Animals
may be stunned by passing an alternating electric current through the
brain. The method is widely used for
stunning pigs and poultry.
Unconsciousness is induced by a wide range of voltages, from about
seventy
volts to several hundred volts. The length of time that the current is
passed
through the brain may be reduced to only one or two seconds if abattoir
workers
are waiting to shackle the pig's hindlimb with a chain, and then to
exsanguinate the animal immediately. With a simple, hand-held electric
stunner,
the current is applied to the pig's head with two electrodes that
protrude from
an insulated handle. The electrodes must be cleaned at frequent
intervals to
ensure good electrical contact with the pig. The transformer which
supplies the
current is usually mounted on a nearby wall. However, large automated
stunning
systems are used in most commercial abattoirs and these may have one of
a
variety of different patterns of electrodes. Some designs have a
current flow
through the chest to stop the heart (although the heart starts to beat
again when the pig is hoisted up on a shackling chain).
High voltage head to back
stunning intended to stop the heart may cause vertebral fractures and
blood
splashes in the meat if the system is not carefully operated and
monitored. Essentially, if the rear
electrode is moved forward there is less damage to the carcass, but
also a
reduced probability of stopping the heart. High
frequency electrical stunning of pig may be used to reduce carcass
damage.
Pigs may be
stunned by placing them in an atmosphere which contains 65% carbon
dioxide.
Carbon dioxide is heavier than air and is trapped in a pit or deep
tunnel into
which the pigs are conveyed. After about one minute, the pigs are
withdrawn in
a cage or on a conveyer belt, and are then exsanguinated as rapidly as
possible. Carbon dioxide stunning requires some major features in
the architecture of the abattoir which make it difficult to
retro-fit whereas automated electrical stunning apparatus is
simple to incorporate because it does not require pits or
tunnels. Modern carbon dioxide stunning apparatus is very
effective because carbon dioxide concentration can be accurately
controlled by solid-state gas sensors. In older system the gas levels
were not accurately controlled.
If the sticking wound is inaccurately placed, exsanguination may be too slow, and it may be almost halted by the formation of large blood clots. The formation of blood clots is accelerated when large areas of tissue are damaged by repeated inaccurate punctures. If the trachea is severed by the sticking wound, blood may be drawn into the lungs as the animal breathes. Later in the slaughter procedure, this may necessitate the trimming of blood clots from the pleural membranes after they have been inspected. If the oesophagus is severed, the vascular system may be contaminated by the entry of food particles into the venous system. If the connective tissues of the shoulder are opened, blood may seep into the shoulder region to form blood clots between the muscles.
Incomplete exsanguination increases the amount of residual blood in
the carcass. The lean meat may then appear unduly dark and the fat may
become streaked with blood.
The sticking knife must be kept clean otherwise bacteria
might
be introduced into the venous system and spread through the otherwise
relatively
sterile muscles of the carcass. Once exsanguination has started, the
pulse
and mean blood pressure rapidly decline because of the reduced stroke
volume
of the heart. Blood pressure changes are monitored physiologically by
baroreceptors
in the carotid sinuses. During exsanguination, respiratory movements of
the thorax may be stimulated, and neurogenic and hormonal mechanisms
attempt
to restore the blood pressure by increasing the peripheral resistance
by
vasoconstriction. The heart keeps beating for some time after the major
blood vessels are emptied, but rapidly stops if exposed and cooled.
Electrical
stunning of pigs may terminate cardiac activity so that, at the start
of
exsanguination, the blood escapes by gravity rather than being pumped
out.
In pigs, cardiac arrest does not affect the rate and extent of
exsanguination.
After exsanguination has started, the heart usually re-starts and
attempts
to pump, until it runs out of energy.
Blood loss as a percentage of body weight differs between species: cows, 4.2 to 5.7%; calves, 4.4 to 6.7%; sheep, 4.4 to 7.6%; and pigs, 1.5 to 5.8%. Blood content as a percentage of live weight may decrease in heavier animals since the growth of blood volume does not keep pace with growth of live weight. Approximately 60% of blood is lost at sticking, 20-25% remains in the viscera, while a maximum of 10% may remain in carcass muscles. Different stunning methods may modify the physiological conditions at the start of exsanguination and, also, the neural responses to exsanguination
Reduction of blood flow to the kidneys causes the release of a proteolytic enzyme, renin, which acts on a plasma protein to produce a polypeptide, angiotensin I. This polypeptide is converted enzymatically to angiotensin II which then causes widespread vasoconstriction. Vasoconstriction is important because it decreases the retention of blood in meat. Angiotensin II vasoconstriction is operative in both conscious and anaesthetized animals. Catecholamines and antidiuretic hormone (ADH) may also enhance vasoconstriction during exsanguination. Speed of exsanguination may modify the balance between neural and hormonal vasoconstrictive mechanisms, with hormonal vasoconstriction predominating in rapid exsanguination. However, asphyxia prior to exsanguination may result in vasoconstriction due to the activity of the sympathetic nervous system.
Factors that regulate the balance between extracellular and intracellular fluid compartments in meat are poorly understood. Fluid is delivered to living muscles by arteries, but it may return to the heart by either of two routes, in the venous system or in the lymphatic system. The route taken by intercellular fluid depends primarily on the extent to which fluid is taken up by capillaries and then passed to the venous system. In living animals, the venous return is far greater than the lymphatic return. The lymphatic capillaries which drain skeletal muscles are mostly located in the connective tissue around bundles of muscle fibres. The small amount of lymph that drains from muscles is increased after neural stimulation, and its lactate dehydrogenase content (LDH - an enzyme from within the muscle fibre) increases dramatically following muscle damage. In sheep, the flow of lymph from lymph nodes increases within 15 minutes of stress due to pain. Haemorrhage may or may not cause absorption of intercellular fluid into the blood stream, depending on the degree of vasoconstriction and consequent hydrostatic pressure in the vasculature.