Received for publication August 5, 1997.
Accepted for publication July 23, 1998.
To whom correspondence should be addressed: sbilgili@acesag.
Abbreviation Key: EEG = electroencephalograms; NELS = New
Enhanced Line Speeds; SEP = somatosensory evoked potentials; SIS =
Streamlined Inspection System.
Recent Advances in Electrical Stunning
S. F. BILGILI
Department of Poultry Science, Auburn University, Auburn, Alabama 36849-5416
ABSTRACT This paper provides an overview of the
electrical stunning systems and their implementation in
modern-day broiler slaughter plants. The application of
low voltage electrical stunning systems in the U.S. is
reviewed and contrasted with European experiences
within the context of differing slaughter technologies,
practices, and regulatory constraints. Finally, the impact
of electrical stunning on traditional carcass and meat
quality attributes of broilers is examined.
(Key words: electrical stunning, slaughter, carcass quality, broiler)
1999 Poultry Science 78:282–286
Electrical stunning is the most universally accepted
and utilized method for immobilizing poultry prior to
slaughter. The equipment is relatively simple, inexpen-
sive, takes up little space, is compatible with the current
kill-line speeds, and for the most part, is easy to operate
Electrical stunning systems for poultry were primar-
ily developed to render the bird unconscious long
enough to allow automated neck cutting and to reduce
carcass damage due to slaughter-induced struggle and
convulsions during bleeding. Although considered more
humane than killing without stunning, electrical stun-
ning has often been questioned, especially in Europe, on
animal welfare grounds (Fletcher, 1993). In contrast to
European recommendations, which require specific con-
ditions to assure that poultry be “instantaneously
rendered insensible to pain until death supervenes”
(Kettlewell and Hallworth, 1990), the Poultry Inspection
Regulations do not mandate stunning conditions in the
U.S. (USDA, 1984).
Regardless of differences in cultural, religious, and
regulatory practices governing slaughter of poultry, the
application of electrical stunning principles in slaughter
plants and the effects of stunning-killing on end-product
quality are of concern to processors worldwide. Excel-
lent reviews have been published on electrical stunning
basics (Richards and Sykes, 1967; Ingling and Kuenzel,
1978; Kuenzel and Walther, 1978; Schutt-Abraham et al.,
1983; Veerkamp and de Vries, 1983; Gregory, 1989;
Kettlewell and Hallworth, 1990; Bilgili, 1992a) and
slaughter-bleeding of poultry (Newell and Shaffner,
1950; Davis and Cole, 1954; Kotula and Helbacka, 1966;
Abram and Goodwin, 1977; Scott, 1978; Heath et al.,
1981, 1983; Heath, 1984; Warris, 1984). The scope of this
review will be to highlight the recent advances in design
and application of electrical stunning technology in
modern-day broiler slaughter plants.
PRINCIPLES OF ELECTRICAL STUNNING
Electrical stunning is accomplished by passing a
sufficient amount of electrical current through the
central nervous system of birds for a given amount of
time (Bilgili, 1992a). The state of unconsciousness
induced by electricity results from the inhibition of
impulses from both the reticular activating and the
somatosensory systems (Heath et al., 1994). Loss of
somatosensory evoked potentials (SEP) and spontaneous
electroencephalograms (EEG) have been directly related
to brain failure and instantaneous insensibility to pain
(Richard and Sykes, 1967; Lopes da Silva, 1983; Gregory
and Wotton, 1989, 1990). The stunning current reaching
the brain must be adequate enough to induce an
epileptic seizure. This current is usually lower than that
required for ventricular fibrillation and, hence, death by
electrocution. Insufficient currents may physically im-
mobilize the bird, but may not prevent perception of
pain, stress, or discomfort by the animal. Hence, if the
bleeding is not rapid, birds may regain consciousness
prior to scalding (Fletcher, 1993).
SYMPOSIUM: RECENT ADVANCES IN SLAUGHTER TECHNOLOGY
and Electrical Circuits
Although there are many makes of commercially
availableelectricalstunners,forthe most parttheir design
and operation are similar. A fiberglass brine-water bath
cabinet is supported under the overhead conveyor line
from which chickens move suspended on shackles. The
cabinet is vertically adjustable and usually set at a height
that allows the heads of the birds to be submerged in
brine-bath water. An electrified metal grate is submerged
in the bottom of the brine-water bath tank. Although the
shackle line isconnectedto earth, aground bar contacting
theshacklesisoftenusedto complete theelectricalcircuit.
procession, typically either 140 or 180 birds per minute in
the U.S., depending upon the inspection system used.
When a voltage is applied between the submerged
electrode and the earth (ground), the current flows
through the immersedchickens in the cabinet to complete
the circuit. Chickens in this type of circuit represent a
series of resistors connected in parallel. Although birds
contacting each other in this circuit can create other
resistive pathways, the significance of these pathways is
not well established (Kettlewell and Hallworth, 1990;
Sparrey et al., 1992).
The amount of current that flows through each
individual bird is dependent upon the voltage applied
and the electrical impedances of the birds in the brine-
bird resistance ofbroilers ranges between 1,000 to 2,600 V.
More recently, sex differences in resistance were also
reported, with females exhibiting higher resistance than
males(Rawles etal.,1995). Asthebirdsenterandleavethe
stunner cabinet, they constantly change the total
resistance of the system. At a given constant voltage, the
birds receive a current in proportion to their own
resistance. In addition, the resistance provided by the
water or brine solution is also critical and has been shown
to vary under commercial conditions (Bilgili, 1992a).
Commercialstunnersprovidea choice ofalternating or
direct currents, either low or high frequency, half or full
rectified, sine or square waveforms, constant or pulsed
currents (Ingling andKuenzel, 1978; Griffiths and Purcell,
1984; Bilgili, 1992a; and Heath et al., 1994). The effective-
ness of an electrical stunning system is dependent upon
notonly the electrical variables used(i.e.,current,voltage,
waveform, frequency, and duration), but also the biologi-
cal factors that affect bird impedance (i.e., size, weight,
sex, composition, and feather cover) (Kettlewell and
Hallworth, 1990). Woolley (1986a,b) has shown that
individual birds, as well as different tissues within a bird,
vary in their resistance. Given the typical biological
variationobserved amongthebirds withinor between the
flocks processed, it is not surprising that the research and
development of stunning technology have been driven
primarily by defining and standardizing the electrical
variables used in stunning.
Overview of Stunning
and Slaughter Practices
Itis important toemphasizethat stunning, neck cutting
(killing), and bleeding operations are inseparable and
interrelated steps in the slaughter process. The evolution
of electrical stunning technology in modern-day broiler
plants, for the most part, has been driven by other factors
in the slaughter process, such as type of neck cut
performed, bleed time, scalding and picking efficiency,
isimportantto highlight thevaryingcommercial practices
in the stun-slaughter process.
evisceration lines in the U.S. Typically, each kill line
supplies carcasses for two evisceration lines. Depending
upon the inspection system utilized, evisceration line
speeds are limited to either 70 or 91 birds per minute, for
Line Speeds (NELS), respectively. A U.S. plant with four
NELS evisceration lines will typically operate two kill
lines, each at 180 birds per minute, which is in contrast to
European plants,in whicheach evisceration line is served
bya separatekillline, usuallyoperatingat 100to140birds
per minute. The kill line speeds are important in terms of
dwell time in the stunner (i.e., length of the stunner
cabinet), as well as efficiency of kill and bleeding
IntheU.S., the blood vessels withinthe neck of the bird
(both carotid arteries and jugular veins) are severed
usually by a deep ventral cut within 8 to 12 s of stunning.
This methodology is accomplished by automatic neck
cutters and by back-up personnel (Heath et al., 1994). The
ensuing rapid drainage of blood causes anoxia and often
prevents birds from regaining consciousness during the
subsequent80 to 90 sbleedtime.In Europe, the neck cut is
rate of blood loss is slower, the bleed times are usually
blood vessels thatsupply thebrain intact, giving birdsthe
opportunity to regain consciousness when the cut or
bleeding is incomplete (Gregory, 1992). This potential for
regaining consciousness has been the major reason from
the humane standpoint that current levels of 120 to 150
mA per bird have been suggested in Europe to insure an
instantaneous and irreversible stun, i.e., “stun-to-kill”
(Fletcher, 1993). Contrasting with Europe, the electrical
currents used in the U.S., have been traditionally much
lower (25 to 45 mA per bird). Also, the deep bilateral neck
cuts often severe the trachea and cause the heads to come
offinthepickers.Concernovertheremovalofa section of
trachea that is usually left attached to the neck has
prevented many processors in the Europe from using
deep ventral neck cuts in the past.
Recently, new evisceration systems have been ap-
proved in the U.S., wherein the viscera are completely
removed from the carcass during evisceration and
inspection (Bilgili, 1997). These systems operate at
Stork Gamco Inc., Gainesville, GA 30503.
Cantrell-Meyn, Gainesville, GA 30503.
evisceration line speeds of up to 140 birds per minute.
Because the transfer from kill to evisceration shackles is
automated with these new systems, each evisceration line
is suppliedby aseparate killline, operating at similar line
speeds. The two systems currently being used, Nu-tech
both utilize automated kill machines to
severe the neck on one side, leaving the trachea and
esophagus intact. The bleed time is extended to 150 s, as
the rate of blood loss is reduced. Both systems require
specialized head pullers to separate the head together
with trachea and some crops. This separation of head,
crop, and trachea is a crucial step in these total
evisceration systems, whereby the visceral organs are
loosened for later separation by the specialized eviscera-
in Electrical Stunning Systems
There have been significant advances in electrical
stunning systems in the last few years. Development and
implementation of low voltage (10 to 14 V, Pulsed direct
current, 500 Hz, 10 to 12 mA per bird) stunning systems
forbroilershasbeenwellreceivedby the industry(Wayne
Austin, 1997, Simmons Engineering Co., Dallas, GA,
30132, personal communication). This low level of
stunning has been accomplished by significant changes
not only in electrical circuitry, but also in the actual
stunning process. The most significant change has been
ft(1.8 to 4.3m),in anattempt toincreasedwelltimeandto
reduce thetotal resistance in the stunner. The cabinets are
designed with rump-bars to limit the movement of the
birds and to prevent birds from avoiding the brine-bath.
eliminated by elevating a secondary entry ramp. This
secondary in-feed ramp is extended 4 to 5 cm over the
primaryrampto allow quick capture ofbirds at entry into
the brine solution. Breast rub pads are utilized industry-
wide to calm the birds from live-hanging through the
stunning. The feet-shackle contact is sprayed with water
or brine solution to assure current flow. The metal grate at
the bottom of the stunner cabinet is immersed roughly 1
cmin brine solution(1% NaClrecommended) at the entry
end of the cabinet. More importantly, ground bars are
designed to assure continuous and uninterrupted flow of
current through the system. The stunner control panels
have also been redesigned for continuous display and
monitoring of the voltage and current levels.
The popularity of low voltage electrical stunning in the
U.S.is evidentinasurvey publishedbyHeathetal. (1994).
Of the 329 poultry plants surveyed in the U.S., 92.1%
utilized electrical stunning as the method of preslaughter
immobilization. Low voltage (10 to 25 V) and high
frequency (500 Hz) systems were used in 77.4% of these
plants. Suchlow voltageis incontrast to high voltage and
current systems utilized in Europe and other parts of the
By design, all the “communal brine-bath” stunning
systems suffer from the same fundamental constraint in
that many birds are connected to the same circuitry at the
experienced by individual birds cannot be controlled.
Severalattempts have beenmade inrecentyears todesign
“constant current” stunners for broilers. The electrical
circuitry is available to measure the resistance of in-
1995). However, the application of these systems in
commercial slaughter lines has been limited. On the kill
line, the birds are suspended on shackles approximately
15 cm apart. Given the commercial kill line speeds, it is
extremely difficult, if not practically impossible, to isolate
each bird long enough to measure its resistance and
deliver precisely the preset current.
and Product Quality
The quality of end-products, whether whole birds,
parts, or boneless-skinless meat, is of great importance to
the processors. Bruising, discolorations, and broken or
dislocated bones are the primary defects often attributed
tothestunning-bleeding stageofslaughter (Bilgili,1992a).
Application of high voltages during stunning has been
associated with broken bones (Gregory and Wilkins,
1989), exploded or damaged viscera, bruised wing joints,
and red wing tips (Heath, 1984), hemorrhages on the
breast meat (Veerkamp and de Vries, 1983; Veerkamp,
1988), and split wishbones and separation of shoulder
muscle tendons (Sams, 1996).
Under commercial conditions, it is extremely difficult
byother factors,suchas catching, hanging, wing flapping,
typeofcut, efficiency ofbleeding, andpicking (Gregory et
al.,1989; Gregoryand Wilkins, 1993). Although there isno
clear relationship between stunning current and the
traditional whole carcass quality attributes (Griffiths,
1985; Bilgili, 1992b,c), hemorrhages on deep breast
musclesofbroilershave beenshownto increase withhigh
stunningcurrents(Veerkamp,1988;Gregory and Wilkins,
1989). On the other hand, high stunning voltages have
been linked to increased incidences of red wing tips and
tails (Veerkamp and de Vries, 1983) and broken bones
(Walther, 1991). High stunning current frequencies have
been shown to reduce the severity of thigh and breast
hemorrhages, and result in fewer broken bones (Gregory
et al., 1990; Hillebrand et al., 1996). It has been postulated
(Veerkamp, 1992; Bilgili, 1993) that causes of muscle
hemorrhages in broilers were multifactorial and may
(Kranen et al., 1996), the extent of disturbances in blood
circulation created by low rearing temperatures did not
SYMPOSIUM: RECENT ADVANCES IN SLAUGHTER TECHNOLOGY
correlate with occurrence of hemorrhages in breast and
thigh muscles. In the same study, whole-body stunning
caused more severe hemorrhages than head-only stun-
The electrical stunning systems currently being used
in modern-day broiler slaughter plants have evolved in
response to regulatory, as well as technological changes,
in processing technology. Low voltage electrical stun-
ning systems have been an effective method for
immobilizing broilers prior to slaughter in the U.S.
Personal observations in commercial broiler processing
plants indicate that traditional whole carcass defects
often attributed to stunning damage arise only in the
presence of day-to-day operational problems.
With the expansion of further-processed poultry
products in recent years, there has been a renewed
interest on the influence of electrical stunning on end-
product quality. The influence of stunning current
(Craig and Fletcher, 1997) and its relationship to
electrical stimulation during bleeding on early rigor
development and final meat quality attributes have been
the topic of recent research (Lyon et al., 1989; Sams et al.,
1989; Papinaho and Fletcher, 1995a,b; Sams, 1995).
There is no question that stunning technology will
continue to evolve as quality standards for poultry meat
change in parallel to the development of new product
forms and ever increasing consumer expectations.
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