Caballero B. (ed.) Encyclopaedia of Food Science, Food Technology and Nutrition. Ten-Volume Set

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troponin C (TnC), which is the calcium-binding sub-

unit. Troponin I (TnI), the inhibitory subunit, con-

nects TnC to the thin myofilament. At sufficient levels

of calcium, the interaction between the troponin, tro-

pomyosin, and actin exposes the myosin-binding site

of actin.

0016 In the thick myofilament, the best-characterized

protein is myosin. Myosin has been called a mechano-

enzyme as it not only plays a structural role in the

thick myofilament, but also acts as an adenosine tri-

phosphatase (ATPase). Myosin’s globular head has

both enzymatic and structural roles. It breaks down

adenosine triphosphate (ATP) to provide the energy

necessary for contraction, while it also interacts with

actin to form a cross-bridge between the two proteins.

The tail portion of the myosin molecule plays a struc-

tural role, and is involved in the formation of the

thick myofilament. Other proteins in the thick myo-

filament help maintain the structure of the myofiber

and also play regulatory roles.

0017 During contraction, the globular head of myosin

breaks down ATP to adenosine diphosphate (ADP)

and inorganic phosphate, binds to actin, and subse-

quently changes the angle of the actomyosin cross-

bridge from 90



to 45



. When another molecule of

ATP binds to actomyosin, myosin detaches from

actin. This process is cyclical and results in the thin

myofilament sliding inward past the thick myofila-

ment. Contraction ceases when calcium ions are

sequestered back into the sarcoplasmic reticulum.

Contraction of an antagonist muscle is required to

return the original muscle to resting length.

0018The signal for muscle to contract begins as a wave

of depolarization, or action potential, that sweeps

down a nerve cell. When the action potential reaches

the neuromuscular junction (the connection between

a nerve cell and a muscle fiber), acetylcholine is

released from the nerve cell, crosses the synapse (the

gap between the nerve and muscle cell), and attaches

to receptors located on the sarcolemma. The sarco-

lemma depolarizes in a similar manner to the nerve

cell, and the message is carried into the t-tubule

system in order to carry the message deep into the

myofiber. The sarcoplasmic reticulum is stimulated

by the t-tubules to release calcium which interacts

with regulatory proteins to initiate contraction.

0019All myofibrils within a muscle cell contract in con-

cert. By moderating the number of muscle cells re-

cruited for contraction, the strength of a particular

muscle can be controlled. Thus, fine motor move-

ments and considerable physical strength can both

be demonstrated by the same muscle.

Low-resolution

electron

micrograph

(× 10 000)

Z-

line

M-

line

Z-

line

Thick filament

(A) (myosin)

Pseudo

H-zone

High-resolution electron micrograph (× 100 000)

Cross-section

Thin filament (I) (actin)

Myosin cross-bridges

Schematic

A-band I-band

fig0004 Figure 4 Fine structure of a sarcomere along with portions of two adjacent sarcomeres. From Pearson AM and Young RB (1989)

Meat and Muscle Biochemistry. San Diego, CA: Academic Press, with permission.

3764 MEAT/Structure

Postmortem Changes in Meat

0020 In the conversion of muscle to meat, several changes

occur that impact meat quality. During slaughter and

processing, the muscle cell environment undergoes

changes like: declining energy levels, pH, and tem-

perature, increasing ionic strength, and changes in

redox potential. The pH declines as lactic acid is

produced as a byproduct of anaerobic metabolism.

Lower muscle pH results in reduced water-holding

capacity and influences meat color.

0021 An important contributor to the increase in ionic

strength is calcium. As energy sources are depleted,

the sarcoplasmic reticulum loses its ability to seques-

ter calcium. With increased calcium levels in the

sarcoplasm, conditions for contraction exist (as long

as sufficient energy is available). Eventually, the

cross-links between the thick and thin myofilaments

(myosin and actin) are sustained. These attachments

(rigor bonds) cause muscle rigidity (rigor mortis) and

reduce meat tenderness. Shorter sarcomeres have a

greater number of rigor bonds and create less tender

meat.

0022 Meat is usually held at refrigeration temperatures

for a time postmortem to modify meat palatability

(flavor and tenderness). This storage period is called

aging. Meat undergoes several structural changes

during this period which lead to an increased fragility

of the myofibrils (as measured by an increase in meat

tenderness). The decrease in the intensity of the Z-line

and subsequent loss of Z-line integrity is the most

notable change in structure. Additionally, the sarco-

lemma separates from the underlying myofibrils. Ad-

jacent myofibrils also separate from each other, and

some researchers have observed discontinuities in the

I-band.

0023Despite considerable research, a unified theory on

the mechanism of postmortem tenderization in meat

has not been embraced. Tenderness increases most

rapidly and to the greatest extent during the first

3–4 days postmortem. Changes in tenderness are

believed to be associated with the degradation of

structural proteins in the myofibers. Early researchers

attributed postmortem tenderization to the cathep-

sins, a group of hydrolytic proteins found in the lyso-

somes, while other researchers have suggested that

the change in ionic strength within the cell is the

cause of postmortem tenderization. More recently,

the calpain system has been implicated as a link in

postmortem tenderization. This group of proteases

Costamere: vinculin, spectrin,

dystrophin, talin, ankyrin, etc.

IF: desmin, spectrin, ankyrin

Sarcolemma

M-filaments

ECM

Titin

M-Line Z-line

Myosin

Actin

Z-line−IF anchorage,

synemin, filamin,

spectrin, ankyrin, etc.

fig0005 Figure 5 Structural arrangement of costameres, intermediate filaments, and myofibrils. ECM, extracellular matrix; IF, intermediate

filaments; S, skelemins. Adapted from Price MG (1991) Striated muscle endosarcomeric and exosarcomeric lattices. In: Malhotra SK

(ed) Advances in Structural Biology, vol. 1, pp. 175–207. Greenwich, CT: JAI Press.

MEAT/Structure 3765

requires calcium for activation. They maintain activ-

ity in the cellular environment that predominates in

meat. There are three calpain isoforms: m-calpain, m-

calpain and skm-calpain. All three forms have been

identified in skeletal muscle; however, skm-calpain

(also called p94) is only found in skeletal muscle.

skm-calpain is difficult to purify, and therefore has

not been fully characterized. The other two isoforms

are activated at different calcium levels, and the

typical conditions during postmortem aging favor m-

calpain. The calpain system is subject to control by a

powerful inhibitor, calpastatin. It is believed that one

molecule of calpastatin can inhibit multiple calpain

molecules. The calpains have been shown to degrade

several proteins like titin, nebulin, desmin, and TnT,

which may influence postmortem tenderization.

0024 Collagen is another contributor to meat tender-

ness. Collagen molecules are cross-linked in order to

stabilize the molecule and to impart the strength

associated with connective tissue. In young animals,

collagen cross-links are reducible, and therefore less

stable. Collagen containing reducible cross-links will

hydrolyze to form gelatin if heated in the presence of

moisture. The conversion to gelatin, a softer material,

results in more tender meat. However, in older

animals, the collagen cross-links become heat-stable

and nonreducible, thus contributing to tougher meat.

Pyridinoline appears to be one of the more significant

nonreducible cross-links.

0025 The flavor of meat is determined by two major

components: the meaty flavor is associated with pro-

tein, while the species flavor is associated with fat.

During aging, proteins are altered to impart the

characteristic flavor of aged meat. The breakdown of

protein and fat produces flavor compounds, including

hypoxanthine, xanthine, hydrogen sulfide, ammonia,

acetylaldehyde, and acetone. (See Flavor (Flavour)

Compounds: Structures and Characteristics.)

0026 The structure of living muscle is inexorably linked

to the structure and characteristics of meat; therefore,

an appreciation of all three is required to understand

meat-based food products.

See also: Fats: Production of Animal Fats; Flavor

(Flavour) Compounds: Structures and Characteristics;

Meat: Eating Quality; Analysis; Protein: Food Sources

Further Reading

Bechtel PJ (1986) Muscle as Food. Orlando, FL: Academic

Press.

Huff-Lonergan E and Lonergan SM (1999) Postmortem

mechanisms of meat tenderization. In: Xiong YL, Chi-

Tang H and Shahidi F (eds) Quality Attributes of Muscle

Foods, pp. 229–251. New York: Kluwer Academic.

Judge MD, Aberle ED, Forrest JC, Hedrick HB and Merkel

RA (1989) Principles of Meat Science, 3rd edn. Dubu-

que, IA: Kendall/Hunt.

Pearson AM and Young RB (1989) Muscle and Meat Bio-

chemistry. San Diego, CA: Academic Press.

Price MG (1991) Striated muscle endosarcomeric and exo-

sarcomeric lattices. In: Malhotra SK (ed.) Advances in

Structural Biology, pp. 175–207, vol. 1, Greenwich, CT:

JAI Press.

Swatland HJ (1994) Structure and Development of Meat

Animals. Englewood Cliffs, NJ: Prentice-Hall.

Taylor RG, Geesink GH, Thompson VF, Koohmaraie M

and Goll DE (1995) Is Z-disk degradation responsible

for postmortem tenderization? Journal of Animal

Science 73: 1351–1367.

Slaughter

K Troeger, Bundesanstak f_r Fleischforschung,

Kulmbach, Germany

Introduction

0001Meat consumers, especially young people in largely

urbanized regions of developed countries, are increas-

ingly demanding that animals be reared, handled,

transported, and slaughtered using humane practices.

At slaughter, this means that stunning should render

animals insensible to pain to insure that they do not

suffer needlessly during slaughter. Because of animal

welfare considerations in European law, all animals

must be stunned before they are slaughtered. The

second aspect of slaughtering is the quality of the

final product. This article describes technical param-

eters of the slaughter process in relation to both meat

quality and aspects of animal welfare.

Animal Handling

0002The moving, handling, and unloading of animals

within a slaughterhouse should be carried out with

care. Good handling procedures will reduce pale,

soft, exudative (PSE) meat, bloodsplash, and injury

losses. Animals should be unloaded as soon as possible

after arrival at the slaughterhouse. The unloading

facilities (ramp, bridge, gangway) should be fitted

with siderails and the slope should not exceed 20



0003While cattle should if possible be slaughtered im-

mediately after unloading to avoid poorer meat qual-

ity (dark-cutting beef or DCB), lairage times for pigs

of at least 2 h and at most 4 h have proved to be

successful. The shape and construction of the pens

3766 MEAT/Slaughter

should permit most of the animals to lie against a

wall. Holding pens should therefore be long and

narrow rather than square. The following conditions

of lairaging have proved to work well:

0004 Group penning: groups of animals should be kept

together, not mixed with ‘strangers’

0005 Number of animals per pen: each animal must have

enough space to lie down and turn around

0006 Sprinkling: showering, at least during hot weather,

is particularly recommended for pigs

0007 Water: drinking water must always be available

0008 Cattle and pigs are driven to the stunning place

along passageways and races. Sticks and electric

prods are used commonly as guiding instruments.

According to European law, personnel involved in

preslaughter handling must be trained and certified.

Suitable passageways allow the use of gates on rollers

or guiding boards, so a more gentle driving of pigs

is possible. Races should be short, straight, horizon-

tal, and wide enough to enable two animals to walk

side by side for as long as possible. Shadows and

sparkling reflections should be eliminated. Animals

tend to move from a darker area towards a lighter

one, but they will avoid blinding light. Animal welfare-

relevant problems mostly occur if it is necessary to

line up pigs in a single race. A new Danish carbon

dioxide (CO

) stunning system (Backloader) makes it

possible to handle pigs in a group into and through

the stunning unit. The principle of the new concept is

that 15 pigs are driven forward using a series of push-

hoist gates or, alternatively, manually driven to the

area in groups of 15. There are three main elements to

this concept: first, there is an area where the group of

15 hogs is divided into smaller groups (e.g., five);

second, there is automatic transfer of these smaller

groups to and through the stunning equipment; and

third, there is a system for presenting the stunned

hogs for shackling and sticking. As the hogs are

stunned in groups, the concept assumes that CO

stunning will be used and this has several advantages.

Hogs do not like to be isolated away from one an-

other and stunning in groups eliminates this problem.

Moreover, hogs find restraint stressful and in modern

systems hogs are not restrained at all during the

stunning itself (Figure 1).

Stunning

General Aspects

0009Stunning aims to bring the animal immediately to a

state of insensibility that lasts until it is dead. Another

aim is to produce sufficient immobility to facilitate

exsanguination. These two aims are not necessarily

complementary. Some methods of stunning result in

violent reactions by the animals after they have been

rendered insensible. Conversely, certain procedures,

such as low-voltage application, may produce total-

body immobility without causing insensibility. In the

light of present scientific and practical knowledge,

acceptable stunning methods, if applied properly,

are electrical stunning, CO

stunning or stunning

with alternative gas mixtures (e.g., argon), and per-

cussive stunning (penetrative; nonpenetrative with

restriction). Irreversible stunning methods, such as

cardiac arrest stunning, induced by an electrical cur-

rent or longer exposure to gas mixtures, is preferred

from the animal welfare point of view as well as for

meat quality reasons.

Electrical Stunning

0010Electrical stunning is important first for slaughtering

pigs and poultry; sheep and cattle are also stunned

electrically in some countries. The electrodes must be

placed so that they span the brain. Effective electrical

stunning results in immediate insensibility, which is

exhibited as an epileptiform seizure. The effectiveness

of electrical stunning depends on a minimum current

level passing through the brain. Using a constant

stunning current of 1.3 A, for pigs effective stun can

be induced within a minimum current flow time of

0.3 s. This time requirement seems to be appropriate

Access

fig0001 Figure 1 Backloader CO

stunning plant for group stunning of

hogs (Butina, Denmark). a, Stunning box; b, pushing gate;

c, conveyor belt.

MEAT/Slaughter 3767

for animal welfare-conforming electrical stunning.

There are two main types of electrical stunning: head-

only and head-to-body. In head-only systems elec-

trodes are applied only to the head, whereas in

head-to-body stunners an (additional) electrode is

placed elsewhere on the animal’s body (back, breast)

so that the flow of current passing through the body

should additionally cause cardiac ventricular fibrilla-

tion. With this method, all species will be rendered

immediately and permanently insensible.

0011 Besides manual stunning with a pair of tongs or

single-handed prods, fully automated electrical stun-

ners are available for use on both sheep and pigs.

With older systems, animals are fixed and conveyed

in a v-conveyor (restrainer) to the point of stunning

(bilateral electrode contact on the head). The stunning

voltages used in these systems are about 450–1000 V

(high-voltage systems). Recent developments have led

to improvements in automatic electrical stunning

devices. The Midas system (Stork, Netherlands) in-

corporated a new conveyor belt transport system; the

pig rides on this belt with its feet hanging down on

both sides and becomes relatively relaxed in this pos-

ition. This is a good condition for an effective stun.

The system has the capacity for 200–600 hogs per

hour. It is not a high-voltage system. The induction of

cardiac ventricular fibrillation is carried out by an

automatic breast electrode, so the animals show little

or no convulsions. After a current flow through the

head, the third electrode is contacted pneumatically

with the left side of the breast and an additional

current flows head-to-body.

0012 The current flow through the brain leads to de-

polarization of neurons, with subsequent repolariza-

tion, which causes uncoordinated motor activity

(tonic–clonic spasms). These muscle contractions

stimulate glycolysis, with production of lactic acid

in the musculature. The consequence is an accelerated

pH drop postmortem, which favors the incidence of

PSE meat. Fewer muscle spasms occur after cardiac

arrest stunning. Other adverse effects of electrical

stunning can be fractures of bones or bloodsplash

(intramuscular hemorrhages) and speckling (petech-

ial hemorrhages in subcutaneous tissues). Factors im-

portant for a careful slaughter process including

electrical stunning are summarized in Table 1.

Gas Stunning

0013Gas stunning with high concentrations of CO

in air

is used primarily for pigs; turkeys and chickens are

also sometimes stunned or killed with CO

.CO

colorless and heavier than air, so stunning can be

done in a pit. The gas atmosphere for stunning con-

sists of about 70–95% CO

and air. CO

has the

general effect of depressing neuronal function by

both hypoxia and lowering tissue pH < 7.0. There

are two main types of CO

stunning or killing

systems: a dip-lift system, where pigs are lowered

continuously into the gas, and the paternoster system,

where pigs are lowered successively into the gas with

stops as pigs enter or leave the equipment. The pater-

noster system is most commonly used. The latest

development is an automatic Danish system for

group CO

stunning of up to 800 hogs per hour.

This system has the advantage that lining up and

restraint of animals are no longer necessary. Up to

five pigs enter the stunning box simultaneously.

Adequate stunning can be achieved using high CO

concentrations (> 90%) and a long exposure time

(> 150 s).

0014The welfare implications of CO

stunning are still

under debate. Some studies have shown that most

pigs will avoid an atmosphere of high concentrations

of CO

. This aversion was found to be greater than

the motivation to obtain a reward in the CO

atmos-

phere, even after 24 h fasting. On the other hand,

exposure to CO

was less aversive than electrical

shocks. Scientists have also interpreted the increase

in rate and depth of respiration that occurs during

inhalation of CO

as respiratory discomfort. In con-

trast to stunning of pigs with CO

, the animals show

no aversive reactions when exposed to gas mixtures

of either 90% argon in air or 60% argon and 30%

tbl0001 Table 1 Electrical stunning: important factors for animal welfare and meat quality

Factors Criteria

Minimum of excitement and physical strain Internal muscle temperature (ham) 45 min post mortem 40.5



Immediate unconsciousness No vocalization (squealing) if the stunning current is interrupted

after 0.3 s

Two-step cardiac arrest system:

1. Constant current (1.3 A) through the head, followed by

2. Head-to-body current

Little or no convulsions

Death within 60 s (no eye reactions, dilatation of the pupils)

Insignificant bloodsplash

Horizontal bleeding table Conveying till muscles are relaxed (1.5–2 min)

Short stun-to-stick interval Horizontal bleeding <10 s after stunning

Effective sticking Amount of blood > 3 l per pig

3768 MEAT/Slaughter

in air. The phase before unconsciousness

happens without stress. The practicability of these

gas mixtures under commercial conditions was tested

in the UK.

0015 CO

anesthesia results in fewer carcass defects than

electrical stunning. The incidence of bloodsplashes,

fractures, and PSE meat is lower.

Percussive Stunning

0016 Percussive stunning methods can be divided into two

categories: penetrative and nonpenetrative. The first

category includes free bullets, captive bolts, and high-

pressure water jets. Nonpenetrative methods deliver

a controlled obtuse blow to the head (mushroom

head, manual blow). In all cases kinetic energy is

transferred to the brain, which causes neuronal dys-

function and subsequent unconsciousness. While

penetrative methods, which produce a trauma of

brain tissue, cause permanent insensibility, uncon-

sciousness after the use of nonpenetrative methods is

only temporary.

0017 For penetrative stunning – in general of cattle,

sheep, goats, and horses – captive bolt apparatus

and pistols are mostly used. These consist of a steel

rod with a collar and piston (bolt) at one end con-

tained in a chamber. On activation the bolt is pro-

pelled forward by the energy of a cartridge or by

compressed air and penetrates the head of the

animal. Water jet stunning is the use of a high-

pressure water jet with 2500–4000 bar and an

injection time of 20–100 ms for stunning or killing

slaughter pigs. This method is in an experimental

phase. The only method of nonpenetrative percussive

stunning commonly used is the ‘mushroom head’

fitted to a captive bolt and used almost exclusively

on cattle, especially in cases of ritual slaughter. The

‘mushroom’ is a metal convex disk approximately

4 cm in diameter and the impact is applied to the

frontal region of the head.

0018 After the correct use of a captive bolt, corneal

reflexes cannot be evoked and respiration ceases.

The effect of a nonpenetrative percussive technique

depends on the size of impulse delivered. Studies have

shown that only about 80% of animals are being

stunned effectively by such methods.

0019 Percussive stunning, like other stunning methods,

causes an increase in blood pressure, violent muscular

contractions, and elevated levels of catecholamines.

These factors can result in bloodsplash and the

occurrence of PSE in pigs. In some European coun-

tries, where cases of bovine spongioform encephal-

opathy (BSE) have occurred, the use of captive

bolts for stunning cattle is under debate. There is

a clear risk, because of the destruction of brain

tissue, including blood vessels, with this method,

that in the case of BSE infectious brain tissue might

be dispersed into the blood stream, and especially to

the lungs.

Killing

0020Killing of slaughter animals is usually done by exsan-

guination (bleeding). An incision (sticking) which

severs the major arteries of the neck or anterior thor-

acic cavity causes a rapid loss of blood, resulting in

a lack of oxygen to the brain. Irreversible cellular

changes occur and the animal dies. Because most

stunning methods are reversible as a rule, prompt

and accurate (effective) sticking is of high importance

from the animal welfare point of view.

0021Bleeding can be done with the animal in a vertical

or horizontal position. The most common technique

is to shackle a hind leg and to elevate the stunned

animal on the bleeding rail, where sticking is done in

a hanging position. There is a significant positive

effect on pork quality (reduction of PSE), if prone

bleeding is practiced. Sticking is done immediately

after electrical stunning on a movable conveyor.

For a bleeding time of about 2 min, the pigs are

conveyed in a horizontal position. As a result, muscle

spasms have a less detrimental effect on meat quality,

and internal ham bruising and bloodsplashing are

obviated.

0022Severing of the common carotid arteries or the

vessels from which they arise during sticking can be

done in different ways. A transverse incision of the

neck is a common method of bleeding sheep. This

method is also used for ritual slaughtering of sheep

and cattle.

0023Ritual slaughter is performed by a throat cut, which

severs all soft tissues of the throat (muscles, blood

vessels, esophagus, trachea, nerves) without pre-

slaughter stunning. It is necessary to distinguish be-

tween kosher (Jewish) slaughter and halal (Muslim)

slaughter. For kosher slaughter, there exist exact rules

for the slaughter process, the instruments used, and

the qualifications of the slaughterman. A straight,

razor-sharp knife (chalaf) that is twice the width of

the throat is required, and the cut must be made in a

single continuous motion. For halal slaughter, no

special knife design is required.

0024Another sticking method, lateral stab incision of

the neck, avoids incision of the trachea and esopha-

gus, because this may result in aspiration of blood and

pollution of the blood by stomach contents. Finally,

thoracic sticking is commonly used for cattle and

pigs. The knife is inserted in the midline, cranial of

the sternum bone, so that the bicarotid trunk and

anterior vena cava are incised.

MEAT/Slaughter 3769

Skinning, Scalding/Dehairing,

Evisceration, and Splitting

0025 In industrial slaughterhouses skinning, including

scalding and dehairing (hogs), evisceration and split-

ting of carcasses occurs continuously within a slaugh-

ter conveyor (Figure 2). The capacities of slaughter

lines in Europe may be up to about 80 cattle and 800

pigs per hour. There is a great deal of mechanization,

automatization, and robotization in slaughterhouses

worldwide.

Skinning (Cattle, Sheep)

0026 After bleeding, the free hanging leg is skinned; this leg

is then used temporarily for hanging, so that the

slaughter chain can be taken away from the other

leg. This leg is skinned too. The gambrel is provided

and the belly, chest, and neck part are skinned. Fur-

ther skinning is done mechanically by means of a

skinning machine. For this, the front legs are fixed

and the skin is drawn off from the lower to the upper

side. This is the traditional method of depelting sheep.

A lower carcass microbial level can be achieved if the

sheep are depelted from the shoulder to the hind leg;

this process is most easily done by suspending the

animal from the front legs (inverted system). For

cattle, with a roller-mounted hide stripper, skinning

is possible either from top to bottom or from bottom

to top. If the head remains on the carcass and is

skinned too, the carcass has to be stabilized by an

electrical current (electrical stimulation with about

150 V, electrodes on the back). A disadvantage of

this method is that the force needed for dehiding is

so high that overstretching of the hide can happen,

which lowers its value for processing.

Scalding, Dehairing, Singeing, Polishing (Pigs)

0027The conventional process of hair removal from

slaughtered hogs utilizes hot water for scalding and

a gas flame for singeing. The traditional scalding

method is vat scalding. Using a water temperature

of 58–62



C, the scalding process requires 4–6min.

On economic grounds, many hogs are scalded in the

same hot water; this results in rapid pollution of the

scalding water. From a bacteriological point of view,

therefore, it is important that the scalding tempera-

ture exceeds 60



C, but it should not exceed 62



because of the coagulating effect on pig skin, which

results in damage to the skin by the dehairing

machine. A disadvantage of vat scalding is the possi-

bility that scalding water can enter the lungs and,

via the sticking wound, into deeper tissues of the

carcass. Investigations have revealed that the risk of

bacterial internal contamination of lung, liver, heart,

and muscles depends on the scalding and dehairing

technology. From a hygienic point of view, vat

scalding with simultaneous dehairing (mechanical

treatment by dehairing implements with pressing

and pumping effects on the thorax) was the most

dubious method.

0028An alternative to vat scalding is the scalding tunnel.

The pigs are brought through the tunnel by means of

(a) (b) (c) (d) (e) (f) (g) (h)

fig0002 Figure 2 Slaughter line for cattle with hanging up to bleed. (a) Stunning; (b) sticking; (c) decapitation; (d), (e) transfer to slaughter

line, removal of hind legs, and preliminary skinning; (f) removal of remaining skin and opening of breast bone by sawing; (g), (h)

removal of paunch and breast organs, and cleaving. Reproduced from Meat: Slaughter. Encyclopaedia of Food Science, Food Technology

and Nutrition, Macrae R, Robinson RK and Sadler MJ (eds), 1993, Academic Press.

3770 MEAT/Slaughter

a chain-conveyor in a hanging position. Scalding is

done either by spraying carcasses with hot water that

is pumped around the system or by condensed steam.

To obtain a low bacterial count on the carcass sur-

face, it is still necessary that the pigs pass through a

prewashing machine prior to the scalding tunnel.

0029 After scalding, the next process in a pig slaughter

line is dehairing. Dehairing (scraping) machines are

constructed from heavy U-shaped bars, a steel frame,

and one or two shafts to which belt scrapers are

attached. The hogs are sprayed with water as they

pass lying and rotating through the dehairer. If the

water in the machine circulates, the water tempera-

ture should be at least 55–58



C for hygienic reasons.

After dehairing, the pigs are conveyed to the prepol-

isher. The prepolish machine is needed to remove any

remaining hair and to dry the surface of the pig. In

this way the effect of the singeing furnace will be

optimal and gas consumption is reduced. The singe-

ing furnace is intended to reduce surface bacteria and

to singe hair stubble. The surface bacterial count can

be reduced by 1–2 powers of 10 by the heat effect.

This is not the case if the carcasses are still very moist

after singeing. This is followed by the polishing ma-

chine, which cleans the singed pigs by brushing. Here

the mechanical treatment of the pig skin comes to

an end. The washing and polishing process usually

causes recontamination of the carcass surface. This is

due to the whips inside the washing machine, which

are hard to clean.

Evisceration

0030 This follows the ‘clean’ side of the slaughter line

where, when working hygienically, the surface bacter-

ial count of the carcasses is no longer greatly affected.

Opening and evisceration of the slaughtered animals

take place on the slaughter conveyor manually by

means of knife, axe, or saw. First the aitch-bone is

split at its midpoint. The next step is loosening the

anus and the reproductive tract. For this step, mech-

anical rectum looseners are frequently used. After

splitting the sternum and opening the belly cavity,

the gut set and the stomach are taken out once the

esophagus has been cut approximately 2 cm from

the stomach (in pigs). In the USA and the Far East,

the guts and the pluck (contents of the thoracic

cavity) are taken out together.

0031 To avoid the hygiene risk of an outflow of rumen

content in cattle, the esophagus should not be cut but

rather it should be separated from the trachea and

shut with a special instrument (rodding). Removing

the pluck is the last step of evisceration.

0032 Hog opening can also be done by a robotic system

that adapts itself to the physical shape of the hog. The

slaughtering robot cuts loose the rectum, splits the

aitch-bone, and separates the ham. The front unit

cleaves the sternum and opens the abdominal wall

of the hog.

Splitting

0033Splitting of carcasses (cattle, pigs, horses) can be exe-

cuted manually by axe or saw or in automatic car-

cass-splitting systems. The automatic systems work

within the continuous dressing conveyor. The car-

casses are split by a circular saw. Backbone guides

and holding jaws on each side stabilize the carcass

and thus prevent miscuts. During the cutting oper-

ation the saw blade is cooled with cold water. After

splitting, the saw blade as well as the holding jaws are

sterilized with hot water (82



C). For pig carcasses,

automatic chopper machines are also in use.

Waste Product Handling

0034Waste products are those parts of a slaughter animal

that are not suitable for direct utilization. This in-

cludes condemned meat and offals, obligatorily con-

fiscated tissues, e.g., meat of the sticking cut, eyes,

tonsils and gut, stomach and rumen contents, glands,

hair, inedible fat and blood. As a result of the BSE

epidemic in some European countries, in the Euro-

pean Union the so-called specific risk material (SRM)

has to be extirpated. This includes the following

tissues of cattle, sheep, and goats greater than 12

months old: skull, including brain and eyes, tonsils,

and spinal cord; the spleen of sheep and goats and the

ileum (part of the small intestine) of cattle greater

than 12 months old. In comparison to live weight,

waste material accounts for about 20% for sheep (live

waste weight 45 kg), 14% for bulls (live weight

450 kg) and 10% for pigs (live weight 90 kg).

0035With modern technologies, nearly all the slaughter

waste can be economically transferred to useful prod-

ucts (Table 2). (See Offal: Types of Offal.)

0036Precleaning of waste water is frequently done

within the plants. With mechanical methods such as

rotating filter tubes or paternoster filter grids, up to

60% of sedimentable particles can be separated.

More complete cleaning can be achieved by a flota-

tion process, whereby suspended substances (col-

loids) are coupled with microgas bubbles so that

they come to the surface by flotation and can be

removed. (See Effluents from Food Processing: On-

Site Processing of Waste; Composition and Analysis.

Effect on Meat Quality

0037There is a close relationship between meat quality

and the treatment of animals before slaughter.

Rough handling of pigs and fighting between animals

MEAT/Slaughter 3771

prior to stunning both increase the incidence of PSE

meat. When the glycogen level in a stressed pig is

high, much lactic acid is produced after killing,

while muscle temperature is still high (about 40



C).

The combination of high lactic acid level and high

temperature leads to denaturation of muscle proteins

and thus produces PSE meat. The next severe stress

factor is stunning. Many stunning methods cause

vigorous clonic spasms (with stimulation of glycolysis

and accelerated pH drop) and high release of stress

hormones such as epinephrine (adrenaline) and nor-

epinephrine (noradrenaline). By activating enzyme

systems, epinephrine causes increased breakdown of

muscle glycogen to lactic acid. If sticking of the

stunned animal is delayed, the glycolytic effect of

stress hormones is more pronounced and the inci-

dence of PSE meat is higher.

0038 The treatment of the carcass can influence the

postmortem biochemical reactions in the muscula-

ture. It has been found that pigs that had been

dehided instead of scalded had musculature that was

darker, with better water-binding properties and

greater tenderness. The reason is that scalding and

singeing of pig carcasses produce muscle tempera-

tures that are about 1



C above those that apply

during dehiding. This results in a quicker pH drop

postmortem. Also, the intensive mechanical treat-

ment of the carcasses from hair scrapers and other

mechanical machines may contribute to the produc-

tion of PSE meat.

0039 Long times of stress, long fasting times (transport,

lairage), and mixing animals from different farms

produce dark, firm, dry (DFD) meat. In these cases,

at the time of slaughter most of the muscle glycogen

reserves are already exhausted. The remaining glyco-

gen is therefore at a much lower level, so that the

amount of lactic acid, which is produced via glycoly-

sis, is also low. Consequently, the ultimate pH value

in DFD musculature is relatively high (> 6.2). Refer to

individual meats.

See also: Beef; Meat: Eating Quality; Hygiene; Pork;

Sheep: Meat