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Electrical Inputs and MeatProcessing

PHILIP E. PETCH

MIRINZ Centre AgResearch, Hamilton, New Zealand

I. INTRODUCTION

II. ELECTRICAL STUNNING

III. ASPECTS OF ELECTRICAL STUNNINGA. Head-Only Electrical StunB. Deep StunC. Electrical Stunning Parameters and EquipmentD. Efficacy of Electrical StunningE. Placement of ElectrodesF. Blood-Splash and Broken BonesG. High-Frequency Stunning

IV. ELECTRICAL TREATMENTS DURING DRESSINGA. Low-Voltage ImmobilizationB. Spinal DischargeC. Bleeding TreatmentsD. Hide Pull

V. ELECTRICAL STIMULATIONA. OverviewB. Low-Voltage Electrical StimulationC. High-Voltage Electrical Stimulation

VI. SOME PROBLEMS WITH ELECTRICAL TREATMENTSA. OverviewB. Contact and Carcass ResistanceC. Special Problems with High-Voltage Stimulation

VII. CONTROLLED CURRENT TECHNOLOGY

VIII. WORKER HEALTH AND SAFETY

IX. MONITORING SYSTEMS AND PROCEDURESA. Process Monitoring EquipmentB. The Need for Processing Auditing

Copyright © 2001 by Marcel Dekker, Inc. All Rights Reserved.

C. Process-Auditing EquipmentD. Equipment Calibration

X. AREAS OF RESEARCH

XI. SUMMARY

APPENDIX: BASIC ELECTRICAL THEORY

REFERENCES

I. INTRODUCTION

Pioneering experiments with electricity and muscle began in the 1600s when Swammer-dam stimulated an innervated frog muscle with a low voltage, causing the muscle to con-tract (Bendal, 1980). In 1780 Luigi Galvani carried out a range of electrical experiments(Fig. 1), including hanging freshly killed frogs’ legs on an iron fence during a thunder-storm. When the legs touched the iron railing they twitched violently, even when there wasno lightning (Wilson, 1965).

In the 1950s, several workers explored the use of electrical stimulation as a means ofimproving tenderness of meat (Harsham and Deatherage, 1951; Rentschler, 1951). Theadoption of high-speed blast freezers by the New Zealand frozen meat trade in the late1960s led to serious problems with cold shortening, resulting in unacceptably tough meat.Electrical stimulation of carcasses was found to accelerate the onset of rigor so the car-casses could be frozen quickly while avoiding cold shortening. The success of electricalstimulation in New Zealand sparked major scientific interest and led to the commercial ap-plication of this technology by meat-exporting nations.

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Figure 1 Early experiments with electrical stimulation, by Galvani.


During the 1970s and 1980s, electricity gained further uses in meat processing as ameans of ensuring humane slaughter via electrical stunning, and of enhancing workersafety by preventing reflex movements of the carcass during dressing.

This chapter discusses the application of electrical inputs during slaughter and dress-ing from an engineering perspective rather than a meat quality perspective. It discusses theapplication of electrical stunning, methods for immobilizing the carcass to prevent carcassmovement during dressing, and the application of electrical stimulation to promote tender-ness. Some gaps in the knowledge, and topics of current research are also briefly discussed.The chapter concludes with a brief outline of elements of electrical theory that may be help-ful when reading the chapter.

II. ELECTRICAL STUNNING

Seeking to kill a domestic animal in the most painless way possible is the final act of wel-fare that humankind can offer it. When animals are slaughtered in meat packing plants, theyare generally stunned before slaughter. If stunning is properly carried out, the animal’sdeath will be much less distressing to it than could ever be expected in the wild. This is asit should be.

A properly stunned animal will be insensible, so it feels no pain or discomfort duringthe slaughter procedure. To satisfy this requirement, stunning must itself be acceptably freeof pain or stress, and insensibility must be maintained during slaughter until unconscious-ness due to hypoxia in the brain occurs. Throughout the world, the meat industry uses var-ious methods of stunning animals, including free bullets, captive-bolt pistols, percussionbolts, carbon dioxide gas, and electrical stunning. Internationally, the captive-bolt pistol ismost commonly used to stun cattle; electrical stunning is most widely used for sheep andpigs (Gregory, 1998).

The use of electricity to stun animals prior to slaughter has an extended history. Ben-jamin Franklin is reputed to have carried out experiments with electrical stunning in theeighteenth century (Hill, 1935). The Slaughter of Animals Act, passed in the United King-dom in 1933, authorized the use of electrical stunning for pigs as one way to ensure that“every such animal shall be instantaneously slaughtered or shall be rendered insensible topain until death supervenes.” This lead to the widespread use of electrical stunning inBritish pig plants during the 1930s, and subsequently in many other countries (Warrington,1974).

The objective of electrical stunning is to pass a current through the brain to tem-porarily disrupt normal brain function. If done correctly, the animal is rendered uncon-scious and completely unresponsive when the slaughter cut is made. By the time that theanimal would have recovered from the stun if not slaughtered, its brain has ceased to func-tion due to oxygen deprivation caused by loss of blood flow to the brain. Electrical stun-ning technologies can be divided into two main forms: head-only stunning and deep (car-diac arrest) stunning. These technologies are now discussed.

III. ASPECTS OF ELECTRICAL STUNNING

A. Head-Only Electrical Stun

The head-only electrical stun, in which the electric current is passed through the brain butno other vital organs, induces a seizure similar to a grand mal epileptic seizure. Electroen-

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cephalograms (EEGs) recorded immediately after stunning are very similar in form to thoserecorded during human grand mal epileptic seizures (Devine et al., 1986; Jones et al, 1988).Typical EEG waveforms recorded before and after stunning are shown in Fig. 2. Based onthe human experience, it is assumed that the animal exhibiting this brain activity is uncon-scious in essentially the same manner as a human is during the seizure (Gregory, 1998).

Because no electric current passes through the heart, the heart continues to beat dur-ing the stun-induced seizure and if slaughter does not proceed, the animal will eventuallyrecover with no ill effect (Leach et al., 1980). Therefore the head-only stun is consideredacceptable for Muslim (Halal) slaughter (Gilbert et al., 1986).

When a successful head-only stun is applied, the animal displays a sequence of ex-ternal responses (Anil, 1991; Gregory, 1998). The animal first becomes rigid (the tonicphase), with the head raised and the hind legs tucked up into the body. Breathing stops. Thetonic phase typically lasts 10 to 25 seconds. Thereafter, if the animal is not slaughtered,there is a transition to the clonic phase, in which kicking movements take place for 15 to45 seconds. A quiet phase then sets in, breathing restarts, and the animal shows signs of re-gaining awareness. Consciousness can be assessed from corneal reflexes (Gregory, 1998).If the animal responds to corneal contact, it is probably conscious.

Associated with the physical responses, the brain undergoes a series of chemicalchanges (Cook et al., 1995). The degree and duration of these changes are linked to the du-ration of the stunning current, and to the duration of the seizure and subsequent analgesia.

Because animals subjected to a head-only stun can begin to recover consciousnesswithin 25 to 30 seconds, it is important that the slaughter cut be made as soon as practica-ble after the stun has been applied. The cut should sever the major blood vessels (arteriesand veins) to ensure that blood flow to the brain is stopped. The time to permanent insen-sibility after the cut varies with species: it can be as short as 7 seconds with sheep but up to60 seconds with calves (Hoenderken et al., 1980; Newhook and Blackmore 1982a,b; Black-more et al., 1983; Grandin, 1999). Cattle have a significant blood supply to the brainthrough the vertebral arteries, which are not affected by a throat cut. There was thereforeconcern that in cattle post-stun consciousness could begin to return before permanent un-consciousness resulted from the throat cut. This caused the humaneness of head-only stun-ning to be questioned for cattle, preventing its early introduction. This issue was resolvedby Cook and coworkers in 1993 (Cook et al., 2000), who showed that the effects of stun-ning and reduction in blood flow due to the slaughter cut combine synergistically to hastenbrain death. Therefore head-only stunning gives an adequate length of unconsciousnessduring slaughter for all species including cattle. However, a chest stick is preferred for op-timum welfare assurance when slaughtering cattle.

Provided the slaughter cut is made during the tonic phase, an additional benefit ofelectrical stunning is that the animal is rigid. This makes the cut easier to carry out and helpsprotect the slaughter personnel from kick or knife injury.

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Figure 2 Typical EEG waveforms (a) prior to stunning and (b) immediately after stunning (samescale).


B. Deep Stun

The so-called deep stun (also known as the cardiac arrest stun) involves passing the elec-tric current through both the brain and the heart. The amount of current is chosen so thatthe heart fibrillates, causing blood circulation to stop. With this type of stun, the animal willnot recover. Thus the stun is not suitable for Halal slaughter. Two main current pathwaysare used for deep stunning: head to back and head to lower chest or leg.

In terms of animal welfare, deep stunning is acceptable for the slaughter of calves(Grandin, 1999) and is favored over head-only stunning for other animals (Wotton and Gre-gory, 1986; Grandin and Smith, 1998), because the animal dies from the stun. The slaugh-ter cut is important only as a means of allowing the blood to drain from the carcass. Whenstunning cattle, a head-only stun must be applied first to ensure insensibility before the car-diac arrest stun is applied (Grandin, 1999).

Another advantage of using a deep stun compared with a head-only stun is that theanimal tends to be more still during the slaughter cut and dressing (Gregory, 1998). This isbecause the current flow through the animal’s body disables the nervous system. Hence thespinal reflexes such as kicking do not occur (Wotton et al., 1992), as discussed later, andthe animal quickly becomes flacid.

C. Electrical Stunning Parameters and Equipment

Electrical stunning can be applied in several ways. In many cases, the animal is restrainedin a crush and the electrical current is passed through its head (head-only stun). The currentmay pass across the head, from top to bottom or from the nose region to the neck. Refer toFig. 3. Alternatively, the current may pass from the head to the lower chest (thoracic stun)


Figure 3 Head-only stunning of sheep.


or to the back (head to back stun), as shown in Figs. 4 and 5. For cattle, a head-only stun isapplied immediately before the thoracic or head-to-back stun to achieve a deep stun. Insmaller European slaughterhouses, pigs are often stunned with hand-held tongs while theanimal is free-standing.

Stunning is most often carried out using waveforms derived from the normal electri-cal supply (50 or 60 Hz) via a transformer, although commercial systems are available thatuse higher frequencies. Automated systems generally use between 400 and 1000 volts a.c.(Troeger, 1991), and restrict the current flow using some form of current control (Gregory,1998). Where stunning tongs are applied manually, worker safety usually dictates that thevoltage be less than 250 volts a.c. (Troeger, 1991). Under these conditions, current controlis not practical. This can lead to insufficient current flow and there is no way to ensure thatevery stun is successful, as discussed later.

The duration of the stun can be controlled manually or automatically. The absoluteminimum acceptable stun duration is 0.2 second (Cook et al., 1995), but this is too short toapply reliably in practice. Typical parameters for stunning current and duration as used forvarious types of animals in New Zealand are given in Table 1 (Gilbert, 1993). These arebroadly in line with international practice.

Electrical stunning of pigs is difficult because the modern pig grown for meat is sus-ceptible to stress, and to muscle hemorrhage and broken bones during handling, stunning,and slaughter (Troeger, 1991). This subject is discussed under blood-splash and brokenbones later in the chapter, and has been reviewed by Anil et al. (1997). However, a mini-mum stunning current of 1.25 to 1.3 amps applied for more than one second has gained ac-ceptance from a number of researchers as being suitable for pigs.

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Figure 4 Head-to-chest (or foreleg) stunning of sheep.



Figure 5 Head-to-back stunning of sheep.

Table 1 Summary of Time and Current Parameters of Electrical Stunning

Head-only electrical stunning

Sheep Lambs Calves Cattle Deerb

Min. currenta (A) 1.0 0.7 0.9 1.1 1.0Range (A) 1.0–1.5 0.7–0.9 0.9–1.5 1.1–2.5 1.0–2.0Min. time (s) 1.0 0.8 1.0 1.0 1.0Range (s) 1.0–4.0 0.8–1.5 1.0–4.0 1.1–4.0 1.0–3.0

Head-to-body electrical stunning

Sheep Lambs Calves Cattle Deerb,c

Min. currenta (A) 1.0 0.7 0.9 1.1 1.0Range (A) 1.0–3.5 0.7–1.3 0.9–1.5 1.1–6.0c 1.0–2.0Min. time (s) 1.0 0.8 0.9 1.0 1.0Range (s) 1.0–4.0 0.8–4.0 0.9–4.0 1.1–18c 1.0–3.0

a Minimum current required for humaneness but not necessarily movement control.b Hinds only at this stage.c These values have been used in practical situations without definitive studies. Heavier cattle should

be given 2.5 amps or more (Council of Europe ruling).


The stunning tongs used in Europe for stunning pigs consist of scissorlike devicesabout 750 mm long, with electrodes attached to the ends (Sparrey and Wotton, 1997). Theoperator is required to place the electrodes on each side of the head and apply the stunningcurrent.

Figures 3, 4 and 5 show some manual stunning equipment. Manual stunning isgenerally used for sheep when throughput is low to moderate. However, automated stun-ning systems may be used with sheep at high rates of slaughter (9 to 10 animals per minute).

Compared with sheep and pigs, more complex systems of mechanical restraint are re-quired when stunning cattle. While manual electrical stunning is widely used in some coun-tries, automated stunning systems are usually safer and more cost-effective. One systemused in New Zealand is the automated electrical stunning system illustrated in Fig. 6 (pro-totype version). The animal is restrained using a neck bail, which closes from either sideonto the narrow part of the neck behind the head. The neck bail forms one electrode, and isconnected to the stunner power source. The head is then lifted by a metal plate or bar whilea second electrode (the nose electrode) is moved down to contact the head at the base of thenose. The current passes from the nose electrode through the head and brain to the neckelectrode. If a thoracic stun is required, current can then be applied from the nose to the

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Figure 6 Automatic beef stunning. U.S. patent 4748719 is held by Jarvis Products Corporation,Connecticut.


chest via a third electrode that makes contact with the lower part of the chest between theforelegs. This current is applied after the head-only stun so that the animal is insensible be-fore the start of the thoracic stun. The stunning sequence is controlled automatically andprovides a very reliable stun (Gilbert, 1993; Petch and Gilbert, 1997).

D. Efficacy of Electrical Stunning

When applied properly, electrical stunning is a highly reliable way of protecting an animalfrom suffering during slaughter. However, animals come in a wide range of shapes, sizes,and temperaments. It is not as easy as might be imagined to design automated or manualsystems capable of delivering a consistent, effective stun to every one of the hundreds orthousands of animals that can pass through a meat packing plant in a single day. Factorsthat are critical to ensuring a successful and humane stun while preserving the economicvalue of the carcass include the following:

The animal must be properly presented to the stunner.The equipment must be designed so that sufficient current is available to ensure in-

stantaneous insensibility at the start of the stun.The electrodes must make a good electrical contact with the animal (contact resis-

tance needs to be low).Enough of the available current must pass through the brain to ensure instantaneous

insensibility.Each animal should be stunned once only, i.e., no interrupted or “double stuns.”The current must not damage valuable parts of the hide.Blood-splash and speckle bruising must be minimized.Bone breaks must be minimized.

The issues of sufficient current and acceptably low contact resistance are commonto all the electrical treatments applied during slaughter and dressing, and are discussed un-der contact and carcass resistance later in the chapter. The remainder of this section willdiscuss the design and positioning of the electrodes, and efforts to minimise blood-splashand other carcass damage that can arise from electrical stunning.

E. Placement of Electrodes

The head of any animal is a complex, anisotropic structure, containing a brain that is sur-rounded by membranes and regions of bone, fat, muscle, skin (plus hair or wool), and fluid.The whole structure is permeated by a highly complex network of blood vessels and nerves.This complex structure makes it difficult to predict precisely how the electric current willspread out from one electrode, pass through an animal’s head, and then come togetheragain at the second electrode.

With present technology it is not practical to directly measure the amount of currentflowing in specific regions within a living animal’s head. However, the current pathwayshave been modeled for pigs’ heads (Koch et al., 1996), and Lambooij (1994) has exploredplacement of electrodes directly against the brain. This allowed satisfactory stunning withonly 25 volts (approximately 0.13 amps) at 150 Hz, a finding that suggests that with con-ventional stunning techniques, a significant proportion of the current flows through thestructures surrounding the brain rather than directly through it.

Because direct measurements of current flows are lacking, present recommendationson electrode positioning are therefore based on common sense reasoning combined with



observation of the animal and analysis of the chemical or electrical activity in the brain.While this situation may not appear ideal, literally billions of animals have been stunnedsuccessfully using this empirical approach.

The electrodes should be placed so that the brain lies in the most direct path from oneelectrode to the other. Figure 6 shows a good example, in that the current passes from thenose through the whole head and out through the neck electrodes. Alternatively, the elec-trodes can be placed on either side of the head between the eyes and the ears, or one on thetop of the head and the other underneath (Grandin and Smith, 1998). With the head-onlystunner shown in Fig. 3, both electrodes are placed on the top of the head. In this case thecurrent field can be expected to fan out in a roughly hemispherical shape that includes thebrain. In all cases, positioning of the electrodes is less critical and a successful stun is eas-ier to achieve if the stunning voltage is high, so that the current meets or exceeds the rec-ommendations in Table 1.

Both electrodes must not be placed low down on the neck. If the electrodes are placedacross the neck and further back than approximately 50 mm behind the ears, the spinal cordmay be affected, resulting in the expected tonic and clonic responses, but the current willbypass the brain, leaving the animal paralyzed but still conscious. This is unacceptable interms of humane slaughter. If kicking starts immediately after the current stops, the stunwas unlikely to have been successful.

F. Blood-Splash and Broken Bones

Conditions known as blood-splash and speckle bruising can occur as a result of electricalstunning. The contraction of the muscles during the tonic phase is generally powerful, lead-ing to rupture of fine capillaries. Blood can be forced from these capillaries into the mus-cle tissue creating large blotches (blood-splash), or into the muscle cover and fat in a rash-like pattern (speckle bruising). The seepage is aggravated by the surge in blood pressureassociated with electrical stunning. The muscular contraction during stunning can alsocause bones to break, particularly in heavily muscled, weak-boned animals such as pigs.These defects can be adequately controlled for sheep and cattle but remain a significantproblem with pigs.

Because of their economic consequences (Morgan et al., 1993; Smith et al., 1994),blood-splash and bone breakage have been widely studied (Gilbert, 1993; Grandin, 1994;Grandin and Smith, 1998; Gregory, 1998). Techniques to avoid these problems include:

Elimination of interrupted and double stunsMinimization of the interval between stunning and stickingWhere possible, use of deep stunning so circulation is stopped immediatelyUse of high voltage, constant current stunning systems and avoidance of unnecessar-

ily long stunsProper maintenance of all equipment to ensure the contact surfaces are clean and all

electrical wiring is in good orderMinimization of stress prior to slaughter, particularly with pigs and deerEnsuring that the restraining system allows the animal’s body to move during the

muscle spasm caused by electrical stunningUse of high-frequency stunning, particularly with pigs

If carcass damage remains a problem, an alternative method of stunning should beconsidered, such as CO2 gas (pigs) or captive bolt (beef).

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Another similar problem associated with electrical stunning is shoulder bruising(Gilbert, 1993), caused when a sheep is still in the tonic phase of a head-only stun. As thesheep is put onto the spreaders used to convey the carcass along the slaughter chain, the re-quired spreading of the forelegs causes tearing of muscle tissue and blood vessels in theshoulder region. This problem can be virtually eliminated by improving the shackling andhanging techniques.

G. High-Frequency Stunning

High-frequency stunning (�60 Hz) can alleviate problems with blood-splash and brokenbones, particularly with pigs (Simmons, 1995). The muscle spasm induced by high fre-quency currents is less powerful than with 50 or 60 Hz currents, and the incidence ofblood-splash is often reduced. Warrington (1974) reviewed work done by Koledin in1963, in which he investigated the use of frequencies in the range 2400 to 3000 Hz. How-ever, van der Wal (1977) found when stunning pigs that square wave currents in the 2000to 3000 Hz range did not cause immediate unconsciousness and appeared to be verypainful. Anil and McKinstry (1992) found that sine waves at 1592 Hz and squarewavesat 1642 Hz caused immediate unconsciousness at 150 volts, but recovery times weresomewhat shortened. Lambooij et al. (1996) found that a head-only stun (800 Hz, 240volts, 3 seconds) followed by a cardiac arrest stun (50 Hz, 125 volts, 3 seconds) achievedan efficient and humane stun for pigs.

High-frequency stunning is accepted in Europe as a reliable method for stunningpigs. However, more research is needed with the larger U.S. pigs to confirm its acceptabil-ity unless the animal is first stunned with a 1 second 50 or 60 Hz stun (Grandin, 1999).

IV. ELECTRICAL TREATMENTS DURING DRESSING

The movements of a live animal are under the control of the brain. However, there area number of reflex pathways mediated by the spinal cord, and once the animal has beenhead-only stunned, these pathways are less likely to be inhibited by the brain. Violent kick-ing and walking movements may occur during the first few minutes after stunning, partic-ularly if the animal is being handled. These movements can pose a significant hazard toworkers responsible for dressing the animal, both from kicks and from induced knifeinjuries.

Two electrical techniques are available to prevent these reflex movements.The first, called low-voltage immobilization, uses a low-voltage electrical current thatcauses the muscles to contract moderately and maintain a rigid carcass. The second,called spinal discharge, uses a much larger current to discharge the neurons of the spinalcord so the reflex pathways are disabled. These techniques are widely used in NewZealand in plants that specialize in head-only electrical stunning (Halal slaughter) andhave been described by Gilbert and coworkers (Gilbert et al., 1983; Gilbert, 1993). Apartfrom this work, these techniques have not been systematically studied or widely reported.Importantly, these treatments are known to induce significant reductions in muscle pHand hence they interact with the effects of electrical stimulation (Petch and Gilbert,1997).

This section (A–D) is written largely based on personal observation, unpublished re-search (Petch and Zhang, MIRINZ Centre), and discussions with Gilbert and associates.



A. Low-Voltage Immobilization

A low-voltage immobilization current stimulates the (still functioning) nervous system,causing the muscles to contract and the carcass to become rigid. The electric waveformused is usually similar to the current used during low-voltage stimulation, described laterin the chapter. If applied for 45 to 60 seconds (depending on the current), the resulting mus-cle contraction depletes the energy reserves in the muscles, which become exhausted, andno further movement is possible. This system is widely used in venison and some beef pro-cessing plants in New Zealand. It simultaneously stimulates and immobilizes the carcass.Beef carcasses can be immobilized on the slaughter table using a short metal bar or strip(known as a rubbing bar electrode) that contacts the hindquarters, and a second rubbing barthat contacts the neck region. Alternatively a battery clamp can be attached to the lip, withthe return current carried by a clip or probe attached at the anus, or via the chain that is usedto hoist the animal via the hock.

In a high-throughput operation typical of sheep processing plants there is unlikely tobe enough space on the chain at the point of slaughter to allow a full immobilization/stimu-lation to be carried out. Instead, the immobilization current is usually applied for only 5 to15 seconds during bleeding so that the carcass is still during the initial stages of dressing.The current can be applied by using a rubbing bar that contacts the carcass at the inside ofthe hind legs. The current passes through the body and forelegs of the carcass to the trans-port chain and back to the stimulator. The contact is likely to be poor and variable due to thepresence of the pelt. This means that the current flow is generally lower than recommendedfor low-voltage stimulation and the muscle contractions are correspondingly weaker.

B. Spinal Discharge

The brief period of low-voltage immobilization applied in a high throughput operation maynot be sufficient to ensure that all carcasses will be completely still. To further immobilizethe carcasses, a second stage of treatment, known as spinal discharge, is often used. Spinaldischarge typically involves a 0.6 to 1.5 amp, 50 to 60 Hz current, applied via a rubbing barelectrode that contacts the shoulder region of the carcass for 2 to 4 seconds. The currentpasses into the shoulder, along the length of the spinal cord and out via the hind legs andgambrel. This depolarizes the motor neurons in the spinal cord that control the reflexive ac-tivity and effectively prevents further carcass movement.

As for stunning, the carcass will enter a tonic phase for up to 20 seconds after spinaldischarge is applied. During this phase the carcass will be quite rigid, but will subsequentlybecome limp and unresponsive. (The deep stunning techniques discussed earlier also causea significant current to flow along much if not all the spinal cord, depolarizing the motorneurons in the same way. This is why carcasses stunned in this way are much more still thanhead-only stunned carcasses.)

The electrical circuitry used for producing spinal discharge currents is very similar tothat used for stunning. A transformer delivers 400 to 550 volts a.c., and the current is con-trolled by an inductor connected in series with the carcass. The amount of current flowingthrough the carcass and its duration can be adjusted to produce the required degree of car-cass stillness. However, spinal discharge cannot be applied indefinitely, because spinal dis-charge will result in a significant fall in muscle pH and therefore complicates the applica-tion of electrical stimulation. There is also a risk of contamination if spinal discharge isapplied for more than a few seconds, due to contraction of the rumen forcing its contentsout through the esophagus.

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C. Bleeding Treatments

Some beef plants, particularly in the United States, use a high voltage pulse unit to improvebleeding after slaughter (D. Grose, personal communication). The unit produces a shortelectrical pulse about once a second, causing the carcass muscles to alternately contract andrelax. This acts to squeeze the blood vessels and helps to expel blood. The pulses are sim-ilar to those produced by an electric fence; i.e., 200 �s to 300 �s long, with a peak voltagein the range 2000 to 8000 volts.

D. Hide Pull

The process of removing the hide from a beef carcass can generate large stretching forces,particularly in the lumbar region of the spine as the hide is drawn over the shoulders andhead. This can result in tearing of the ligaments holding the vertebrae together, damage tothe vertebrae themselves, and tearing of the valuable longissimus dorsi (LD) muscle.

Some beef plants use an electrical device similar to a low voltage stimulator to passa small electric current through the LD muscles during hide pulling, causing them to con-tract (Fig. 7). The combined strength of the contracted muscles and spinal column is oftensufficient to prevent damage during hide pulling.


Figure 7 Electrically induced stiffening of the back during hide pulling. Current flows along theLD muscle between the anterior and posterior contacts.


V. ELECTRICAL STIMULATION

A. Overview

The earliest reported use of electricity to improve meat quality is its purported use by Ben-jamin Franklin in 1749 to electrocute turkeys with the result that they were “uncommonlytender” (Lopez and Herbert, 1975). Although studies in the 1950s investigated the use ofelectrical stimulation as a means of improving tenderness of meat (Harsham and Deverage,1951; Rentschler, 1951), it was not commercially applied at that time. However, the tech-nique was widely adopted when its benefits in avoiding cold shortening (Chrystall andDevine, 2000) and improving tenderness (Bendall, 1980; Taylor and Marshall, 1980) be-came known.

Two forms of electrical stimulation are widely used: low voltage and high voltage.While their purpose is essentially the same, they are quite different processes. Their namesprovide one distinction: commercially available low-voltage stimulation systems generallydeliver voltages below 150 volts, whereas commercial high-voltage stimulation operates atup to 1130 volts peak and beyond. However, this distinction is often not clear in the re-search literature, where the voltages used in experiments range from 2.5 volts (Taylor andMarshall, 1980) to 9000 volts (Smulders and Eikelenboom, 1985). A more useful distinc-tion between low and high voltage stimulation lies in their mode of operation: stimulationvia the nervous system (low voltage) versus direct stimulation of the muscles (highvoltage).

The types of waveform and the frequencies useful for electrical stimulation vary evenmore widely than the voltages. The most important variables that have been studied andtheir ranges can be summarized as follows:

Frequency: 0.5 Hz to 60 HzDuration of stimulation: 0 to 4 minutesWaveform: sine waves, pulse trains derived from sine waves, rectangular pulses,

short duration (300 �s) impulsesBurst length: 0.5 to 8 seconds or continuous current

European and New Zealand researchers have tended to use continuous stimulationfor the duration of the treatment, whereas researchers in the United States have focusedmore on bursts of stimulation current with a period of relaxation between. Australian re-searchers have made wide use of both techniques.

In a seminal study, Chrystall and Devine (1978) showed the relationship betweenpulse frequency and pH fall in the muscles. These workers used unidirectional pulses de-rived from a 50 Hz a.c. supply (Fig. 8). Pulse frequencies between 5 Hz and 17 Hz resultin the greatest rate of pH fall, and frequencies below 5 Hz are much less effective. Above17 Hz, the effectiveness of stimulation also falls, but even at 100 Hz the pH fall is morethan 70% of that at 15 Hz. Several groups have noted that different muscles respond dif-ferently to electrical stimulation (e.g., Houlier et al., 1980). Bouton et al. (1980) found thatthe longissimus dorsi responds better to 14 Hz and the semimembranosus responds betterto 40 Hz. Differences in pH fall have been linked to the ratio of slow-twitch fibers and fast-twitch fibers present in each muscle (Swatland, 1980; Devine et al., 1983). Tornberg (1996)discusses this issue further in her review of aspects of meat tenderness.

Several groups have obtained satisfactory stimulation using bursts of 60 Hz a.c.waveforms, with a range of burst lengths from 0.5 to 8 seconds (Smith et al., 1977; Rileyet al., 1980). Such waveform bursts are widely used in industry.

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A variety of pulse widths have been tested as well. Bendall (1980) notes that pulsewidths of 2 ms or shorter are unlikely to trigger all the muscle fibers when stimulating beef,but pulse widths of 5 ms or more are known to be satisfactory.

In summary, stimulation waveform parameters can vary quite widely without seri-ously reducing the efficacy of electrical stimulation in causing a fall in muscle pH. Thechoice as to which type of stimulation, where to site it in the meat plant, and which wave-form parameters to use will be determined as much by practical considerations such asphysical space and the overall parameters of the processing plant as by the direct effect onpH fall.

As a generalization, high voltage stimulation is most suitable for high carcassthroughput rates, where several carcasses will be stimulated simultaneously. Because of thehigh voltage and current used, elaborate safety systems are required. These requirements,combined with high-power electronics, mean that a typical high-voltage installation isabout 20 times the price of a low-voltage installation. As a result, many high-throughputsheep and beef processing plants use high voltage stimulation to stimulate between 7 and120 carcasses (or sides) at once, whereas smaller plants tend to use low voltage stimulation,stimulating only 1 or 2 whole carcasses at once.

B. Low-Voltage Electrical Stimulation

1. Description

Low-voltage stimulation operates by stimulating the nervous system, which then causes themuscles to contract (Chrystall et al., 1980; Chrystall and Devine, 1983). It therefore relieson a functioning nervous system and loses much of its effectiveness once the nervous sys-tem ceases to function (approximately 5 minutes postslaughter). However, low-voltage


Figure 8 Effect of stimulation frequency on the fall in pH. Stimulation consisted of 200 volts(peak) unidirectional pulses derived from the 50 Hz a.c. mains supply for 120 s.


stimulation can still produce a useable pH fall up to 20 minutes post slaughter. When ap-plied this long after slaughter, it appears to stimulate the muscles directly as does high volt-age stimulation, although the muscle contractions are weaker and the pH fall is less than forhigh-voltage stimulation because the current density is lower. This form of stimulation isused in some smaller processing plants, as a “poor-man’s” high-voltage stimulation.

As outlined earlier, there are a great many possible low-voltage stimulation wave-forms. Figure 9 shows two alternative waveforms, widely used in New Zealand, and inparts of the United States and Europe (Fig. 9a), and in Australia (Fig. 9b). Table 2 summa-rizes parameters for low-voltage stimulation.

A wide range of specifications exists for low-voltage electrical stimulators in theUnited States, but most operate with an output voltage of 20 to 90 volts, with the currentapplied either in bursts or continuously, for 15 to 20 seconds (Savell, 1985).

2. Application

Low-voltage stimulation can be applied in a number of ways. Beef carcasses to be stimu-lated before dressing (within 5 minutes of slaughter) are generally suspended by one hock

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Figure 9 Low-voltage stimulation waveforms use in (a) New Zealand and the United States and(b) Australia.

Table 2 Low-Voltage Stimulation Parameters Used in New Zealand

PeakPeak current (A)

Pulse Duration ofvoltage Beef Beef width Period stimulations(V) Lamb Sheep (side) (whole) (ms) (ms) (s)

20–90 �0.2 �0.2 No spec. �0.3 5–10 58–80 30–90


using a chain and shackle. A large spring-loaded clip or a spear is used to connect the livelead from the stimulator to the neck wound or lip. The current returns to the stimulator viathe chain and shackle, or preferably via a hook or clip attached to the tail. The clip or hookmay include a length of metal rod that is curved so that it is easily inserted into the anus.The anus provides a particularly good contact because the surface membrane in this regionis thin and moist. This improves the reliability of stimulation.

Carcasses that are to be stimulated using low voltages after dressing (approximately20 minutes post slaughter) are usually suspended from a gambrel. The live lead is attachedto the neck of the carcass using a clip or a spear, while the current returns to the stimulatorvia the gambrel. However, recent research in the author’s laboratory has shown that thelower hind leg and the junction between the leg and the gambrel offer a high electrical re-sistance. This may cause the stimulation current to be too low, resulting in ineffective stim-ulation. See Sec. VI.B for more details.

C. High-Voltage Electrical Stimulation

1. Description

High-voltage stimulation acts directly on the muscles to induce contraction. High-voltagestimulation involves much higher voltages and currents than low-voltage stimulation andis generally applied 30 to 60 minutes post slaughter.

As for low-voltage stimulation, there are a great many possible waveforms and pa-rameters (Table 3). Figure 10 shows one waveform, widely used in New Zealand, Aus-tralia, and parts of Europe. The waveform is bipolar: the pulses are alternately positive andnegative and are derived from the a.c. power supply using a switching system.

The U.S. industry has followed their American researchers in favoring bursts of 60Hz current, with a typical specification being 550 to 600 volts, 2 second long bursts withone second between, for 15 to 20 bursts (Savell, 1985).


Figure 10 High-voltage stimulation waveform used in New Zealand, Australia, and parts of Eu-rope.

Table 3 High-Voltage Stimulation Parameters

PeakPeak current (A)

Pulse Duration of Modevoltage Beef Beef width Period stimulation of(V) Lamb Sheep (side) (whole) (ms) (ms) (s) operation

990–1150 �1.6 �1.8 �2.5 �7.5 7.5–10 60–80 60–120 Continuous


2. Application

High-voltage stimulation is often applied using a rubbing bar electrode. In a typical high-throughput packing house, the carcasses (or sides) are suspended from a gambrel and skidwhile the rubbing bar contacts them in the shoulder region (Fig. 11). The skid is pulledalong by the chain so that the carcass is drawn along the length of the live electrode. Theduration of the stimulation is controlled by the length of the electrode and the speed of thechain. In a variation on this, the rubbing bar can be maintained at ground potential while asecond live rubbing bar contacts the hock region of the carcass (Smulders and Eikelen-boom, 1985). This is possible only if the chain supporting the carcass has an insulating sec-tion to isolate the live part of the suspending chain from the transport chain.

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Figure 11 High-voltage stimulation of beef sides using a rubbing bar (Photograph courtesy ofAuckland Meat Processors and Jarvis Equipment (NZ) Ltd., New Zealand).


Another technology widely used in the United States employs a continuous belt ofmetal plates, linked by insulating material. The plates move with the carcasses, and arelivened only as they make contact with the carcasses to provide the current pathway(Fig. 12).

In lower-throughput operations, the carcass may be pushed into a small booth withentry and exit doors. The live lead is attached to the carcass by a clip or a spear inserted intothe neck, or by a metal plate that is raised until it makes contact with the carcass (Cuth-bertson, 1980).

Because of the danger posed to personnel by high voltages, the whole installationmust be isolated by systems designed to prevent access to the electrically live carcasses orelectrodes. Where a rubbing bar or continuous belt system is used, the whole assemblyshould be enclosed within a tunnel or room, with physical barriers and warnings at thepoints where carcasses enter or leave the tunnel. Pressure pads or photoelectric sensors areused to disconnect the live electrode if personnel are detected inside the tunnel (Anon.,


Figure 12 High-voltage stimulation using the Lecto-Tender apparatus.


1995, Anon., 1996). Interlocks are also required on all doors or access hatches, so that theelectrode is disconnected if these doors are opened. Most importantly, strict safety proce-dures are required for shutting the system down when personnel need to visit the protectedzone, and restarting it after personnel have left the protected zone.

VI. SOME PROBLEMS WITH ELECTRICAL TREATMENTS

A. Overview

With all the electrical treatments discussed so far, the most critical problems lie in ensur-ing suitable currents flow through the animal or carcass. Several factors can cause varia-tions in the current flow, as follows:

1. Contact resistance can be large and/or variable. This is a particular problem withlow-voltage treatments and where the hide or pelt has not been removed. Sheepwith long wool are especially problematic.

2. Animals and carcasses vary in size, shape and conformation. This can compli-cate the positioning of electrodes, particularly with automated systems.

3. Animals and carcasses vary in their response to electrical treatments.4. Animal or carcass movement can cause serious problems, particularly with stun-

ning. If an animal moves immediately before the stunning equipment makes con-tact with it, a poor or unexpected contact position may result, causing an unsat-isfactory stun.

5. Manually controlled procedures are subject to human failings such as boredom,error, and fatigue.

6. Equipment failure can cause serious disruption to plant operations.

Many meat processing plants are critically dependent on electrical stimulation. Whileproperly designed and maintained stimulation equipment is very reliable, should the equip-ment fail, the result is often an immediate and complete stoppage of work, with all em-ployees idle on full pay. Experience shows that such an event attracts the focused attentionof plant managers more quickly and completely than virtually any other problem that canoccur. Proper maintenance and having some spare parts on hand represent cheap insurance.

Of the problems listed above, variations in contact and carcass resistance are proba-bly the least understood in industry, perhaps because they are not readily observable.

B. Contact and Carcass Resistance

Electrical resistance is a problem only if it is high enough to significantly limit the flow ofelectric current or if physical damage such as burning results. This is a matter of context. Ifthe required current flow is large or the available voltage is small, the effect of high resis-tance may be very significant. Similarly, the contact position can be important if unin-tended conductive pathways are introduced, which allow the current to bypass the expectedcurrent path.

The key issues are:

The hide or pelt of an animal is comparatively highly resistive, particularly if coveredwith long, dry hair or wool.

The resistance of the hide or pelt is highest with low voltages; it reduces as the volt-age is increased.

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Some parts of the carcass, such as bone, connective tissue, and fat, are also relativelypoor electrical conductors.

Perhaps surprisingly, both the tongue and the nose are poor connection points, prob-ably due to their high connective tissue content.

When a current flows through a resistance, heat is released in the resistance. This cancause localized burning.

Different connection points cause the current to flow through different pathways,each with a possibly significant difference in resistance. This may mean, for in-stance, that the brain is effectively by-passed during stunning.

Contact and carcass resistances are particularly important with electrical stunningand low-voltage stimulation, because of the risk to animal welfare (stunning) and becauseresistance problems are often exacerbated if the available voltage is low.

Consider a practical example. The current specified to stun a large beef animal isat least 2.5 amps. In a trial with 811 beef animals (Petch and Gilbert, unpublished), anautomatic stunner made contact between the muzzle and the neck immediately behind thehead. Of the head resistances measured during stunning, 95% lay between 44 and 109ohms. If a hypothetical 180 volt stunner was used and the current not controlled, the cur-rent flow would lie between 4.1 amps and 1.7 amps, in which case many of the animalswould not receive a satisfactory stun. In this example, the contact resistance is likely tobe a major contributor to the variation in the measured resistance, and would pose a se-rious welfare problem. In another plant tested during the same trial, a slightly differentstunner made contact between the top of the head at the midpoint between the muzzle andthe eyes, and the neck behind the head. For 95% of the 367 head resistances measuredwith this stunner, the resistance lay between 67 and 185 ohms. While these resistancepopulations overlap, they are significantly different. Without current control, the currentflow expected with a hypothetical 180 volt stunner at that plant would be between 2.7amps and 0.97 amps, and almost all of these animals would not receive a satisfactorystun. Fortunately, both groups of animals were actually stunned using well-designed con-trolled-current stunners, so all received a satisfactory stun. These examples illustrate howinadequate stunner voltage can combine with variable contact resistance to create signif-icant welfare problems.

With head-to-back stunning of sheep, the contact resistance between the electrodeand an animal’s back can be large if the pelt is unwetted, and particularly if the wool is long.The heat dissipated can lead to burning and costly damage to the pelt. Similar problems canapply to cattle.

Low voltage stimulation is also very vulnerable to high contact resistance. One com-mon method of applying low voltage stimulation is to use the hock, shackle, and chain asthe return path for the electric current (Fig. 13). In a trial involving low-voltage stimulationof 345 bull carcasses (Petch and Gilbert, unpublished), the current flow in more than 97%of the carcasses fell below the New Zealand specification of 300 mA, and the median was219 mA. The current had to flow through the hock to reach the hoof region, and then passthrough the hide. These regions combined to produce excessive resistance. The live con-nection to the head was also poor, with the clamp being attached to the hide at the cheek.The overall result was completely unsatisfactory.

The following methods can be used to minimize the impact of contact resistance:

Use a large contact areaUse contact sites that have a moist, thin surface: for example, anal probes



Wet the contact area with water or saline solutionAlways make the connection in the same wayUse controlled-current equipment if at all possibleUse automation where practical to minimize variation

One further approach to reducing contact resistance during low voltage stimulationhas been developed by Smulders and Eikelenboom (1985). These researchers intersperseda low voltage waveform (35 volts, 14 Hz) with short 3000 volt impulses (1.5 ms long, 1pulse per second). Smulders and Eikelenboom hypothesized that the high-voltage pulseslowered the hide resistance so that the low-voltage current could flow freely. Commercialequipment was developed based on this principle in the early 1980s.

C. Special Problems with High-Voltage Stimulation

High-voltage stimulation with a rubbing bar electrode involves several issues that must beovercome:

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Figure 13 Low-voltage stimulation of cattle suspended by one hock.


1. Electrode Positioning

The electrode that rubs against the shoulder must be positioned so that the carcass is dis-placed from the vertical and presses gently but firmly against the electrode. If the carcasspresses too lightly against the electrode, it may repeatedly flick away from the electrodeand fall back like a pendulum as the current flows in surges. This is known as carcassbounce, and in severe cases can continue for most of the length of the electrode.

Alternatively, if the carcass presses too hard against the rubbing bar, the gambrel mayturn in the skid, allowing the carcass to roll along the electrode. This can disrupt the stim-ulation current and may cause the carcass to fall off the gambrel.

In addition to correct positioning of the electrode, carcass bounce can be controlledby using an insulated bar fitted on the opposite side of the carcass to limit the distance thatit can swing (Fig. 11). Similarly, carcass rolling can be prevented by fitting a stabilizing barthat prevents the gambrel from turning.

2. Carcass Staining

In some cases, the carcass can develop a brown stain where it contacts the stimulation elec-trode. This discoloration is primarily due to rust forming on the electrode, then rubbing offonto the carcass. Staining can be severe when the electrode is new, but it usually reducesafter a few hours as a glaze forms over the electrode surface.

Staining can be minimized by using a corrosion-resistant grade of stainless steel(U.S. Grade 316 or similar), by mounting the electrode with a fall or rise of approximately100 mm along its length so that the contact point with the carcass changes along the lengthof the electrode, and by allowing the surface glaze to accumulate over time. For example,the glaze should not be scoured away during cleaning. Finally, the electrode can be passi-vated with concentrated nitric acid.

3. Equipment Failure

Failure of the switch units that control the stimulation waveform can cause unexpected ef-fects. In particular, should both switches fail permanently on, the current flow in the car-cass will be the continuous a.c. frequency rather than the pulsed waveform. Localized con-tact resistance between the gambrel and the hock can then cause the Achilles tendon to meltand the carcasses to fall off the gambrel in the middle of the stimulation tunnel. It is mostdisconcerting to find that carcasses are entering the system at one end, but nothing is emerg-ing at the other.

VII. CONTROLLED CURRENT TECHNOLOGY

With any of the electrical treatments discussed, the desired effects are caused by the flowof electrical current rather than the voltage that drives it. This means that it is important toensure that the current flows is appropriate for the treatment concerned.

Current flow can be controlled in a number of ways. These can be separated into twoclasses:

Passive current control, in which the maximum current that can flow is limited by afixed electrical impedance such as an inductor, or some other limit built into thesystem.

Active current control, in which the current flow is controlled by some variablemechanism that continually adjusts the output to achieve the desired current.



Provided that worker safety can be ensured, high-voltage current-controlled equip-ment can offer very significant advantages (Gregory, 1998; Grandin, 1999), and often withonly slight increases in cost and complexity. For the first beef stunning example outlinedin the previous section, the voltage could be set at 550 volts, with a single inductor includedto limit the current to no more than 3 amps. This simple passive control system eliminatesthe risk of large current spikes that have been associated with blood-splash and brokenbones. The inductor also dominates the total impedance of the circuit and reduces the ef-fects of contact and carcass variability on the stun current. The stun current for the resis-tance range quoted (44 to 109 ohms) would therefore range from 2.9 amps to 2.5 amps,rather than between 4.1 amps and 1.7 amps as before. This represents a huge improvementin stunning repeatibility, with advantages in terms of both animal welfare and reducedblood-splash.

Higher voltages also help break down total carcass resistance including the resistanceof the hide (Anon., 1991), and allow a more appropriate current to flow. This is useful withhead-only stunning, where localized burning of the hide or pelt on the head is of little con-sequence.

Active control systems can act in a variety of ways to limit the current flow. Providedthey are properly designed, active systems can achieve very precise current control. Petchand coworkers (Petch and van Royen, unpublished; Petch, 1999) have developed systemsthat can deliver a wide range of controlled-current waveforms. These are being used for on-going research into controlled-current stunning, immobilization, and stimulation. A num-ber of organizations manufacture controlled-current stunners.

VIII. WORKER HEALTH AND SAFETY

There are several potential areas of risk for workers involved with electrical treatments ap-plied during slaughter and dressing. The first is the obvious risk of injury or death due toacute electric shock. This risk is clearly greatest with high voltage treatments such as stun-ning, spinal discharge, and high-voltage stimulation. Considerable effort has been directedat developing systems to prevent mishaps with high voltage stimulation (Anon. 1995;Anon. 1996). Rather less attention has been paid to ensuring safety with stunning or spinaldischarge, although European legislation requires that the voltage used with manual stun-ning of pigs be maintained below 230 volts to protect worker safety (Troeger, 1991).

The second area of risk is more subtle: the potential risk of injury or disability re-sulting from accumulated damage caused by long-term exposure to low levels of electriccurrent, as may be experienced when working on carcasses as they receive a low-voltagetreatment. This is a difficult issue to manage because of the difficulties in obtaining enoughinformation to accurately quantify the risks. A realistic assessment will require extensiverecords covering many workers for many years.

To further complicate the issue, many operations during slaughter and dressing canbe carried out more efficiently and safely on carcasses while they are being immobilizedusing electric currents. There are many incidences of workers being severely injured fromtheir own knives when a carcass moved unexpectedly.

Without a solid base of knowledge about the long-term risks from low level electri-cal currents, it is difficult to make judgments about best practice for such electrical treat-ments. Current policy in many areas is to minimize exposure to electrical currents abovevery low levels. Recent legislation in Europe (Anon., 1999) seeks to prevent large-area

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contact in potentially damp areas with voltages exceeding 6 volts a.c. or 15 volts d.c. Thiseffectively prevents workers from touching carcasses undergoing any of the electrical treat-ments outlined in this chapter. If risk due to carcass movement is to be minimized at thesame time, a significant redesign of the slaughter area of many meat processing plantswould be required, particularly those carrying out Halal slaughter procedures where thehead-only stun does not extinguish spinal reflexes.

IX. MONITORING SYSTEMS AND PROCEDURES

Any industrial production system involves a process that converts raw materials into oneor more products that meet specific market requirements. In the case of meat products,these market requirements will include a variety of measures, including tenderness, texture,taste, color, wholesomeness, and storage life. Animal welfare is also a key issue.

The electrical treatments applied during slaughter and dressing can affect all of theseparameters. It is therefore important to monitor and audit the electrical treatments as theyare applied. Furthermore, systems and procedures must be in place to ensure that any defi-ciencies are immediately detected and corrected.

A variety of equipment is available to help meat processors and regulatory authori-ties carry out monitoring and auditing procedures.

A. Process Monitoring Equipment

Electrical devices are commercially available that analyze the electrical voltages and cur-rents applied during stunning and stimulation (Petch and Peach, 1994; Coulton, personalcommunication; Ross, 1999). These continually provide feedback to operators that theelectrical process is being carried out properly, and warn of any problems. Some stunningmonitors (Coulton, personal communication) also check that the electrical contact with theanimal’s head is good before the stunning current is applied. This ensures that the stun willbe effective and humane.

B. The Need for Process Auditing

Regular checking is essential to ensure that the electrical treatments continue to be appliedcorrectly. Many factors can alter the effectiveness of the treatments, including:

1. Changes in employees, leading to changes in the way that treatments are applied2. Partial or complete failure of the electrical equipment3. Variations in the animals, due to seasonal variations or different suppliers4. Fluctuations in the quality of the electrical supply caused by routine or extraor-

dinary operation of plant facilities such as chillers, water heaters, etc.5. Changes in the processing plant, such as the installation of new equipment,

which may affect the quality of the electrical power supply6. Changes in the local electricity supply demand (for example, construction of a

major new industrial or residential complex), which may alter the quality of theelectrical power supply to the packing plant as a whole

Extended records should be kept and reviewed to detect seasonal or longer-term vari-ations, so corrective action can be taken.



C. Process-Auditing Equipment

Quite sophisticated equipment has been developed that allows a detailed analysis of bothhigh and low voltage stimulation waveforms. The waveform analyser (Petch and Peach,1994) analyzes the current waveform passing through individual carcasses undergoingelectrical stimulation, and records the number of pulses that meet the New Zealand speci-fication for high and low voltage stimulation. This device is used to audit the operation ofelectrical stimulation equipment. A waveform recorder (Petch and Peach, 1994) is used torecord the current flowing through a carcass. The current waveform is then downloadedonto a computer and analyzed to determine the nature of any problems with the stimulator.The waveform recorded is used primarily as a diagnostic tool for meat processors that arehaving problems with stimulation.

A sophisticated datalogger system has been developed (Petch and Lynch, 1997) thatcan record and analyze all of the electrical currents applied during slaughter and dressingfor large numbers of animals. This system was used to record the pulse width, pulse fre-quency, peak current and peak voltage, and the duration where relevant, of the stunning,immobilization, and stimulation treatments applied to nearly 20,000 sheep and 3000 cattlein four plants. The information gained is continuing to provide insights into the perfor-mance of the electrical equipment used to apply these treatments. Analysis revealed, amongother things, the erratic performance of a cattle stunning box operator who was chronicallydrunk.

D. Equipment Calibration

All equipment used for monitoring or auditing electrical treatments must be regularly testedto ensure that it still meets its specified accuracy and precision. There is little point is mon-itoring a process with faulty equipment. Where regulatory bodies are involved, regular cal-ibration using test equipment that is itself traceably calibrated against national standards isessential.

X. AREAS OF RESEARCH

Most of the standards set down for electrical stimulation during the 1970s were based onthe assumption that stimulation was the only electrical treatment applied during slaughterand dressing. While this assumption was largely true at that time, it is known that the elec-trical treatments introduced subsequently can cause a significant pH fall. Furthermore,these treatments suffer from significant variability.

Present research is focused on minimizing the variability of the electrical treatments,understanding the interactions between the various treatments, quantifying and under-standing the variability in the carcass response to the treatments and developing ways tocontrol the treatments to achieve the desired meat quality outcomes. This research is par-ticularly important for the chilled meat trade.

The determination of the current pathways during electrical treatments is difficult.Some work has inferred the pathways by measuring the voltage at different points(Houlier and Sale, 1984), while both magnetic resonance imaging and electromagnetictomography hold some promise for the direct measurement of the current density andpathways.

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XI. SUMMARY

This chapter describes electrical treatments applied during slaughter and dressing, includ-ing stunning, immobilization, and stimulation.

Electrical stunning is used to ensure that animals are insensible during slaughter. It isan important method of preserving the welfare of animals and has been successfully appliedthroughout the world. However, it needs to be applied with care to ensure that every ani-mal is properly stunned and either remains insensible until death through loss of blood oc-curs (head-only stunning), or is killed by the stun (deep stunning). Electrical stunning canresult in meat quality problems, particularly with pigs.

Immobilization is used to prevent carcass movement during dressing. Its primaryfunction is to protect workers from injury.

Electrical stimulation is used to induce the early onset of rigor. It is a vital tool inbringing about the rapid ageing and conditioning of carcasses. Two main forms are widelyused in industry: low-voltage stimulation and high-voltage stimulation.

A key issue with any of these treatments lies in ensuring that the current that flows iscorrect and follows the desired pathway. Contact resistance can cause significant problemsand must be managed effectively. All the electrical treatments discussed can have impor-tant effects on meat quality attributes and must be properly monitored.

APPENDIX: BASIC ELECTRICAL THEORY

To appreciate what happens when an electrical treatment is applied to a carcass, it is help-ful to understand some basic electrical concepts. This section will briefly discuss those con-cepts needed to understand the essence of these electrical treatments.

Electric current: Electric current is the nett movement of electrons through a mate-rial. An electric current can be viewed as the electrical equivalent of water movingfrom point to point through a pipe. The ampere (amp) is the unit of electric current.

Conductor: Any material that allows electrons to flow through it freely is called aconductor. Copper wire is a good example of a conductor.

Insulator: Any material that does not allow electrons to flow through it freely iscalled an insulator. Plastics are usually good insulators.

Resistance: Any material has a tendency to oppose, or resist the movement of elec-trons. This effect is known as resistance. The resistance of a material can be verysmall, such as in a superconductor, or very large, as a piece of plastic. The unit forresistance is the ohm. Copper is widely used as a conductor because it has a lowresistance to the flow of electrons. Plastics used to insulate wires have a high re-sistance to the flow of electrons.

Voltage: Voltage describes how much electrical “pressure” is being applied to elec-trons between two given points. The unit for voltage is the volt.

Ohm’s law: Ohms’ law defines the relationship between voltage, current and resis-tance. If a piece of material has a resistance of one ohm between two points, thena voltage of one volt will be needed to drive one amp of current between thosepoints. Ohms law is usually written:

Voltage � amperes � resistance

Electric circuit: An electric circuit is any path that an electric current can flowthrough. An electric circuit is always a complete loop, with current always mov-ing at every point within that loop. See Fig. 14 for an example.



Load: Most electrical circuits contain some form of intentional electrical load, inwhich some useful function is carried out. In this chapter, the animal or carcass isusually the load for the circuit concerned.

Direct current: In many circuits, the supply voltage or current is essentially constantwith time. This is referred to as a direct current (abbreviated d.c.). A good exam-ple is the voltage (or current) from a dry cell as used in a flashlight

Alternating current: In many circuits, the voltage or current continually alternatesbetween a positive peak value and a negative peak value, typically following a sinewave pattern as shown in Fig. 15. This is referred to as an alternating current (ab-breviated to a.c.). The number of times the pattern is repeated per second is calledits frequency, measured in hertz (Hz). Almost all domestic and industrial powersupplies are a.c. supplies, and Fig. 15 shows the voltage waveform expected froma 110 volt 60 Hz a.c. supply (continuous curved line) as typically used in theUnited States.

Peak voltage: The peak voltage of an a.c. supply refers to the maximum positiveor negative voltage reached by that supply. In Fig. 15, the peak voltage is 156volts.

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Figure 14 A simple electrical circuit.

Figure 15 Waveform of the U.S. 60 Hz domestic electric power supply.


Power in a resistor: When an electrical current flows in a resistor, heat is dissipatedin the resistor as energy is expended to drive the current through. This is the prin-ciple used in an electric heater.

The power dissipated in a load is often written:

Power � voltage � current

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dk1792 ch14

Education

electrical treatments

electrical stunning

aspects of electrical

cattle electrical stunning

use of electrical stimulation

highvoltage stimulation

highfrequency stunning

meat industry