Br. vel .
3. (1991) . 147, 7 1
EFFECT OF ELECTRICAL STUNNING FREQUENCY ON VENTRICULAR FIBRILLATION, DOWNGRADING AND BROKEN BONES IN BROILERS, HENS AND QUAILS
N . G . GREGORY, L. J . WILKINS and S. B . WOTTON Department of Meat Animal Science, University of Bristol, Langford, Bristol BS18 7DY
SUMMARY The effect of stunning current frequency on the incidence of ventricular fibrillation was determined in broilers and hens . When a pulsed unipolar square wave (DC) was used the incidence of ventricular fibrillation decreased as the frequency was increased beyond 125 Hz . When a sinusoidal AC was used, 50 Hz killed some of the birds whereas at the corresponding r .m .s . currents no birds experienced a ventricular fibrillation with 1500 Hz . In quail, it was found that between 45 and 110 mA (50 Hz) all birds experienced a ventricular fibrillation . The effect of 50, 200 and 350 Hz square wave DC on carcass downgrading was examined in broilers, and there were no differences between the groups . It was concluded that there were few advantages from using high frequency stunning currents in poultry .
INTRODUCTION There is growing interest in the poultry processing industry in using high frequency stunning currents . If their popularity increases it will be important to provide a recommendation for the minimum current which ensures a humane kill and acceptable carcass quality . Since there are welfare advantages in achieving a ventricular fibrillation at stunning (Gregory & Wotton, 1986 ; Gregory, 1989), this study set out to examine the effect of frequency on the incidence of ventricular fibrillation in chickens . In quail, the neck is not usually cut at slaughter and instead the carcass is processed as if it were a game animal, and so it is essential that all the birds have a ventricular fibrillation at stunning . This paper also examines the effect of current and frequency on the incidence of ventricular fibrillation in this species . High frequency currents tend to flow close to the surface of a conductor instead of through its middle (Kennelly & Alexanderson, 1910) . In view of this, high frequency stunning currents might be expected to produce a milder tonic muscular spasm at the onset of current flow . The duration of the initial tonic phase can be reduced in chickens when frequencies above 75 Hz are used (Vernadakis & Burkhalter, 1965), but there is little information on whether the reduced spasm results in less muscle haemorrhaging or a better quality carcass . Kuenzel (1978) and Kuenzel et al. (1978) reported that bleeding
BRITISH VETERINARY JOURNAL, 147, 1
efficiency declined when the frequency of a pulsed DC square waveform was increased beyond 240 Hz . Gregory et al. (1990) found that there were fewer broken bones when hens were stunned with a 1500 Hz sinusoidal AC in comparison with a 50 Hz sinusoidal AC . This present study makes a further contribution to this subject by examining the effect of frequency on carcass haemorrhages and broken bones .
MATERIALS AND METHODS
In the first experiment the effect of stunning frequency on cardiac function and carcass downgrading in broilers was examined . In the second and third experiments, the effects of frequency and current on cardiac function in hens and quails were determined . The different stunning frequencies were generated by two commercially manufactured stunners .
Broiler experiment Eight hundred broilers (479 Ross and 321 La Belle Rouge) were subjected to waterbath stunning using a Linco BA4 (Lindholst, Denmark) control box . Current flow lasted for 4-5 s and the waveform of the current was a pulsed unipolar square wave (DC) bearing a sinusoidal ripple at peak voltage (Fig . 1) . The frequencies examined were 50, 125, 200, 275 and 350 Hz and there were between 159 and 161 birds in each treatment . The voltage at each frequency was adjusted so that approximately the same r .m .s . (root mean square) current was used in each treatment . The r.m .s . current was recorded for each bird using a Fluke 8060A True r.m .s . meter (Fluke Holland BV), and it equalled the square root of the sum of the squares of the AC and DC components .
Fig. 1 .
Voltage waveform used in stunning the broilers and quail .
The birds were bled within 25 s of stunning by a manual neck cut, and during the initial part of the bleeding period the electrocardiogram (ECG) was recorded using a Siemens Mingograf 34 to determine whether the heart had been put into ventricular fibrillation . After bleeding for 170 s the carcasses were scalded and plucked using a Cope & Cope Ltd processing line, and allowed to cool for 4-5 h before placing in a chiller (2°C) for overnight storage .
ELECTRICAL STUNNING OF BIRDS
The uneviscerated carcasses from the Ross broilers which had been subjected to 50, 200 and 350 Hz electrical stunning were graded for carcass blemishes at 24 h after slaughter . The external appearance of these carcasses was assessed on a 0, 1, 2 subjective scoring scale for red pygostyles, red wing tips, wing haemorrhaging (on the medial surface of the wing), shoulder haemorrhaging and red breast feather tracts . An increasing score corresponded to a worsening of the condition, with 0 indicating a carcass that would be acceptable for that trait in the fresh whole bird or portion market . A 1 indicated that, the carcass would have been downgraded from those markets with respect to that particular condition and a 2 would require either trimming or rejection . The internal appearance of the carcasses was assessed after the skin had been removed by hand . The carcasses were scored for superficial haemorrhages on the legs and breasts . The breasts were then removed with a knife and incisions made to allow inspection for deep muscle haemorrhages ; a common site for these was the ventral aspect of the pectoralis minor muscle . The legs were inspected in a similar manner except that the incisions were made whilst the muscles were attached to the bones . The scapula, coracoid and furculum bones were graded for complete fractures . A 0 represented no fractures ; 1 indicated that the bone on one side of the skeleton was broken and 2 indicated that both the left and right bones were broken . Hens experiment Seven hundred and seventy end of lay hens were subjected to waterbath stunning with either a 50 Hz sinusoidal AC or a 1500 Hz sinusoidal AC (Bristol Abattoir Ltd, UK) . The ECG was recorded immediately after the stun in each bird to determine whether the current had induced a ventricular fibrillation . The r.m .s . current was recorded for each bird . Quail experiment This experiment was mainly concerned with the effect of current at 50 Hz on the incidence of ventricular fibrillation . One hundred and fifty-four quail were subjected to waterbath stunning with either a 50 Hz sinusoidal AC constant current or a Linco BA4 stunner generating a DC 200 Hz square wave (Fig . 1) . The ECG was recorded immediately after the stun in each bird to determine whether the current had induced a ventricular fibrillation . The r.m .s . current was recorded for each bird . RESULTS Broiler experiment The mean r .m .s. stunning current was approximately 112 mA per bird, and there was little difference in the currents between each frequency treatment (Table I) . In general the incidence of ventricular fibrillation declined with frequency, and beyond 125 Hz over 50% of the birds survived the current . In the birds which were subsequently used for the appraisal of carcass downgrading, the incidence of ventricular fibrillation decreased from 71% at 50 Hz to 9% at 350 Hz (Table II) . In spite of this there were no obvious effects on any of the carcass quality features, and there was little variation in the incidence of downgrading between the three frequencies . However, since the overall level of downgrading for haemorrhages in the wings, shoulder, sternal feather tracts and pygostyle were low, little improvement with the higher frequencies could be expected .
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ELECTRICAL STUNNING OF BIRDS
With the 50 Hz current the incidence of ventricular fibrillation increased with increasing current (Table III) . The overall range in the currents that were used was 17189 mA per bird . With the 1500 Hz current none of the birds fibrillated over the current range 13-107 mA per bird . Table III Effect of stunning frequency and current on ventricular fibrillation in hens Frequency 50 Hz Current (mA per bird) 0-40 41-60 61-110 > 110
Number of birds
Number of birds
60 133 177 149
2 20 65 99
79 89 83
0 0 0
All the birds fibrillated when the 50 Hz sinusoidal AC was used (Table IV) . Even with currents as low as 45 mA per bird there was 100% fibrillation . With the 200 Hz square wave, all the birds were fibrillated at 50 mA per bird, but with currents less than this there was a sharp decline in the proportion of birds that were fibrillated .
Table IV Effect of stunning current and frequency on ventricular fibrillation in quail Current (mA per bird) 50 Hz sinusoidal wave 45 60 75 90 110
Number of birds
25 25 25 25 25
100 100 100 100 100
200 Hz square wave
BRITISH VETERINARY JOURNAL, 147, 1
On welfare grounds it is desirable to induce a fibrillation at stunning, particularly where no attempt is made at cutting both carotid arteries during the killing procedure (Gregory & Wotton, 1986) . In the absence of inducing a ventricular fibrillation or cutting both carotid arteries, the time to brain failure is unduly long and there is a danger that the birds might regain consciousness before they die (Gregory & Wotton, 1990) . In this study it was found that increasing the frequency of a square wave DC stunning current beyond 125 Hz resulted in a sharp decline in the incidence of ventricular fibrillation in broilers . In addition, a 1500 Hz sinusoidal AC failed to induce a ventricular fibrillation in hens, and field trials in the UK have shown that a 500 Hz alternating square wave was also associated with a low incidence of ventricular fibrillation in broilers (unpublished observations) . None of these alternative frequencies, therefore, is associated with a high incidence of ventricular fibrillation at stunning . The preliminary conclusion is that it would be inadvisable to use these alternative frequencies where neck cutting does not take in both carotid arteries . If, however, the high frequencies were found to produce a particularly long lasting stun, then they would be acceptable from the welfare point of view . There are two manufacturers of high frequency poultry stunning equipment which have an international standing . Both companies claim that high frequencies lead to better bleeding and better quality carcasses . Experimental work from the USA has shown that increasing the frequency of a square wave DC from 30 to 240 Hz resulted in an improvement in bleeding efficiency, and that this waveform gave better bleeding than sinusoidal AC waveforms (Kuenzel & Ingling, 1977 ; Kuenzel, 1978) . The present study, however, did not confirm that increasing frequency was associated with a corresponding reduction in the incidence of haemorrhages in the carcasses . Such haemorrhages often arise when badly bled carcasses pass through the pluckers (Gregory & Wilkins, 1990a) . The conclusion, therefore, is that there were no obvious carcass quality advantages from increasing the frequency of a DC square wave between the limits of 50 and 350 Hz . Unlike a previous study in hens where the incidence of broken bones was lower when a 1500 Hz sinusoidal AC was used in comparison with a 50 Hz sinusoidal AC, there was no effect of frequency (when using 50-350 Hz DC square wave) on the incidence of broken pectoral bones, which are the bones that are most often damaged by stunning (Gregory & Wilkins, 1990b) . Carcass haemorrhaging could not be attributed to the presence of a ventricular fibrillation at stunning, since the majority of the birds receiving 350 Hz failed to develop a fibrillation whereas over 70% of those experiencing 50 Hz were fibrillated, and yet the incidence of haemorrhaging was the same in both treatments . The broilers in this study showed a high incidence of deep breast and leg muscle haemorrhaging and a lower incidence of skin haemorrhaging in comparison with an earlier study where the same r.m .s . current was used but with a 50 Hz sinusoidal AC instead of a 50 Hz square wave DC (Gregory & Wilkins, 1989a) . The reason for the difference is not clear, and further investigations would be required before any weight could be placed on this finding . Quail are usually treated as a game species when they are slaughtered, that is, they are processed without being bled by cutting the neck . It is important that quail experience a cardiac arrest at stunning if they are not to regain consciousness during subsequent processing . In this study it was found that this species is very susceptible to a ventricular
ELECTRICAL STUNNING OF BIRDS
fibrillation at stunning . Currents as low as 45 mA were lethal in all birds, whereas in broilers and turkeys approximately 150 mA per bird is needed to cause fibrillation (Gregory & Wotton, 1987 ; Gregory & Wilkins, 1989b) . The likely reason for the quail's susceptibility is its small size and hence higher current density . Unlike the larger species the heart would be exposed to a higher proportion of the applied current . The implication is that it is not necessary to recommend a minimum current for quail on the basis of inducing a fibrillation . Instead the minimum current need only be based on the level which guarantees an effective stun . The overall impression from the experiments in this and previous studies is that there are few if any welfare or commercial advantages from using high frequency currents when stunning poultry . In fact the higher frequencies have the welfare disadvantage of inducing a lower incidence of ventricular fibrillation, thus introducing a greater need for prompt neck cutting which severs both carotid arteries .
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