Stunning is a key operation in the processing plant and has a significant impact on the quality of processed birds. To ensure that stunning is performed optimally, there are two parameters that need be carefully controlled: the number of properly stunned birds exiting the stunning bath; and, that there is only minimal residual blood in the wings, thighs and breast of stunned birds.

Monitoring should take place daily, and there are certain operations that need particular attention. These include: the time that broilers are kept in the receiving area; hanging; stunning; slaughter; and, bleed out.

Waiting at the plant

Chickens arriving from the farm should rest, and under normal circumstances this should be for 30 minutes to a maximum of two hours. This will reduce the stress experienced by the birds during transport and unloading, which can lead to blood accumulating in the wings and thighs.

Hanging on the overhead conveyor

The hanging area should be lit with blue, red or green light to create a peaceful environment. Birds should be handled with the utmost care, irrespective or processing speed, to not stress them. Should they become stressed, then blood in the extremities will increase.

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The correct lighting will help to keep stress levels down as birds are hung on the shackles.

Prior to bird hanging, the shackles should be wetted to facilitate electric current flow. When birds are lifted into the shackles, they should be held by the lower leg instead of the thigh, as this can result in bruising.

Legs must be correctly placed into the shackles to ensure the maximum electricity flow, and it should be remembered that birds with cuts to their legs or feet should not be allowed to pass through the stunning bath, as the electric current will not flow properly.

If birds are hung roughly, then not only can this damage the thighs, but it will result in intense wing flapping. It is as important that birds are placed into the shackles properly, and that two legs are not placed into the same support as this will also impact quality.

The overhead conveyor must be fully loaded with birds. Should gaps occur, then birds entering the bath will receive additional current, reducing the quality of the processed bird.

Passage to the stunner

The passage from the hanging area to the stunning area should be as straight as possible. Any curves will disorient the birds, raising stress levels.

The breast comforter should be made from smooth materials and solidly constructed to avoid vibrations, as these also increase stress levels. If the comforter does not properly correspond to the birds’ size, they will again flap, and more blood will be forced into the wings.

At the stunner

Birds should enter the stunning bath as they do the scalder. They should never touch the equipment, with the head entering the water first. This will help to prevent pre-shock, which occurs when a part other than the head touches the electrified water. This can also lead to wing flapping and lifting the head away from the bath, meaning that birds exit the stunning area without being stunned.  Flapping against the equipment and other birds can cause damage both internally and externally to their wings.

When birds enter the stunning bath, the shackles must be kept in constant contact with the metal guides. This ensures that the circuit is closed, and that the current exits through the birds’ legs. If this contact is not continuous, then birds will exit the stun bath still awake.

Birds should be submerged no deeper than the halfway point of their necks. Should the water reach the upper level of the breast, a series of contractions in the pectoral muscles can occur, fracturing the small thorax bones. If this happens, the capillaries will be cut, leak blood, and hemorrhages will be formed around the broken bones.

Birds should remain in the bath for 10-12 seconds. Less time will require more voltage and amperage, while more means that chickens will receive more current. In both cases, the quality of the processed birds can be compromised.

The metal grid that is submerged in the water should be placed in such a way as to never be closer to the submerged birds’ heads than 5 centimeters. This helps to ensure that current is evenly distributed in the water, so aiding stun effectiveness.

Electrical conductivity in the bath will vary due to the water’s mineral content. To improve conductivity, common salt can be added up to 0.1 percent at the start of each shift. It is worth remembering that dirt and fecal matter accumulating in the bath will also aid in current flow due to their mineral content.

The stunning bath exit must be kept free from obstructions as, if not, birds may be exposed to additional electric current.

Stunning effectiveness should be more than 95 percent. It will never reach 100 percent, due to the fact that it will set to handle average weights.

Two stages will be observable for a chicken that has been stunned well. The tonic stage is when shaking occurs, and to reduce this, a breast comforter can be installed. Second is the clonic stage, where chickens are completely relaxed. On occasion, however, reflex actions may be seen, but this is normal.

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A special breast comforter to reduce any shaking can be installed at the stunner exit.

Physical characteristics of a well-stunned birds include relaxed thighs, wings kept close the body, a slightly arched neck, brilliant eyes, and total relaxation.

Given that chickens’ necks are slightly curved, in many processing plants, an electrically charged stainless steel sheet is placed at the bath exit. This extends the period during which the bird is stunned, helping entrance into the guides leading to the slaughter equipment.

Getting stunning right is one of the most important challenges in processing plants, and paying attention to the points raised above can help to reduce carcass quality problems.