Components & principle of function of an electrical waterbath

Every year millions of birds are reared for food for humans and the slaughter of these birds must be carried out in a way that causes no avoidable pain or suffering. Several systems have been developed to facilitate the humane stunning of poultry. The main principle of these methods is to stun each bird so that it becomes unconscious and therefore insensible to pain; this condition must persist until the bird is dead.

Large-scale abattoirs stun poultry using either electrical waterbath or controlled atmosphere systems. Electrical waterbaths are the most commonly used commercial stunning method. Birds are unloaded from their transport containers, inverted and hung by both legs, at the shank, onto a moving shackle line which conveys them to an electrical waterbath (Figure 1). An electric potential difference must be generated across the circuit to produce a steady flow of current that overcomes the total resistance, including that of the birds. In a conventional system, the electrode submerged in the water is maintained at a higher electric potential than the earthed rubbing bar. When the head of a bird enters the electrified water, the electrical circuit is completed and the electric potential difference causes the electrons, and therefore the current, to flow from the submerged electrode in the waterbath up through the water and through the head of the bird, through its body and legs, to the metal shackle in which the bird is restrained and finally up into the earthed rubbing bar.

The aim of electrical stunning is to pass sufficient current through the brain in order to disrupt its normal function and immediately render the animal unconscious (known as electronarcosis) and insensible to pain until death supervenes. The electrical parameters (voltage, current, frequency and waveform) of a waterbath system can be set to either stun or stun-kill the birds. A bird may be electrically stun-killed by applying a current at a frequency and amplitude that causes unconsciousness and simultaneously stimulates cardiac muscle into ventricular fibrillation and causes death by cardiac arrest. Alternatively, a bird can be electrically stunned and then killed by exsanguination (blood loss due to severance of the major blood vessels between the heart and the brain). No matter whether a system is intended to achieve a stun or a stun-kill, as soon as possible after they exit a waterbath, all stunned birds should have their two common carotid arteries severed, preferably by using a ventral neck cut to sever all the major blood vessels ventral to the spine and to enable easy post-cut verification of which blood vessels are severed. Prompt and accurate neck cutting will benefit animal welfare and meat quality.

Despite the increasing complexity and highly-automated operation of some stunning and killing equipment, it remains the responsibility of the operator to ensure that every bird is humanely stunned and killed. Humane electrical stunning of animals requires a sound understanding of electrical parameters, their influence on an animal’s brain and how to deliver those parameters efficiently. Effective waterbath stunning depends on the control and management of several elements in order to maximise bird welfare. The welfare of poultry is directly affected by many variables including the waveform and frequency of an electric current, the amount (amplitude or magnitude) of current applied to each individual bird, the optimisation of the flow of electrical current through each bird and the time, and quality, of neck cutting.

Next: A note of caution about conventional electrical waterbaths