Alan R. Sams
Contents
Introduction . . . 19 Slaughter . . . 20 Unloading . . . 20 Stunning . . . 20 Killing . . . 22 Feather removal . . . 22 Scalding . . . 22 Picking . . . 23 Evisceration . . . 25 Chilling . . . 31 Summary . . . 33 References . . . 34 Selected bibliography . . . 34Introduction
The processing plant is a highly coordinated system of mechanized operations that kill the birds, remove the inedible portions of the carcasses, and package/preserve the edible por- tions of the carcass for distribution to the consumer. The efficiency of processing is largely dependent on the uniformity of the birds, so that each machine can do a repeated move- ment with little or no adjustment between birds. Another important factor is the logistical coordination of carcass flow and production lines so that adequate birds are present to make maximum use of the personnel and equipment. These fixed costs are incurred by the plant regardless of the presence of birds, and therefore need to be paid by the production of poultry meat. This necessitates that every shackle be occupied to produce the maximum amount of product.
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Slaughter
Unloading
After their arrival at the processing plant, the birds are unloaded for processing. The coops of birds are removed from the truck and “dumped” onto a conveyor or placed in a position for them to be manually unloaded. The “dumper” can be a source of carcass damage such as bruising and broken bones, because the birds are allowed to freely fall one or more meters to the conveyor belt below. Minimizing this distance can reduce the damage. Manual unloading can also cause carcass damage if the birds are handled roughly. Proper training and supervision are critical to minimizing damage. When the birds are manually unloaded from the coops, they are usually directly hung on a shackle and not placed on a separate conveyor belt. Because of bird size and numbers, dumpers have become the industry norm in the U.S., with manual unloading still occurring in some other parts of the world. Because of their large size and poor body control, turkeys are usually still unloaded manually worldwide.
The ergonomics and safety of the unloading process has become an issue in recent years. Coops and/or workers are on platforms of adjustable heights, maintaining the birds at an optimal position to minimize the bending and lifting required by the worker. The industry has determined that such ergonomic innovations can yield benefits from reduced medical claims and better worker performance/retention. Proper ventilation is also impor- tant in the unloading and hanging areas to further improve worker welfare. These are par- ticularly dusty areas, and respiratory health of the workers can be a concern. The hanging areas have traditionally been dark, lit only with “black lights” or dim red lights. This dark- ening was thought to calm the birds, reducing their struggle against hanging and thereby reducing the damage to their bodies during handling. However, processing companies are increasingly realizing that this benefit may not be as great as once thought, and that improving the working environment with brighter lighting and ventilation more than off- sets any increase in carcass damage.
Stunning
The first step in humane slaughter is “stunning” to render the bird unconscious prior to killing. Several methods have been developed to accomplish this goal. The most common and one of the simplest is electric shock. While hanging by their feet, the heads of the birds contact a saline solution (approximately 1% NaCl) that is charged so that an electrical cur- rent flows through the bird to the shackle line which serves as the ground (Figure 3.1). A proper electrical stun will produce about 60 to 90 sec of unconsciousness during which the bird is unable to stand or right itself when removed from the shackle and placed on the floor. This is a suggested method of evaluating the effectiveness of the stun. Immediately after contact, the legs are extended, the wings are tight against the body, and the neck is arched. Several seconds after leaving the stunner contact, the bird’s posture relaxes and the body becomes almost limp. In addition to humane slaughter, there are other benefits to be gained from proper stunning, such as immobilization for improved killing machine effi- ciency, more complete blood loss, and better feather removal during picking. Inadequate stunning can result in carcass defects such as incomplete bleeding, while excessive stun- ning can cause quality defects such as broken clavicles (wishbones) and hemorrhages from ruptured arteries and capillaries. Some commercial poultry is not stunned because some cultures specifically prohibit preslaughter stunning, requiring the birds to be conscious when slaughtered (see Chapter 17).
There are different conditions used for electrical stunning, depending on the region of the world. Although poultry is not required by law to be stunned before slaughter in the U.S., virtually all commercial poultry is stunned for humane, efficiency, and quality rea- sons. The birds receive 10–20 mA per broiler and 20–40 mA per turkey for 10 to 12 seconds. These conditions yield adequate time of unconsciousness for the neck to be cut and suffi- cient blood to be lost so as to kill the bird before it regains consciousness. In most European countries, laws require poultry to be stunned, and with much higher amperages (90 mA per broiler and 100 mA per turkeys for 4–6 seconds). These laws and high amperages are intended for humane treatment to ensure that the birds are irreversibly stunned so that there is no chance they will be able to recover and sense any discomfort. Essentially, these European electrical stunning conditions kill the bird by electrocution and cardiac arrest, stopping blood flow to the brain. Thus, death is by loss of blood supply to the brain for both stunning conditions, but one is by removal of blood and the other is by stopping blood flow to the brain. The harsher European electrical conditions also result in higher incidences of hemorrhaging and broken bones.1, 2
Other methods of stunning have been developed to replace electrical stunning in areas such as Europe, which require the higher electrical conditions. Exposing the birds to gases to induce either anesthesia or anoxia is one method in commercial use. Carbon dioxide is an anesthetic gas used to induce rapid unconsciousness by altering the pH of the cereb- rospinal fluid.3Argon and nitrogen are inert gases that displace the air and cause uncon- sciousness through lack of oxygen.4, 5There are two main types of gas stunning systems for poultry. First, systems using mixtures of carbon dioxide (10 to 40%) and air (60 to 90%) are shorter duration (30 to 45 sec) and intended to render the bird unconscious but alive for the killing machine. Systems using mixtures of argon (55 to 70%), nitrogen (0 to 15%), and car- bon dioxide (30%) are longer duration (2 to 3 min) and intended to render the bird dead at the time of neck cutting. Thus, the carbon dioxide systems would be most analogous with the low amperage, reversible electrical stun, while the argon stun would be most analogous with the higher amperage, irreversible electrical stunning. However, both gas stunning procedures reduce carcass damage relative to the high amperage but not the low amperage electrical stunning.2This is because the low amperage electrical stunning has an equally low incidence of carcass damage. An additional note on gas stunning is that these birds are flaccid on the shackles when entering the killing machine. This differs from the stiffer, elec- trically stunned bird and must be accommodated with minor machine adjustments for bird orientation.
Figure 3.1 Electrical stunner cabinet containing an electrode covered by a saline solution. Bird movement is from left to right.
Another stunning system that has received attention for reasons of humane animal treatment is captive bolt stunning.6In this stunning method, the head is immobilized and a metal pin or probe is shot into the skull and brain causing immediate and irreversible unconsciousness. The humane and carcass quality effects of this method are still under investigation.
Killing
Within seconds after stunning, the shackle conveyor moves the bird to the killing machine (Figure 3.2). A series of rotating bars grab the wattles and lower neck skin to hold and guide the head into the machine for proper presentation to the cutting blade. The killing machine uses a rotating circular blade to cut the jugular veins and carotid arteries on one or both sides of the neck of the bird. Most killing machines cut both sets of blood vessels by rota- ting the head from the bird’s left to right as it passes over the cutting blade. If the cut is too deep and the spinal nerve cord is cut, the resulting nervous stimulation “sets” the feathers and makes picking more difficult. Conversely, if the cut is too shallow, there will be insuf- ficient bleeding and the residual blood will cause engorged vessels and can discolor the skin. Once the neck has been cut, the bird is allowed to bleed for 2–3 minutes. During this period the bird loses about 30 to 50% of its blood, which eventually causes brain failure and death. If the blood loss is insufficient to cause death or if the neck cut is missed altogether, the bird may be still alive at the end of the bleeding period when it enters the scalder. In this case, the blood rushes to the skin surface in response to the scald water heat, imparting a bright red color to the carcass.