Agriculture Reference
In-Depth Information
When cardiac arrest is induced, these signs are shorter
lasting and less pronounced and are followed by a com-
pletely limp carcase, no breathing (absence of abdominal
movements in the vent area), loss of nictitating membrane
reflex and dilated pupil. The comb reflex can also deter-
mine whether sensibility has resumed after stunning or
neck cutting.
Although cardiac arrest is preferable from a welfare
standpoint, the use of high stunning currents on the
quality of the carcase is said by some to be associated
with wing haemorrhages, red skin condition, including
red wing tips and pygostyles, poor plucking, broken
bones (in particular furculum) and ruptured blood
vessels causing blood splashing in the breast muscle.
birds rub, also has a quietening effect. It is important that
this sheeting extends into the stun bath.
Turkeys must not be suspended for more than 3 minutes
and other birds for 2 minutes before being stunned or
killed.
Stunning and slaughter
Electrical stunning
Stunning is usually carried out in an electrically charged
water bath by dragging the heads of the birds through
water in which an electrode is submerged. The shackles
of the killing line simultaneously touch an earth elec-
trode, causing an electric current to run through the body
of the bird. Effective stunning requires careful observa-
tion of the birds and adjustment of the equipment.
The water level is critical, and it is essential to avoid
water flowing down the inlet chute and causing a pre-stun
shock, which may make the birds raise their heads, thus
avoiding contact with the water of the actual stunner.
Research has shown that a water bath stunner using a
50 Hz AC supply and providing a current of 148 mA per
bird will result in a stun/kill of 99% of birds, with only
1% of birds leaving the water bath stunned but still alive.
However, 105 mA per broiler would be acceptable pro-
vided both carotid arteries are severed within 15 seconds
to ensure death before birds can begin to recover.
Problems with conventional water bath stunners have
been identified through observation in commercial pro-
cessing plants and experiments in the laboratory. Owing
to differences in the electrical resistance of the individual
birds, there is little control over the stunning current and
hence the effectiveness of stunning. A prototype poultry
stunner has been developed which controls the current
delivered to individual birds. The machine is capable of
operating at typical commercial speeds of 6000 birds/
hour. It can provide a constant current to each individual
bird provided there is no significant current pathway
between adjacent birds. A constant current stunning sys-
tem will control the current flow through individual
birds at an optimal level which will ensure an effective
stun and at the same time minimise the carcase quality
problems produced by high currents.
Less commonly used stunning instruments are the
dry stunner, usually incorporating an electrically charged
metal grid or plate, and hand-operated stunner.
The most reliable indicator that a bird is properly
stunned by the low-current or high-frequency method is
the electroplectic fit. The characteristics of this condi-
tion are the neck arched with the head directly vertical,
open eyes, dilated pupils, absence of corneal reflex, wings
flexed, rigidly extended legs and constant rapid body
tremors after 4 seconds.
Gas stunning
Although electrical stunning is the most common
method of stunning poultry prior to slaughter under
commercial conditions, methods of stunning/killing
poultry using gases, while the birds are still in their
transport containers, have been approved under the new
EC Directive. Gas stunning/killing will enable shackling
to be performed on the freshly killed birds, and this
would eliminate the live bird handling at the processing
plants. The two gas stunning/killing methods that are
being approved in the United Kingdom are anoxia
induced with 90% argon or other inert gases and a mix-
ture of 25-30% carbon dioxide and 60% argon or other
inert gases in air. In both systems, the oxygen levels must
not exceed 2% (Fig. 10.5).
The birds would be stunned/killed while they are in
their transport containers. This would result in the birds
leaving a stunning unit in large numbers, and all the car-
cases would have to be uncrated and shackled rapidly to
allow prompt neck cutting. It is inevitable that the time
between the end of gas stunning and neck cutting would
be longer than that practised under the conventional
electrical stunning system. Although research has shown
that the efficiency of bleeding in broiler chickens was not
impaired when their necks were cut immediately after
gas stunning/killing, delayed neck cutting after gas stun-
ning of broilers could increase the prevalence of carcase
downgrading conditions associated with a poor bleed
out. This does not happen with turkeys, the difference
being attributable to a difference in carcase cooling rate
or other factors.
Gas stunning/killing methods accelerate the rate of
post-mortem pH fall in poultry, and this would allow
early filleting in broilers (between 2.5 and 4 hours after
slaughter). Generally with anoxia, there are fewer carcase
and meat quality defects. Many fewer defects are found in
the breast region, including haemorrhages, but there is an
increase in broken wings caused by convulsions.
