measured. where m = mass of the bolt (kg) and v = velocity (m/s). When a captive bolt is used with sheep the target area is the highest central point of the head, aiming straight down towards the angle of the jaw. Adjustments are necessary when shooting horned animals to ensure accuracy and penetration. With horned sheep and goats the position is further back, behind the ridge between the horns on the midline and aiming towards the base of the tongue. The horned animals must be bled within 15 s of the shot to prevent recovery. In young pigs, i.e. pork and bacon weight, the shot position is at a point 2 cm above the rear margin of the eyes, on the midline and aiming towards the tail. In larger boars and sows the skull has a more ‘dished’ conformation, which has to be taken into account when positioning the captive bolt pistol. Shot position for adult pigs is at a point 5 cm caudal to a line joining the rear margin of the eyes slightly to one side of the midline. It is recommended that electrical stunning be used for adult pigs, because of the bone and sinus development. Cattle should be shot at the cross-point of two imaginary lines from the rear corners of the eyes to the opposite horn buds. In the event of an ineffective stun, the back-up gun should be repositioned 1 cm caudal and 1 cm lateral to the ideal position. With non-penetrative captive bolt stunning the shot position is 2 cm above that used for a penetrative captive bolt. Following an effective captive bolt stun the animal should immediately collapse, become rigid with its forelimbs extended and hindlegs tucked under the abdomen. The eyes should have a fixed and glazed appearance. There should be no positive corneal reflex and no rhythmic breathing (brainstem reflexes). Heart action, however, does not stop but continues for some time (3–4 min). If bolt penetration occurred there may be enough physical damage to make recovery impossible. When an ineffective stun occurs, the animal’s eye tends to be rolled down, and instead of having a fixed glazed appearance the animal will show a positive eye reflex and can be observed to be breathing rhythmically. In the worst cases the animal may not collapse at all, or if it does so at the outset, it may get back on its feet. Ineffective captive bolt stunning can be caused by incorrect positioning or problems associated with the stunning equipment, for example poor maintenance, incorrect cartridge, etc. The application to red meat animals of alternating electrical currents (AC) of sufficient magnitude will produce epileptiform activity in the brain. This is analogous to a human being undergoing a ‘tonic/clonic’ epileptic fit. During this fit an epileptic human is always unconscious, therefore a similar brain condition in animals is analogous to a stunned state. The production of epileptiform activity in the brain may not be immediate (>200 ms) however, but the application of high-amplitude AC to brain tissue will inhibit normal neuronal function for the duration of current application, thus bridging a possible delay between the start of current application and the initiation of epileptiform activity. The criteria for an ‘immediate’ stun are therefore assured. Cook (1993) demonstrated that epileptiform activity is generated in the brain through the over-stimulation of nerve endings by the stunning current. The ‘over-excitation’ stimulates the release of two neurotransmitters (glutamate and aspartate), which at very high levels of production result in epileptiform activity in the brain. Thus a threshold is reached which, once exceeded, produces brain dysfunction and unconsciousness. This simplified view of what is physiologically a complex series of events does help to explain the results of Anil (1991) and Daly (1990) (Table 5.1). The start of the recovery process can be identified by the return of rhythmic breathing movements (brainstem reflex), which returns when the epileptiform activity in the brain subsides. The times given in Table 5.1 are from the start of the current application. Neither an increase in the stun duration from 3 to 7 s, nor an increase in applied voltage had any effect on the duration of unconsciousness produced by the stun. The results suggest that in both sheep and pigs unconsciousness is produced very quickly after the start of current application and that, provided sufficient current is applied, this period of unconsciousness is unaffected by prolonged application (across the range evaluated) or by increased current amplitude. The recommended minimum current to stun is given in the Table 5.2. Low-frequency (50 Hz) electrical current can be used to initiate a cardiac arrest through ventricular fibrillation. Therefore, stunning systems that include the heart in the electrical pathway between the electrodes, e.g. head-to-back, will promote the start of death to the point of stun and therefore should be promoted. Recent research has shown that the impedance of a live pig’s head is predominantly a function of the stunning voltage, and decreases nonlinearly with increasing voltage. These results suggest that voltage may be a more significant parameter in the production of an effective pre-slaughter electrical stun than was previously thought. In particular, the applied voltage should be in excess of the threshold necessary to break down the initial high impedance, to promote effective and immediate stunning. Table 5.2. Head-only minimum currents to stun for red meat species. Species Minimum current to stun (amps) Pigs 1.3 Sheep/goats 1.0 Lambs/kids 0.6 Calves 1.0 Cattle 1.2 1. The tonic phase starts from the onset of current application when the whole body of the animal will become rigid, rhythmic breathing will stop and the eyes may roll. The head becomes raised and the hind legs are flexed under the body. The forelegs may initially be flexed but then usually straighten out. This is the tonic phase, which usually lasts for about 10–12 s. 2. The clonic phase immediately follows the tonic phase and can be recognized by the presence of uncontrollable involuntary motor activity, i.e. kicking, which generally lasts between about 20 and 45 s. Eye roll or flicker and salivation are also often seen during the clonic phase. Termination of the clonic phase will lead to the return of rhythmic breathing and the subsequent recovery in an unbled animal. The one exception to the above symptoms can be observed following electrical cardiac arrest stunning of cattle, when rhythmic breathing, as a result of residual brainstem activity, can occur in a cortically dead animal. Carbon dioxide is used for killing pigs in the UK, and more widely in Europe and America as a stunning, but not necessarily as a killing, method. UK legislation states that pigs may be killed at a slaughterhouse by exposure to a carbon dioxide gas mixture in a chamber provided for the purpose (DEFRA, 1995). CO2 is an acidic gas that will dissolve in water or saliva to form an acidic substance. The gas is also absorbed through the lungs, where it enters the blood system and is carried in a readily available form. Once in the blood the CO2 compound will cross into the fluid (CSF) bathing the spinal cord and the brain, where it increases the acidity (measured in pH units). When the pH is lowered from its normal value of 7.4 to 7.1 the animal will begin to lose consciousness. If the exposure continues, the pH will drop further and below pH 6.8 the animal will enter a stage of deep anaesthesia followed by death. Concern has been expressed about the humaneness of the induction of anaesthesia with CO2 in all species. Raj and Gregory (1996) showed that pigs would avoid high concentrations of CO2, to the extent that they would not enter an atmosphere of CO2 for a reward of chopped apples, even when fasted for 24 hours. When the CO2 was replaced by an atmosphere containing the inert gas argon, with less than 2% oxygen, the pigs entered, fed and were stunned, recovered and voluntarily repeated the process. The researchers demonstrated that the use of an anoxic atmosphere, produced by an inert gas, to kill pigs was not stressful. Similar results have been obtained with poultry and fish. Anoxic killing using either argon or nitrogen is seen as a more humane method of slaughter, provided animals are killed in the modified atmosphere. However, because recovery from anoxia is very rapid, animals need to be exposed for sufficient time to kill them rather than just to stun them. In a commercial slaughterhouse, pre-slaughter stunning is followed by exsanguination to produce the death of the animal. The majority of stunning systems do not kill, e.g. head-only electrical stunning, but simply render the animal unconscious for a sufficient period to allow the animal to die following the severance of major blood vessels. In the laboratory, death can be defined as the irreversible breakdown of the central nervous system (CNS). Brain failure or death can be diagnosed through the production of an isoelectric EEG or, objectively, through the irreversible failure of specific neural pathways. The visual pathway is a basic pathway with perhaps only a single synapse between the retina and visual cortex. Photic stimulation of the retina with a strobe can be measured in the visual cortex with EEG or ECoG electrodes. Signal averaging techniques permit the identification of the visually evoked response (VER) and through the use of a system of moving averages the time to loss of brain responsiveness to visual stimuli can be identified. The times shown in Table 5.3 are the average values for each sticking method in sheep. The different sticking methods were carried out under full anaesthesia and their accuracy was verified at the end of each recording session. A bilateral neck cut severing both carotid arteries and both jugular veins resulted in the fastest time to loss of brain responsiveness (14 s). Taken in isolation, this method could be considered as the most humane of the four methods tested; however, as we shall see later, these results should not be viewed in isolation. An inaccurate stick that might miss the vessels on one side of the neck prolongs the time to brain death by a factor of five. However, if the carotids are missed altogether the time is extended to nearly 5 minutes. Cardiac arrest produced an average time of 28 s but also promoted the start of death to the point of stun, presenting a distinct welfare advantage over the other methods. Table 5.3. Sheep: time to loss of brain responsiveness. (From Gregory and Wotton, 1984a.) Sticking method Number of sheep Time to loss of brain responsiveness (s) Both carotid arteries and both jugular veins 20 14 One carotid artery and one jugular vein 8 70 Neither carotid artery and both jugular veins 8 298 Electrically induced cardiac arrest 8 28 Effective head-only electrical stunning has been shown to result in a minimum time to return of rhythmic breathing (symptomatic of the start of recovery) of 37 s with pigs. On average, a pig will take 19 s to reach brain death following sticking. However the maximum time was 22 s. Given that sticking would result in a maximum of 22 s to ‘kill’ the pig, in practice this limits the maximum stun to stick interval to 37−22 = 15 s (Anil et al., 1997). The calf showed little difference from sheep and pigs, demonstrating an average time to loss of brain responsiveness of 17 s when both carotid arteries and jugular veins were severed during sticking (Gregory and Wotton, 1984b; Table 5.4). Shechita in adult cattle resulted in the severance of both carotid arteries and both jugular veins. However, the average time to loss of brain responsiveness was 55 s, which was significantly greater than that observed in the other species. The range of times was also seen to vary greatly (20–102 s) (Daly et al., 1988). This can be explained by considering the anatomy of blood vessels in the neck of cattle compared to those of sheep and pigs. In sheep and pigs, the vertebral artery has no direct connection with the brain, whereas in cattle it corresponds directly. Therefore, after severance of the main blood vessels in the neck of cattle this artery can still supply blood to the head. The delayed loss of brain function is also exacerbated by the formation of carotid balloons. This phenomenon was described by Anil et al. (1995 a, b), who examined the relationships between the blood flow through the carotid and vertebral arteries and brain function in calves during slaughter. They concluded that brain function can be sustained following severance of both carotid arteries and both jugular veins in the neck of calves, due to the occlusion of the severed caudal ends of the carotid artery. When the carotid artery is severed at sticking, occasionally the muscle wall retracts within the connective tissue sheath and the sheath forms a blood-filled balloon, which quickly clots, thereby occluding the cut vessel and impeding blood loss. A solution to the problem is the use of a thoracic stick with both calves and adult cattle. If the blood vessels inside the chest close to the heart are severed, then blood cannot flow through the vertebral artery to the brain. Table 5.4. Pigs: time to loss of brain responsiveness. (From Wotton and Gregory, 1986.) Good practice in the poultry lairage and during shackling requires continual monitoring by personnel responsible for bird welfare. Lairage conditions should take into account ambient temperature, humidity and the general condition of birds arriving at the processing plant. Bird activity within and outside the transport containers should be kept to a minimum. Flapping, for example on the shackle line, will increase downgrading and reduce the effectiveness of electrical waterbath stunning. The shackling procedure itself combined with bird inversion has been shown to be painful to birds, therefore the time birds are shackled before stunning should be kept to a minimum (12 s for chicken and 25 s for turkeys). By minimizing this period, the welfare of the birds can be more easily maintained in the event of a line breakdown, in that there will be less birds to remove or, preferably, stun/kill with a back-up device. Research has demonstrated that there are doubts as to the welfare aspects of decapitation and neck dislocation, which has led to the development of an alternative killing system for use in the casualty slaughter of poultry (Hewitt, 2000). A pneumatically powered percussive device has been developed for use either as a back-up to the killer or for the dispatch of shackled birds in the event of a line breakdown. It is hoped that both neck dislocation and decapitation will be phased out. The turkey can be used as a prime example of the welfare problem of pre-stun shocks, as the average incidence of pre-stun shocks in a survey of turkey plants was found to be 45% (range 0–87%). The prevalence of shocks in turkeys is exacerbated by the anatomy of the bird. Turkeys have wings that hang lower than their heads when the bird is inverted and suspended on a shackle line. This means that their wings will enter the ‘live’ water first and the bird will receive a pre-stun shock. It is also important that the water does not overflow at the entrance ramp, creating a wet route through which live contact can be made, otherwise birds will receive a painful pre-stun shock on the way into the bath. This is particularly a problem at slow line speeds and with badly designed waterbath entrances. Contact with a ‘live’ ramp will induce painful muscle contractions, which may result in birds flying the stunner or making and breaking contact throughout the stunner length. Waterbath entry ramp design and manipulation can be the solution for plants by holding back the bird at the top of the ramp for sufficient time to ensure that they swing down into the ‘live’ water in a fast, clean entry. Electrical stunning is the most commonly applied stunning method by the poultry industry. Birds are electrically stunned in waterbath stunners where the water is ‘live’ and the stunning current flows through the head (brain) and body of the bird to ground through an earthed shackle. Sufficient electrical current must penetrate the brain to induce a stunned state that will enable the bird to remain unconscious until it is dead either through cardiac arrest, induced at the point of stun, or by exsanguination. Commonly, electrical stunners apply mains frequency (50 Hz) alternating current (AC) of sinusoidal waveform. A 50 Hz sinewave is one of the optimum frequencies and waveforms for inducing cardiac arrest though ventricular fibrillation. In addition to the induction of brain dysfunction, the applied voltage will also stimulate muscles to contract. This muscle stimulation is brought about in three ways: first, through direct muscle stimulation; second, through stimulation of the motor cortex in the brain; and third, through the stimulation of motor nerves in the periphery. The direct muscle stimulation can result in muscle haemorrhages and broken bones and this has led the industry to apply higher frequencies, which have a reduced effect on muscle stimulation. Laboratory methods: 1. The method of EEG assessment is not without certain limitations; for example, it has failed to provide unequivocal indication of the state of unconsciousness as produced by sleep or anaesthesia. 2. Somatosensory-evoked responses (SERs) represent a basic level of response, which can be used to investigate the patency of a nervous pathway. The presence of an evoked response does not necessarily indicate consciousness, as they occur in conscious and anaesthetised animals (Gregory and Wotton, 1983). However, the abolition of SERs does indicate a profound loss of consciousness in poultry. 3. The return of rhythmic breathing has been used extensively in red meat species and poultry to indicate the start of the recovery process. The presence of rhythmic breathing indicates that the brain stem and spinal cord are still functioning. It is not a proof of consciousness, but indicates the need for further tests to establish whether the birds are conscious. 1. The use of rhythmic breathing assessment is a basic method that can be used in the processing plant to determine the effectiveness of a stunning system. If the bird has been stunned effectively, rhythmic breathing will not resume for about 8 s or more from the bird’s exit from the waterbath. Looking for signs of rhythmic breathing is not a valid test of consciousness and/or death if the spinal cord has been broken or severed by neck cutting. The use of SERs has allowed researchers to measure the effect of increasing current amplitude on the effectiveness of electrical waterbath stunning. The abolition of the SER suggested that 120 mA should be the minimum recommended current level per bird. Measurement of whether a treatment can abolish a multisynaptic response is an objective method; however, it can be argued that it gives a very conservative answer, whereas the return of neck tension more closely follows the bird’s recovery. These results, taken together with the subjective results using the return of neck tension, produced a recommendation for a minimum current of 105 mA per bird for chickens when an AC voltage is applied. Recent research combining EEG analysis by Fast Fourier Analysis and SER abolition has suggested a minimum rms AC of 100 mA per bird for 100 and 200 Hz, and that the current should be increased for frequencies above 400 Hz. The poultry industry has adopted waterbath stunning using a high-frequency pulsed DC waveform with a 25–30% duty cycle, because of the improvements in carcass and meat quality that they can achieve. In addition, the use of low-frequency AC stunners at the minimum current to stun (105 mA per bird) will result in some birds receiving less than the minimum current, due to variation in impedance between birds whereas, with high-frequency pulsed DC stunners, three or four times the minimum current can be applied i.e. 40–50 mA per bird, and all birds should receive more than the minimum recommended current. It is important that the correct blood vessels are severed and that the cut is made as quickly as possible following electrical waterbath stunning. When 105 mA is applied per bird at 50 Hz, about 90% of birds will be killed in the stunner (ventricular fibrillation). However, it is still important that the neck-cutting procedure is accurate. When higher frequencies are applied, the majority of the birds will survive the stunning treatment. The death process starts from the severance of major blood vessels when insufficient oxygenated blood reaches the brain. It is essential that both carotid arteries are severed at neck cutting to ensure the birds do not recover. An additional advantage can be achieved if heads can be removed by the killer for immediate maceration, which is a solution to the welfare concern over: (i) neck cutting procedures; (ii) the duration of unconsciousness produced by the electrical stun; and (iii) misinterpretation of normal post-kill bird movement. Anil, M.H. (1991) Studies on the return of physical reflexes in pigs following electrical stunning. Meat Science 30, 13–21. Anil, M.H., McKinstry, J.L., Wotton, S.B. and Gregory, N.G. (1995a) Welfare of calves – 1. Investigations into some aspects of calf slaughter. Meat Science 41, 101–112. Anil, M.H., McKinstry, J.L., Gregory, N.G., Wotton, S.B. and Symonds, H. (1995b) Welfare of Calves – 2. Increase in vertebral artery blood flow following exsanguination by neck sticking and evaluation of chest sticking as an alternative slaughter method. Meat Science 41(2), 113–123. Anil, M.H., McKinstry, J.L. and Wotton, S.B. (1997) Electrical stunning and slaughter of pigs. Fleischwirtschaft 77(5), 473–476. Cook, C.J. (1993) A guide to better electrical stunning. Meat Focus International (March), 128–131. Daly, C.C. (1990) Stunning and slaughter an overview – meat quality from gate to plate. Proceedings of a two-day course organized by the Meat Technology Service, Division of Meat Animal Science, University of Bristol, Langford, UK. Daly, C.C., Kalweit, E. and Ellendorf, F. (1988) Cortical function in cattle during slaughter: conventional captive bolt stunning followed by exsanguination compared with shechita slaughter. Veterinary Record 122, 325–329. DEFRA (1995) The Welfare of Animals (Slaughter or Killing) Regulations. Statutory Instruments No. 731, HMSO, London. Gregory, N.G. and Wotton, S.B. (1983) Studies on the central nervous system: visually evoked cortical responses in sheep. Research in Veterinary Science 34, 315–319. Gregory, N.G. and Wotton, S.B. (1984a) Sheep slaughtering procedures. 2. Time to loss of brain responsiveness after exsanguination or cardiac arrest. British Veterinary Journal 140, 354–360. Gregory, N.G. and Wotton, S.B. (1984b) Time to loss of brain responsiveness following exsanguination in calves. Research in Veterinary Science 37, 141–143. Hewitt, L. (2000) The development of a novel device for humanely dispatching casualty poultry. PhD thesis, University of Bristol, UK. Raj, A.B.M. and Gregory, N.G. (1996) Welfare implications of the gas stunning of pigs 2. Stress of induction of anaesthesia. Animal Welfare 5, 71–78. Wotton, S.B. and Gregory, N.G. (1986) Pig slaughtering procedures: time to loss of brain responsiveness after exsanguination or cardiac arrest. Research in Veterinary Science 40, 148–151. The general flow of operations during cattle slaughter and dressing is illustrated in Fig. 5.1. Staff working in abattoirs must be adequately trained, with training records suitably maintained. Suitable health records must be maintained for all persons working or visiting abattoirs, including veterinary science students, abattoir workers and veterinarians. Health must be assessed on the basis of transmissible diseases, and the issue of whether workers are healthy carriers of food-borne pathogens (e.g. Salmonella, viruses). Naturally, employees must not work when ill, and should undergo regular medical checks. In addition, staff should not work if immunocompromised, as this status could put them at risk from infectious agents from the livestock/meat. Hair nets, beard covers, knives, steels, lockers, aprons, smocks, boots, etc. should be maintained and handled in a clean and sanitary manner. These measures help to ensure that the meat produced is protected from direct contamination from plant personnel. Proper changing room facilities and suitable facilities for personnel to wash and clean personal equipment must be provided. Storage lockers should be kept clean and free of dirty clothes, rags etc. Shrouds, aprons, gloves and cotton items should be placed in a marked plastic container after use. These must be washed and dried before being returned to the processing plant. Staff must wash their hands on leaving the restrooms. Hot water (82°C) sterilizers for knives/steels must be monitored regularly for temperature. Multiple knives must be used by each worker, to allow adequate time in the sterilizer to ensure proper microbial kill. Hot water sterilization is effective against most bacterial pathogens, but ineffective against prions. Equipment must be cleaned and sanitized at least daily. However, the Official Veterinary Surgeon (OVS) can require that cleaning and sanitation regimes are conducted more often, as necessary. Before work commences in the morning, the OVS must inspect the premises, checking that the abattoir is ready to use, all equipment is in correct working order and that cleaning and sanitation has been conducted properly. Regular maintenance and repair of equipment must be thoroughly conducted. Belts and other meat conveyance surfaces should be inspected frequently for cracks, pitting, etc., which can hamper cleaning/sanitizing. Complex equipment can experience a build-up of bone dust, meat and fat particles, which require special measures to remove. The OVS’ role is to advise on the hygienic acceptability of, but not to remove, any suitable equipment being used.
5.1 Humane Slaughter
Mechanical Stunning Methods
Recommended shooting positions
Sheep
Pigs
Cattle
Recognizing the effectiveness of a captive bolt or concussion stunner
Signs of an effective captive bolt stun
Signs of an ineffective captive bolt stun
Electrical Stunning Methods
How to recognize an effective stun when using electricity
Carbon Dioxide Killing
Mammalian Stunning and Slaughter
When to stick
Poultry Stunning and Slaughter
Electrical stunning
Pre-stun shocks
Electrical stunning equipment
Methods for assessing effective electrical stunning
Effect of stunning current on efficacy of stun
Slaughter
References
5.2 Hygiene of Slaughter – Cattle
General Hygiene Requirements
Staff
Equipment
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Slaughter and Dressing
