MEAT - SLAUGHTERING OPERATIONS AND EQUIPMENTS

Introduction

Slaughtering, the first step in the transformation of muscle into edible meat, affects the quality and quantity of meat. This depends on postmortem biochemical changes, related to sanitation, as well as physicochemical and physical attributes (aroma, color, and texture among others). From the quantity point of view, carcass yield is related to pre- and postmortem handling. However, carcass handling after leaving the slaughter floor, also alters meat quality. A number of operations are included during transformation of muscle into meat, starting from slaughtering through refrigeration, wholesale storage, fabrication and distribution, and retail storage. Each one includes changes in physicochemical and microbial characteristics that finally affect quality. In any case, changes must be directed to improve quality and/or to extend shelf life.

Preslaughter Conditions Affecting Meat Quality

The main factors that affect meat quality are preslaughter stress and halothane genotype. Stress is the inevitable consequence of processes of transferring animals from farm to slaughter. The preslaughter phase includes the conditions and practices that apply during the period when the animals are moved. During this time, animals can be exposed to a range of challenging stimuli: handling and increased human contact, transport, unfamiliar environments, food and water deprivation, changes in social structure, and so on (Ferguson and Warner 2008). In glycolytic muscles, such as Longissimus dorsi, high physical and psychological stress levels associated with the immediate preslaughter period have a greater effect on meat water-holding capacity (WHC) and may promote pale, soft, and exudative development (Hambrecht and others 2004, 2005). These authors studied the physiological changes elicited in response to stress in immediate preslaughter periods and link them to pork quality characteristics. They concluded that a high preslaughter stress leads to impaired pork quality, with high muscle energy levels aggravating the negative effects of preslaughter stress. Monitoring stress level by blood lactate measurement in combination with strategies to control muscle energy during slaughtering may help improve meat quality. An increase in blood lactate concentration is associated with preslaughter stress, such as aggressive handling immediately prior to stunning; these actions show detrimental effects on pork quality and can be used as an indicator of differences in the overall handling experience of individual pigs (Edwards and others 2010).
A number of reports describe the modifications induced in horse behavior by commercial transports, all data underline that the response to stressors is related to type and length of the transport. Micera and others (2010) concluded that plasma cortisol, epinephrine, norepinephrine, and β-endorphin are main stress indicators in horses, suggesting that, under stress, the release of β-endorphin could be different from that of ACTH. There was no adverse effects in rabbits with respect to the loading method, the stress condition evidenced by hematological and biochemical parameters prior to slaughter, did not affect meat quality (Mazzone and others 2010).
Young and others (2009) found that only 1 h of rest after stress by exercise was enough to reduce these effects, but interesting overshooting effect was noted when pigs were allowed to rest for 1–3 h before slaughter. They concluded that quality characteristics such as drip loss and toughness are not related to stress by exercise in pigs. Bertram and others (2010) applied nuclear magnetic resonance to study the effect of preslaughter exercise stress on plasma metabolite profile at the time of slaughter. It resulted in highly correlated muscle temperature until 1 min, mainly due to WHC. Lactate was found to be the most important metabolite in correlation to plasma metabolome and pH, temperature, and WHC.
Texture is one of the most important components of meat quality. In pigs, halothane susceptibility has been repeatedly shown to negatively affect tenderness. Monin and others (1999) studied the effect of three halothane genotype (NN, Nn, and nn HAL), slaughter weight, and the HAL × slaughter weight interaction. The nn allele of HAL gene unfavorably affects pork texture, this effect was maintained throughout for body weight between 101 and 127 kg. However, several methods have been developed to improve tenderness, such as electrical stimulation. It was originally developed to avoid cold shortening and toughening. The effect of high-voltage stimulation on the tenderness of steers and heifers with a range of known B. indicus and B. taurus genotypes enabled the contribution to the genotypes to final meat tenderness (Gursansky and others 2010).

Stunning

Stunning is the ability loss to experience physical experiences due to a mechanical, chemical, or electrical process that temporarily interrupts sensory perception (Rosmini 2006). In animals, stunning is applied in order to establish an unconscious state (Gregory and others 2009) and, in this way, to ensure it will not recover before drying by bleeding. At the same time, stunning ensures animal immobilization to avoid any damage to the operators during handling prior to the animal’s death. The stunning methods must fulfill several characteristics (Rosmini 2006):
• Not affecting the meat shelf life, therefore the method must allow rapid bleeding avoiding contamination of internal tissues.
• Not affecting meat quality regarding its physical, chemical, and sanitary characteristics.
• To stun the animal preventing any unnecessary pain and suffering, but without causing its death.
• Easy to apply, requiring the minimum effort to the operator and avoiding difficulties in the use of stunning devices.
• Producing a rapid and efficient effect, adapting to the processing line and avoiding process repetitions.
• With no risks to the operator, either due to the direct application or to the animal’s defensive movements.
Although sanitary legislations require animal stunning before slaughtering, in almost all countries several exceptions are considered due to religious or ethnic reasons. These are the cases of kosher (Regestein and Regestein 2010) and halal (Che Man and Qurni Sazili 2010) food-processing methods. Slaughtering without stunning is recognized as a religious prerogative, but there have been concerns that the method can comprise the welfare of the animals due to the time it takes to collapse, or for the prevalence of blood in the respiratory tract (Gregory and others 2009, 2010).
Stunning methods can be divided into three groups, according to the principle the main device uses: electricity, contusion, and gas narcosis.

Electric Stunning

Electricity promotes unconsciousness due to an epileptic shock caused by massive neurotransmitters (glutamate and aspartate) discharge to the brain; when efficiently applied, it presents three phases: tonic, clonic, and recovery (McKinestry and Anil 2004). Electric shock stunning is usually applied to pigs and sheep, although in some cases is also used in cattle. The epileptic shock depends on the voltage intensity passing through the brain. If electricity goes through the brain, the animal is only stunned, and bleeding must be carried out within 30 s because the animal can recover consciousness.
The main advantage of electrical stunning is its high efficiency to cause unconsciousness when properly applied according to electric discharge, electrode placing, time, among others. In addition, it is noiseless therefore the animal is usually not affected by equipment vibrations. However, electric stunning causes hemorrhages more frequently than other stunning methods (Channon and others 2003). These are due to the rupture of small blood vessels due to muscle constriction and increase in blood pressure (Lawrie 1998) or to severe myofibrillar contraction (Channon and others 2002). Most large processing plants apply electric stunning restraining the animal in a confined area. The stunning area must be isolated and kept dry and the operator must wear rubber boots and stand on insulated ground.

Contusion Methods

The principle underlying the use of these equipments is the trauma caused in the brain by a blow or by penetrating the skull, causing total or partial unconsciousness. They include captive-bolt and pneumatic pistols. Stunning with a captive bolt pistol consists in projecting a metallic cylinder into the animal’s skull, this penetration is combined with a blow caused by the pistol against the animal’s head; the cylinder is then returned to their original position for the next shot. However, in order to achieve an efficient stunning, the anatomical location where the pistol is placed must be carefully chosen. In calves, swine, horses, and cattle (Bos taurus), the muzzle is applied to the forehead. In Cebu cattle (Bos indicus), sheep and in plants industrializing the brains, it is applied at the back of the head, due to thickness of the forehead in Bos indicus and sheep, or to avoid brain destruction (De la Puente 2002, pers. commun).
Pneumatic pistols impact the animal’s head without penetration, using compressed air at 160–180 psi; the blow permanently damages the brain cortex. Captive-bolt pistols are used in sheep and cattle; when this stunning method is used in pigs, the muzzle is applied just above the eye level, in the center of the forehead. The main problem in using pneumatic pistols is that if compressed air is too high it could harm the operator’s hands, back, or arms.


Carbon Dioxide

It was developed specially for pigs although it is also applied to sheep; the stunning action of carbon dioxide (CO2) is accomplished by blockade of the animal’s neural terminals, reducing the nervous impulses. CO2 concentrations of 65–70% during 45 s work most efficiently (Velazco 2000). CO2 stunning reduced hemorrhage incidence in the carcass and risks to the operators as animals defensive movements are also reduced (Rosmini 2006). It is reported that meat quality is improved (Channon and others 2003). However, Becerril-Herrera and others (2009), comparing the effect of two different stunning methods, electrical stunning and to exposure to CO2, concluded that CO2 stunning leads to a major imbalance because of mineral and acid–base gaseous interchange, compared to electric stunning, thus possibly compromising animal welfare. The main disadvantage of this method is that facilities building and maintenance costs are very high. Gas application must be carefully controlled; if the gas concentration is low, the animals are not adequately stunned. If it is too high, there is a tendency for stiffness, reflex muscular activity, and inadequate bleeding (Grandin 1991). If time of exposure is too long, skin congestion can occur and the carcass can take a bluish hue after scalding. CO2 is heavier than air, so all devices operate on the basis of keeping the gas at low levels, making the animals descend to an area where suitable gas concentrations are fed, such as the oval tunnel (for slaughtering 120–240 animals/h). Vergara and others (2009) studied the effect of different gas stunning methods on light lamb meat quality in order to define the best CO2 concentration (80% and 90% volume in air) and exposure time (60 and 90 s) to maximize the meat quality. The authors concluded that there were no significant differences among treatments with respect to color, a minimum of 90% CO2 for 90 or 60 s was recommended; lower CO2 concentrations (80%) could cause meat hardening.

Bleeding

During unconsciousness, the animal dies by massive bleeding. There are a number of reasons to promote the animal’s death during this period, the main one is for humane considerations. In addition, biochemical reactions proceed to obtain the optimum meat quality when the animal is properly stunned and slaughtered. When stunned, the animal is still alive; therefore, bleeding is achieved when the heart and respiratory functions are still working. Bleeding is carried out by sectioning the carotid arteries and the jugular vein with a sticking knife (15–25 cm long). The point of the knife is inserted about 2 cm in front of the breastbone, an incision is made toward the jaw, penetrating 12–15 cm and sectioning the carotid artery and jugular veins. Care must be taken when introducing the knife to avoid puncturing the pleura, as this could cause the blood to flow into the chest cavity, increasing the risk of microbial contamination. In sheep, the sticking knife is inserted immediately below and behind the ear, severing the jugular vein. In beef, an incision is made just in front of the sternum, also cutting into the main blood vessels (De la Puente 1996). Because blood is a vehicle for microbial distribution throughout the animal body, microorganisms introduced during sticking can be found after a few hours in other parts of the carcass.
Dikeman and others (2003) studied the effects of postexsanguination vascular infusion of cattle with a solution of saccharides, sodium chloride, and phosphates or with calcium chloride on quality and sensory traits of steaks and ground beef, there were no distinct meat quality advantages for infusing cattle with a solution of saccharides, sodium chloride, and phosphates. Infusion with 0.3 M CaCl2 increased dressing percentage, but caused severe muscle contraction during early postmortem, markedly decreased Longissimus lumborum tenderness and reduced flavor.
Room temperature must be reduced to 10°C during slaughtering, bleeding, evisceration, and cleaning, as carcass temperature affects microbial growth. However, bleeding also reduces microbial contamination
to a large extent. It also prevents the formation of “blood-splash” due to pressure built within the muscles, decreasing meat acceptability, and increasing hygienic risk (Palumbo and others 1996).
Blood is also of economic importance as it can be transformed into meal for animal consumption or used in pharmaceutical applications. Extracted blood, on an average, is 3.8–4% of live weight or 6.6–8% of carcass weight, although these percentages vary widely depending on the species: cattle (5–6%), horses (9–10%), pigs (3–3.5%), sheep (7%), and goats (8%) (Rosmini 2006). It is carried out in curbed-in areas with enough space to prevent blood from splashing on stunned animals or on carcasses; the carcass is hung in a rail head down to allow maximum blood drain. Rails 4.8 m high above the floor should be used for beef bleeding; 3.3 m for pig, sheep, and calf; 2.74 m for goats (Guerrero-Legarreta and Pérez-Chabela 2001). Bleeding must be carried within a minimum time from stunning to take advantage of the stunning effect and, at the same time, to avoid hormones freed by stress to distribute throughout the animal’s body, affecting meat quality (Rosmini 2006).

Dressing, Evisceration, and Cleaning

After bleeding, the carcass undergoes cleaning operations by various methods, depending on the species. Cattle, sheep, horse, and goat hides are removed; viscera are also removed from the carcass of all species, and finally the carcass is trimmed and washed with pressurized water. The head, feet, and tail are left attached to the pig carcass, whereas head and hide are removed from beef carcasses.

Dehiding

Beef carcass dressing starts with dehiding making and incision on the neck centerline toward the head. The animal is then scalped, the horns chopped, and the head is skinned out. A cut is made across the larynx and the head is detached by cutting through the occipital joint. The front legs are cut, leaving the knucklebones on the carcass and the hind legs are cut through the tendons; the hide is then removed using a small saw. Legs in sheep carcasses are cut from the knuckle down the front legs; the skin is removed, starting from the neck up to the root of the tail; and the hind legs are cut.

Pork Carcass Hair Removal

For obvious reasons, pig hides are not removed; in this case, carcasses are scalded in tanks with water at 60–65°C heated by steam. The scalding vats are equipped with a cradle to remove the pig after scalding and before scraping. Scalding process of up to 9 min may be needed for adequate hair removal; however, carcass temperature may increase during extended scalding. Reducing the duration of scalding allows for earlier evisceration and entry into the cooler, which may minimize the protein denaturation due to prolonged exposure to high muscle temperatures. The carcass is then hung from the hind legs in a shaving rail equipped with a drained cabinet washer. The remaining hair is then burned off by using a blowtorch or by passing the carcass through a furnace, allowing the skin to become dry and hard. Gardner and others (2006) studied the specific effects of extending the interval between dwell time and duration of scalding on pork quality attributes. Treatments included extending the dwell duration from 5 to 10 min and extending the scald duration from 5 to 8 min. Lengthening the duration of dwell and scalding resulted in a more rapid postmortem pH decline. Reducing the duration of scalding may lead to increased time for manual removal of hair.

Evisceration and Trimming

The most frequently used edible by-products are blood, liver, heart, kidney, tongue, heart, stomach, and lungs. The specific by-product, organ or tissue, considered edible by a given human population group depends on facts such as tradition, religion, and culture (Guerrero-Legarreta 2010). Inedible viscera and by-products are also utilized in other industries such as animal feeds, fertilizers, or pharmacy: gall
bladder, bones, and hooves from beef, sheep, and pigs; horns and feet from sheep and beef; wool and skin from sheep. Beef hide and hair from pigs are inedible by-products. Scraps and condemned parts can also be used for meat meals. Pharmaceutical products are obtained from slaughter by-products: thymus and thyroid of beef, sheep, and pig; pig stomach lining; sheep prostate and intestine; beef and sheep pancreas and suprarenal glands; pig and beef ovaries and spleen; and beef pituitary and pineal glands, corpus luteum, parathyroid.
To remove the viscera, the abdominal cavity is opened by cutting behind the brisket; the liver is removed by cutting it from the diaphragm, which in turn is cut. The content of the chest cavity is removed and the aorta is trimmed. The body is then sawn, dividing the carcass into two sides. To prevent microbial contamination it is important to trim excessive tissue, the area where the sticking knife was introduced if this was the case, and the spinal cord. The carcass is then washed by high-pressure spraying (19 kg/cm2). Inclusion of head, hide, and feet in beef and sheep carcass weight depends on commercial agreements in each country or region.

Storage

Owing to its composition making them a suitable substrate for pathogens and spoilage microorganisms, carcasses and viscera should be stored under refrigeration conditions. Refrigeration is any process that removes heat from a material (Chanona-Pérez and others 2006).
Once the carcass is clean, refrigeration is necessary to 8°C or below to delay the multiplication of psychrophilic organisms associated to spoilage or any microorganisms of public health significance (Manral and Bawa 2010). Refrigeration also restricts enzymatic activity, although if ripening (or aging) is desirable as in the case of added-value beef, storage at higher temperature (12–15°C) during the initial postslaughtering period is necessary with further reduction to 4–6°C. Therefore, the optimum cooling temperature and time vary with the species and desired product (Alarcón-Rojo and Pérez-Chabela 2006).
In commercial slaughtering facilities, the types of installed refrigerators are bulk storage rooms. In these, the air movement around the unwrapped carcasses should be the minimum required to maintain a constant temperature so as to minimize weight loss and appearance changes associated with desiccation. Air distribution must be as homogenous as possible (Manral and Bawa 2010).
In few cases, controlled atmosphere storage rooms ensure optimal quality due to the considerable reduction of aerobic microorganisms; these types of storage rooms must include gas-tight seals to keep atmospheres with low oxygen and high nitrogen and CO2 concentrations, making these facilities expensive. Additional installations are necessary to control gas concentrations (Manral and Bawa 2010)
The average temperature of a cooler must be between 2°C and 6°C. A certain degree of “sweating” occurs when relative humidity is 70% or more. Required building material in coolers must be easy to clean; floors must have a slope to allow proper drain. Rails should be at least 3.3 m above the floor for halves of beef, 9 m for calves and hogs, and 2.25 m for quarters of beef; goat and sheep carcasses should be suspended 1.95 m to the hook. Carcasses must be hung 1 m from the walls and 0.60 m from the refrigeration equipment, with a separation among carcasses of 0.30 m (Guerrero-Legarreta and Pérez-Chabela 2001). Edible organs and offal should be placed in a separate cooler of the carcasses. Retained carcasses or parts should be located in a refrigerated separate compartment.

Slaughter Plant Facilities

It is recommended that slaughtering plants are situated far from areas where objectionable odors or particles are generated, such as dumps or chemical plants; they should also have accessibility; therefore, they should be connected to streets or highways but separated from other plants or buildings. Drinking water supply must be in good quantity because washing is a continuous operation throughout the plant. Carcasses are washed after dressing, so any bacterial contamination in the water supply will be passed to the meat substrate. As water contains high amount of grease, blood, hair, tissue, and bone particles, water disposal must be carefully calculated. In addition, rumen and intestine content must be discarded according to local regulations, as this material is highly polluting. Building materials for walls, floors, drains, ceilings, and equipment are also subject to regulations. All these materials should provide easy cleaning. In general, these materials comprise concrete, ceramic floor tile, floor-glazed brick, glazed tiles, smooth surface Portland cement plaster, plastic, or Portland cement plaster for ceilings. Certain materials in particular are not acceptable, such as lead, porcelain, wood, leather, fabrics, or any material that undergoes chemical reactions.

By Isabel Guerrero-Legarreta and María de Lourdes Pérez-Chabela in "Handbook of Meat and Meat Processing", edited by Y. H. Hui, PhD, CRC Press, Taylor & Francis Group, Boca Raton, Florida, U.S.A., 2012, excerpts p. 407-413.  Adapted and illustrated to be posted by Leopoldo Costa.