meat processing

Introduction

      preparation of meat for human consumption.

      Meat is the common term used to describe the edible portion of animal tissues and any processed or manufactured products prepared from these tissues. Meats are often classified by the type of animal from which they are taken. Red meat refers to the meat taken from mammals; white meat refers to the meat taken from fowl; seafood refers to the meat taken from fish and shellfish; and game refers to meat taken from animals that are not commonly domesticated. In addition, most commonly consumed meats are specifically identified by the live animal from which they come. beef refers to the meat from cattle, veal from calves, pork from hogs, lamb from young sheep, and mutton from sheep older than two years. It is with these latter types of red meat that this section is concerned.

Conversion of muscle to meat

      Muscle is the predominant component of most meat and meat products. Additional components include the connective tissue, fat (adipose tissue), nerves, and blood vessels that surround and are embedded within the muscles. The structural and biochemical properties of muscle are therefore critical factors that influence both the way animals are handled before, during, and after the slaughtering process and the quality of meat produced by the process.

Muscle structure and function

      There are three distinct types of muscle in animals: smooth, cardiac, and skeletal. Smooth muscles (smooth muscle), found in the organ systems including the digestive and reproductive tracts, are often used as casings for sausages. Cardiac muscles are located in the heart and are also often consumed as meat products. However, most meat and meat products are derived from skeletal muscles (striated muscle), which are usually attached to bones and, in the living animal, facilitate movement and support the weight of the body. Skeletal muscles are the focus of the following discussion.

Skeletal muscle structure

      Skeletal muscles are divided from one another by a covering of connective tissue called the epimysium. As shown in Figure 1, individual muscles are divided into separate sections (called muscle bundles) by another connective tissue sheath known as the perimysium. Clusters of fat cells, small blood vessels (capillaries), and nerve branches are found in the region between muscle bundles. Muscle bundles are further divided into smaller cylindrical muscle fibres (cells) of varying lengths that are individually wrapped with a thin connective tissue sheath called the endomysium. Each of the connective tissue sheaths found throughout skeletal muscle is composed of collagen, a structural protein that provides strength and support to the muscles.

      The plasma membrane of a muscle cell, called the sarcolemma, separates the sarcoplasm (muscle cell cytoplasm) from the extracellular surroundings. Within the sarcoplasm of each individual muscle fibre are approximately 1,000 to 2,000 myofibrils (myofibril). Composed of the contractile proteins actin and myosin, the myofibrils represent the smallest units of contraction in living muscle.

Skeletal muscle contraction

      The contraction of skeletal muscles is an energy-requiring process. In order to perform the mechanical work of contraction, actin and myosin utilize the chemical energy of the molecule adenosine triphosphate (ATP). ATP is synthesized in muscle cells from the storage polysaccharide glycogen, a complex carbohydrate composed of hundreds of covalently linked molecules of glucose (a monosaccharide or simple carbohydrate). In a working muscle, glucose is released from the glycogen reserves and enters a metabolic pathway called glycolysis, a process in which glucose is broken down and the energy contained in its chemical bonds is harnessed for the synthesis of ATP. The net production of ATP depends on the level of oxygen reaching the muscle. In the absence of oxygen (anaerobic conditions), the products of glycolysis are converted to lactic acid, and relatively little ATP is produced. In the presence of oxygen (aerobic conditions), the products of glycolysis enter a second pathway, the citric acid cycle (tricarboxylic acid cycle), and a large amount of ATP is synthesized by a process called oxidative phosphorylation.

      In addition to carbohydrates, fats (fat) supply a significant amount of energy for working muscles. Fats are stored in the body as triglycerides (triglyceride) (also called triacylglycerols). A triglyceride is composed of three fatty acid molecules (nonpolar hydrocarbon chains with a polar carboxyl group at one end) bound to a single glycerol molecule. If the fat deposits are required for energy production, fatty acids are released from the triglyceride molecules in a process called fatty acid mobilization. The fatty acids are broken down into smaller molecules that can enter the citric acid cycle for the synthesis of ATP by oxidative phosphorylation. Therefore, the utilization of fats for energy requires the presence of oxygen.

      An important protein of muscle cells is the oxygen-binding protein myoglobin. Myoglobin takes up oxygen from the blood (transported by the related oxygen-binding protein hemoglobin) and stores it in the muscle cells for oxidative metabolism. The structure of myoglobin includes a nonprotein group called the heme ring. The heme ring consists of a porphyrin molecule bound to an iron (Fe) atom. The iron atom is responsible for the binding of oxygen to myoglobin and has two possible oxidation states (oxidation number): the reduced, ferrous form (Fe²⁺) and the oxidized, ferric form (Fe³⁺). In the Fe²⁺ state iron is able to bind oxygen (and other molecules). However, oxidation of the iron atom to the Fe³⁺ state prevents oxygen binding.

Postmortem muscle

      Once the life of an animal ends, the life-sustaining processes slowly cease, causing significant changes in the postmortem (after death) muscle. These changes represent the conversion of muscle to meat.

pH changes

      Normally, after death, muscle becomes more acidic (pH decreases). When an animal is bled after slaughter (a process known as exsanguination), oxygen is no longer available to the muscle cells, and anaerobic glycolysis becomes the only means of energy production available. As a result, glycogen stores are completely converted to lactic acid, which then begins to build up, causing the pH to drop. Typically, the pH declines from a physiological pH of approximately 7.2 in living muscle to a postmortem pH of approximately 5.5 in meat (called the ultimate pH).

protein changes

      When the energy reserves are depleted, the myofibrillar proteins, actin and myosin, lose their extendability, and the muscles become stiff. This condition is commonly referred to as rigor mortis. The time an animal requires to enter rigor mortis is highly dependent on the species (for instance, cattle and sheep take longer than hogs), the chilling rate of the carcass from normal body temperature (the process is slower at lower temperatures), and the amount of stress the animal experiences before slaughter.

      Eventually the stiffness in the muscle tissues begins to decrease owing to the enzymatic breakdown of structural proteins (i.e., collagen) that hold muscle fibres together. This phenomenon is known as resolution of rigor and can continue for weeks after slaughter in a process referred to as aging of meat. This aging effect produces meats that are more tender and palatable.

Properties of meat

Chemistry and nutrient composition

       Nutrient composition of red meatsRegardless of the animal, lean muscle usually consists of approximately 21 percent protein, 73 percent water, 5 percent fat, and 1 percent ash (the mineral component of muscle). These figures vary as an animal is fed and fattened. Generally, as fat increases, the percentages of protein and water decrease. The Table (Nutrient composition of red meats) provides a comparison of the nutrient composition of many meat products.

Protein

      Meat is an excellent source of protein. As is explained above, these proteins carry out specific functions in living muscle tissue and in the conversion of muscle to meat. They include actin and myosin (myofibrillar proteins), glycolytic enzymes and myoglobin (sarcoplasmic proteins), and collagen (connective tissue proteins). Because the proteins found in meat provide all nine essential amino acids to the diet, meat is considered a complete source of protein.

Fat (fat and oil processing)

      Fats, in the form of triglycerides (triglyceride), accumulate in the fat cells (adipose cell) found in and around the muscles of the animal. Fat deposits that surround the muscles are called adipose tissue, while fat that is deposited between the fibres of a muscle is called marbling.

      In the diet the fats found in meat act as carriers for the fat-soluble vitamins (A, D, E, and K) and supply essential fatty acids (fatty acid) (fatty acids not supplied by the body). In addition to their role as an energy reserve, fatty acids are precursors in the synthesis of phospholipids, the main structural molecules of all biological membranes.

      Fatty acids are classified as being either saturated (lacking double bonds between their carbon atoms), monounsaturated (with one double bond), or polyunsaturated (containing several double bonds). The fatty acid composition of meats is dependent on several factors. In animals with simple stomachs, called nonruminants (e.g., pigs), diet can significantly alter the fatty acid composition of meat. If nonruminants are fed diets high in unsaturated fats, the fat they deposit in their muscles will have elevated levels of unsaturated fatty acids. In animals with multichambered stomachs, called ruminants (ruminant) (e.g., cattle and sheep), fatty acid composition found in the lean muscle is relatively unaffected by diet because microorganisms in the stomach alter the chemical composition of the fatty acids before they leave the digestive tract.

      A beneficial characteristic of saturated fatty acids is that they do not undergo oxidation when exposed to air. However, the double bonds found in unsaturated fatty acids are susceptible to oxidation, and this oxidation promotes rancidity in meat. Therefore, products higher in saturated fats can generally be stored for a longer time without developing unpleasant flavours and odours.

Vitamins (vitamin) and minerals (mineral)

      Meat contains a number of essential vitamins and minerals. It is an excellent source of many of the B vitamins, including thiamine, choline, B₆, niacin, and folic acid. Some types of meat, especially liver, also contain vitamins A, D, E, and K.

       Nutrient composition of red meatsMeat is an excellent source of the minerals iron, zinc, and phosphorus. It also contains a number of essential trace minerals, including copper, molybdenum, nickel, selenium, chromium, and fluorine. The Table (Nutrient composition of red meats) provides a comparison of the vitamin and mineral content of different types of meat.

cholesterol

      Cholesterol is a constituent of cell membranes and is present in all animal tissues. Leaner meats typically are lower in cholesterol. veal, however, is an exception: it is lower in fat than mature beef but has significantly higher cholesterol levels.

Carbohydrates (carbohydrate)

      Meat contains virtually no carbohydrates. This is because the principal carbohydrate found in muscle, the complex sugar glycogen, is broken down in the conversion of muscle to meat (see above Postmortem muscle: pH changes). Liver is an exception, containing up to 8 percent carbohydrates.

water

      Water is the most abundant component of meat. However, because adipose tissue contains little or no moisture, as the percentage of fat increases in a meat cut, the percentage of water declines. Therefore, lean young veal may be as much as 80 percent water, while fully fattened beef may be as little as 50 percent. Because water is lost when meats are cooked, the percentages of protein and fat in cooked meats are usually higher than in their raw counterparts.

colour

      In well-bled animals approximately 80 to 90 percent of the total meat pigment is due to the oxygen-binding protein myoglobin. Colour differences in meat are related to the myoglobin content of muscle fibres and to the chemical state of the iron atom found in the myoglobin molecule.

Myoglobin content

      A number of factors influence the myoglobin content of skeletal muscles. Muscles are a mixture of two different types of muscle fibre, fast-twitch and slow-twitch, which vary in proportions between muscles. Fast-twitch fibres have a low myoglobin content and are therefore also called white fibres. They are dependent on anaerobic glycolysis for energy production. Slow-twitch fibres have a high amount of myoglobin and a greater capacity for oxidative metabolism. These fibres are often called red fibres. Therefore, dark meat colour is a result of a relatively high concentration of slow-twitch fibres in the muscle of the animal.

      A second factor contributing to the myoglobin content of a muscle is the age of the animal—muscles from older animals often have higher myoglobin concentrations. This accounts for the darker colour of beef relative to that of veal.

      The size of an animal may also affect the myoglobin content of its muscles because of differences in basal metabolic rates (larger animals have a lower metabolism). Some smaller animals (such as rabbits) typically have a lower myoglobin concentration (0.02 percent of wet weight of muscle) and lighter coloured meat than larger animals such as horses (0.7 percent myoglobin) or deep-diving animals such as whales, which have very high concentrations of myoglobin (7 percent myoglobin) and dark, purple-coloured meat. Myoglobin concentration is also greater in intact males (animals that have not been castrated) of similar age, in muscles located closer to the bones, and in more physically active animals such as game.

Oxidation state of iron

      The oxidation state of the iron atom of myoglobin also plays a significant role in meat colour. Meat such as beef viewed immediately after cutting is purple in colour because water is bound to the reduced iron atom of the myoglobin molecule (in this state the molecule is called deoxymyoglobin). Within 30 minutes after exposure to the air, beef slowly turns to a bright cherry-red colour in a process called blooming. Blooming is the result of oxygen binding to the iron atom (in this state the myoglobin molecule is called oxymyoglobin). After several days of exposure to air, the iron atom of myoglobin becomes oxidized and loses its ability to bind oxygen (the myoglobin molecule is now called metmyoglobin). In this oxidized condition, meat turns to a brown colour. Although the presence of this colour is not harmful, it is an indication that the meat is no longer fresh.

Tenderness

      The tenderness of meat is influenced by a number of factors including the grain of the meat, the amount of connective tissue, and the amount of fat.

Meat grain

      Meat grain is determined by the physical size of muscle bundles. Finer-grained meats are more tender and have smaller bundles, while coarser-grained meats are tougher and have larger bundles. Meat grain varies between muscles in the same animal and between the same muscle in different animals. As a muscle is used more frequently by an animal, the number of myofibrils in each muscle fibre increases, resulting in a thicker muscle bundle and a stronger (tougher) protein network. Therefore, the muscles from older animals and muscles of locomotion (muscles used for physical work) tend to produce coarser-grained meat.

connective tissue

      The amount of connective tissue in a muscle has a complex effect on the tenderness of the meat. The major component of connective tissue, collagen, has a tough, rigid structure. However, even though muscles from younger animals have more connective tissue, the meat derived from those muscles is generally more tender than that from older animals. This is due to the fact that collagen is broken down and denatured during the aging and cooking processes, forming a gelatin-like substance that makes the meat more tender. In addition, collagen becomes more rigid (resistant to breakdown and denaturation) with age, resulting in greater toughness of meat from older animals.

Fat (fat and oil processing)

      A high fat content within the adipose tissue and marbling sites of muscle contributes to the tenderness of the meat. During the cooking process the fat melts into a lubricant-type substance that spreads throughout the meat, increasing the tenderness of the final product.

Postmortem quality problems

      Meat quality may be affected by both the preslaughter handling of the live animals and the postslaughter handling of the carcasses. Psychological or physical stress experienced by the animals produces biochemical changes in the muscles that may adversely affect the quality of the meat. In addition, postmortem muscles are susceptible to adverse biochemical reactions in response to certain external factors such as temperature.

DFD meat

      Dark, firm, and dry (DFD) meat is the result of an ultimate pH that is higher than normal. Carcasses that produce DFD meat are usually referred to as dark cutters. DFD meat is often the result of animals experiencing extreme stress or exercise of the muscles before slaughter. Stress and exercise use up the animal's glycogen reserves, and, therefore, postmortem lactic acid production through anaerobic glycolysis is diminished. The resulting postmortem pH of DFD meat is 6.2 to 6.5, compared with an ultimate pH value of 5.5 for normal meat. The dry appearance of this meat is thought to be a result of an unusually high water-holding capacity, causing the muscle fibres to swell with tightly held water. Because of its water content, this meat is actually juicier when cooked and eaten. Nevertheless, its dark colour and dry appearance result in a lack of consumer appeal, so that this meat is severely discounted at the marketplace.

PSE meat

      Pale, soft, and exudative (PSE) meat is the result of a rapid postmortem pH decline while the muscle temperature is too high. This combination of low pH and high temperature adversely affects muscle proteins, reducing their ability to hold water (the meat drips and is soft and mushy) and causing them to reflect light from the surface of the meat (the meat appears pale). PSE meat is especially problematic in the pork industry. It is known to be stress-related and inheritable. A genetic condition known as porcine stress syndrome (PSS) may increase the likelihood that a pig will yield PSE meat.

Cold shortening

      Cold shortening is the result of the rapid chilling of carcasses immediately after slaughter, before the glycogen in the muscle has been converted to lactic acid. With glycogen still present as an energy source, the cold temperature induces an irreversible contraction of the muscle (i.e., the actin and myosin filaments shorten). Cold shortening causes meat to be as much as five times tougher than normal. This condition occurs in lean beef and lamb carcasses that have higher proportions of red muscle fibres and very little exterior fat covering. Without the fat covering as insulation, the muscles can cool too rapidly before onset of rigor mortis. The process of electrical stimulation (the application of high-voltage electrical current to carcasses immediately postmortem) reduces or eliminates this condition by forcing muscle contractions and using up muscle glycogen. Thaw rigor is a similar condition that results when meat is frozen before it enters rigor mortis. When this meat is thawed, the leftover glycogen allows for muscle contraction and the meat becomes extremely tough.

Heat ring

      Heat ring is a problem associated with beef carcasses and results from differential chilling rates of the muscles after slaughter. A heat ring is a dark, coarsely textured band around the exterior portion of the muscle. In muscles that have a thin layer of external fat, the outer portion of the muscle may chill too fast after death, resulting in a slower pH decline in the outer layer and a dark-coloured ring. This condition is also alleviated by electrical stimulation of beef carcasses after slaughter, causing a more even pH decline throughout the muscle.

livestock slaughter procedures

 The slaughter of livestock involves three distinct stages: preslaughter handling, stunning, and slaughtering. In the United States the humane treatment of animals during each of these stages is required by the Humane Slaughter Act. Figure 2—> represents the general flow of the slaughter process.

Preslaughter handling

      Preslaughter handling is a major concern to the livestock industry, especially the pork industry. stress applied to livestock before slaughter can lead to undesirable effects on the meat produced from these animals, including both PSE and DFD (see Postmortem quality problems (meat processing)). Preslaughter stress can be reduced by preventing the mixing of different groups of animals, by keeping livestock cool with adequate ventilation, and by avoiding overcrowding. Before slaughter, animals should be allowed access to water but held off feed for 12 to 24 hours to assure complete bleeding and ease of evisceration (the removal of internal organs).

Stunning

      As the slaughter process begins, livestock are restrained in a chute that limits physical movement of the animal. Once restrained, the animal is stunned to ensure a humane end with no pain. Stunning also results in decreased stress of the animal and superior meat quality.

      The three most common methods of stunning are mechanical, electrical, and carbon dioxide (CO₂) gas. The end result of each method is to render the animal unconscious. Mechanical stunning involves firing a bolt through the skull of the animal using a pneumatic device or pistol. Electrical stunning passes a current of electricity through the brain of the animal. CO₂ stunning exposes the animal to a mixture of CO₂ gas, which acts as an anesthetic.

Slaughtering

      After stunning, animals are usually suspended by a hind limb and moved down a conveyor line for the slaughter procedures. They are typically bled (a process called sticking or exsanguination) by the insertion of a knife into the thoracic cavity and severance of the carotid artery and jugular vein. This method allows for maximal blood removal from the body. At this point in the process, the slaughtering procedures begin to differ by species.

Hogs

      Hogs are usually stunned by electrical means or CO₂ gas. Mechanical stunning is not generally used in hogs because it may cause serious quality problems in the meat, including blood splashing (small, visible hemorrhages in the muscle tissue) in the lean and PSE meat.

      Hogs are one of the few domesticated livestock animals in which the skin is left on the carcass after the slaughter process. Therefore, after bleeding, the carcasses undergo an extensive cleaning procedure. First they are placed for about five minutes in a scalding tank of water that is between 57° and 63° C (135° and 145° F) in order to loosen hair and remove dirt and other material (called scurf) from the skin. The carcasses are then placed in a dehairing machine, which uses rubber paddles to remove the loosened hair. After dehairing, the carcasses are suspended from a rail with hooks placed through the gambrel tendons on the hind limbs, and any residual hair is shaved and singed off the skin.

      An exception to this procedure occurs in certain specialized hog slaughter facilities, such as “whole hog” sausage slaughter plants. In whole hog sausage production all the skeletal meat is trimmed off the carcass, and therefore the carcass is routinely skinned following exsanguination.

      After cleaning and dehairing, heads are removed and carcasses are opened by a straight cut in the centre of the belly to remove the viscera (the digestive system including liver, stomach, bladder, and intestines and the reproductive organs), pluck (thoracic contents including heart and lungs), kidneys, and associated fat (called leaf fat). The intestines are washed and cleaned to serve as natural casings for sausage products. The carcasses are then split down the centre of the backbone into two “sides,” which are placed in a cooler (called a “hot box”) for approximately 24 hours before fabrication into meat cuts.

cattle, calves, and sheep

      These animals are usually stunned mechanically, but some sheep slaughter facilities also use electrical stunning. The feet are removed from the carcasses before they are suspended by the Achilles tendon of a hind leg for exsanguination. The carcasses are then skinned with the aid of mechanical skinners called “hide pullers.” Sheep pelts are often removed by hand in a process called “fisting.” (In older operations, hides and pelts are removed by knife.) The hides (cattle and calves) or pelts (sheep) are usually preserved by salting so that they can be tanned for leather products. Heads are removed at the first cervical vertebra, called the atlas joint. Evisceration and splitting are similar to hog procedures, except that kidney, pelvic, and heart fat are typically left in beef carcasses for grading. Carcasses are then placed in a cooler for 24 hours (often 48 hours for beef) prior to fabrication into meat cuts.

By-products

      By-products are the nonmeat materials collected during the slaughter process, commonly called offal. Variety meats include livers, brains, hearts, sweetbreads (thymus and pancreas), fries (testicles), kidneys, oxtails, tripe (stomach of cattle), and tongue. Bones and rendered meat are used as bone and meat meal in animal feeds and fertilizers. gelatin, obtained from high-collagen products such as pork snouts, pork skin, and dried rendered bone, is used in confections, jellies, and pharmaceuticals. Intestines are used as sausage casings. Hormones and other pharmaceutical products such as insulin, heparin, and cortisone are obtained from various glands and tissues. Edible fats are used as lard (from hogs), tallow (from cattle), shortenings, and cooking oils. Inedible fats are used in soap and candle manufacturing and in various industrial grease formulations. Lanolin from sheep wool is used in cosmetics. Finally, hides and pelts are used in the manufacture of leather.

Meat inspection

      Meat inspection is mandatory and has the mission of assuring wholesomeness, safety, and accurate labeling of the meat supply. Although inspection procedures vary from country to country, they are centred around the same basic principles and may be performed by government officials, veterinarians, or plant personnel (see ). For example, in the United States meat inspection is administered through the Food Safety and Inspection Service of the United States Department of Agriculture (USDA-FSIS) and is composed of several distinct programs. In general, these programs are representative of the basic inspection procedures used throughout the world and include antemortem inspection, postmortem inspection, reinspection during processing, sanitation, facilities and equipment, labels and standards, compliance, pathology and epidemiology, residue monitoring and evaluation, federal-state relations, and foreign programs.

Antemortem and postmortem inspection

      Antemortem inspection identifies animals not fit for human consumption. Here animals that are down, disabled, diseased, or dead (known as 4D animals) are removed from the food chain and labeled “condemned.” Other animals showing signs of being sick are labeled “suspect” and are segregated from healthy animals for more thorough inspection during processing procedures.

      Postmortem inspection of the head, viscera, and carcasses helps to identify whole carcasses, individual parts, or organs that are not wholesome or safe for human consumption.

Reinspection during processing

      Although previously inspected meat is used in the preparation of processed meat products, additional ingredients are added to processed meats. Reinspection during processing assures that only wholesome and safe ingredients are used in the manufacture of processed meat products (e.g., sausage and ham).

Sanitation

      Sanitation is maintained at all meat-packing and processing facilities by mandatory inspection both before and during the production process. This includes floors, walls, ceilings, personnel, clothing, coolers, drains, equipment, and other items that come in contact with food products. In addition, all water used in the production process must be potable (reasonably free of contamination).

Facilities and equipment

      Facilities and equipment are inspected to ensure that they meet safety requirements. Facilities must have sufficient cooling and lighting, and rails from which carcasses are suspended must be high enough to assure that the carcasses never come in contact with the floor. Equipment must be able to be properly cleaned and must not adversely affect the wholesomeness of the products.

Labels and standards

      Labels and standards regulations assure that products are accurately labeled, that nutritional information meets requirements, and that special label claims (e.g., lean, light, natural) are accurate. Virtually all meat products must have the following components in their label: accurate product name, list of ingredients (in order of predominance), name and place of business of packer and manufacturer, net weight, inspection stamp and plant number, and handling instructions.

Compliance

      Compliance assures that proper criminal, administrative, and civil sanctions are carried out against violators of food inspection laws. These violations include the sale of uninspected meat, the use of inaccurate labels, and the contamination of products.

Pathology and epidemiology

      Pathology and epidemiology programs support the efforts of meat inspectors by working with other public health agencies to minimize the risk from widespread food-poisoning outbreaks. These agencies work to identify the causative agents of food poisoning and prevent repeated occurrences by improving prevention techniques (e.g., proper handling and cooking and prevention of cross-contamination of raw and cooked products).

Residue monitoring and evaluation

      Residue monitoring and evaluation programs identify animals containing harmful residues and remove them from the food chain. These residues include toxins from natural sources, from pesticides, from feeds, or from antibiotics administered to animals too soon before slaughter.

Meat grading

      Meat grading segregates meat into different classes based on expected eating quality (e.g., appearance, tenderness, juiciness, and flavour) and expected yield of salable meat from a carcass. In contrast to meat-inspection procedures, meat-grading systems vary significantly throughout the world. These differences are due in large part to the fact that different countries have different meat quality standards. For example, in the United States cattle are raised primarily for the production of steaks and are fattened with high-quality grain feed in order to achieve a high amount of marbling throughout the muscles of the animal. High marbling levels are associated with meat cuts that are juicier, have more flavour, and are more tender. Therefore, greater marbling levels—and especially marbling that is finely textured and evenly distributed—improve the USDA quality grade (i.e., Prime, Choice, or Select) of the beef. However, in Australia cattle are raised primarily for the production of ground beef products, and the highest quality grades are given to the leanest cuts of meat.

      Some of the characteristics of meat used to assess quality and assign grades include: conformation of the carcass; thickness of external fat; colour, texture, and firmness of the lean meat; colour and shape of the bones; level of marbling; flank streaking; and degree of leanness.

Retail meat cutting

      In the American style of meat cutting, whole carcasses are usually fabricated into more manageable primal (major) or subprimal (minor) cuts at the packing plant. This preliminary fabrication eases meat merchandising by reducing variability within the cuts. Primal and subprimal cuts are usually packaged and sold to retailers that further fabricate them into the products that are seen in the retail case.

pork fabrication

 Hogs are slaughtered at approximately 108 kilograms (240 pounds) and yield carcasses weighing approximately 76 kilograms (70 percent yield of live weight). Pork carcasses are usually divided into two sides before chilling, and each side is divided into four lean cuts plus other wholesale cuts. The four lean cuts are the ham, loin, Boston butt (Boston shoulder), and picnic shoulder. Figure 4a—> shows the major wholesale cuts of pork and the retail cuts derived from each.

beef fabrication

 Steers and heifers average 495 kilograms at slaughter and produce carcasses weighing 315 kilograms (63 percent yield of live weight). Beef carcasses are split into two sides on the slaughter floor. After chilling, each side is divided into quarters, the forequarter and hindquarter, between the 12th and 13th ribs. The major wholesale cuts fabricated from the forequarter are the chuck, brisket, foreshank, rib, and shortplate. The hindquarter produces the short loin, sirloin, rump, round, and flank. Figure 4b—> shows the major wholesale cuts of beef and the retail cuts derived from each.

lamb fabrication

 Live sheep averaging 45 kilograms yield 22-kilogram carcasses (50 percent yield of live weight). Lamb carcasses are divided into two halves, the foresaddle and hindsaddle, on the fabrication floor. The foresaddle produces the major wholesale cuts of the neck, shoulder, rib, breast, and foreshank. The hindsaddle produces the major wholesale cuts of the loin, sirloin, leg, and hindshank. Figure 4c—> shows the major wholesale cuts of lamb and the retail cuts derived from each.

veal fabrication

      Veal is classified into several categories based on the ages of the animals at the time of slaughter. Baby veal (bob veal) is 2–3 days to 1 month of age and yields carcasses weighing 9 to 27 kilograms. Vealers are 4 to 12 weeks of age with carcasses weighing 36 to 68 kilograms. Calves are up to 20 weeks of age with carcasses ranging from 56 to 135 kilograms.

 After slaughter, veal carcasses are split on the fabrication floor into two halves, the foresaddle and hindsaddle. The foresaddle produces the major wholesale cuts of the shoulder, rib, breast, and shank. The hindsaddle produces the major wholesale cuts of the loin, sirloin, and round. Figure 4d—> shows the major wholesale cuts of veal and the retail cuts derived from each.

Meat cookery

      The physical changes associated with cooking meat are caused by the effects of heat on connective tissue and muscle proteins.

Colour changes

      In beef, changes in cooking temperatures ranging from 54° C or 130° F (very rare) to 82° C or 180° F (very well done) correspond to changes in colour from deep red or purple to pale gray. These colour changes are a result of the denaturation of the myoglobin in meat. Denaturation is the physical unfolding of proteins in response to such influences as extreme heat. The denaturation of myoglobin makes the protein unable to bind oxygen, causing the colour to change from the bright cherry red of oxymyoglobin to the brown of denatured myoglobin (equivalent to metmyoglobin).

Structural changes

      The colour changes during cooking correspond to structural changes taking place in the meat. These structural changes are due to the effects of heat on collagen (connective tissue protein) and actin and myosin (myofibrillar proteins). In the temperature range between 50° and 71° C (122° to 160° F) connective tissue in the meat begins to shrink. Further heating to temperatures above 71° C causes the complete denaturation of collagen into a gelatin-like consistency. Therefore, tough meats with relatively high amounts of connective tissues can be slowly cooked under moist conditions to internal temperatures above 71° C and made tender by gelatinization of the collagen within the meat, while at the same time maintaining juiciness.

      The myofibrillar proteins also experience major changes during cooking. In the range of 40° to 50° C (104° to 122° F) actin and myosin begin to lose solubility as heat denaturation begins. At temperatures of 66° to 77° C (150° to 170° F) the myofibrillar proteins begin to shorten and toughen. Beyond 77° C (170° F) proteins begin to lose structural integrity (i.e., they are completely denatured) and tenderness begins to improve.

      The effects of heat on both connective tissue and myofibrillar proteins must be balanced in order to achieve maximum tenderness during cooking. Meats with low amounts of connective tissue are most tender when served closer to medium rare or rare so that muscle proteins are not hardened. Conversely, meats with heavy amounts of connective tissue require slow cooking closer to well done in order to achieve collagen gelatinization.

Meat microbiology, safety, and storage

      When the conversion of muscle to meat begins, biological degradation of meat also commences. In the absence of a living immune system, microorganisms are unchecked in their ability to grow and reproduce on meat surfaces.

Food-borne microorganisms

      Generally, food-borne microorganisms can be classified as either food-spoilage or food-poisoning, with each presenting unique characteristics and challenges to meat product safety and quality.

Food-spoilage microorganisms

      These organisms are responsible for detrimental quality changes in meat. The changes include discoloration, unpleasant odours, and physical alterations. The principal spoilage organisms are molds (mold) and bacteria.

      Molds usually appear dry and fuzzy and are white or green in colour. They can impart a musty flavour to meat. Common molds in meat include the genera Cladosporium, Mucor, and Alternaria. Slime molds produce a soft, creamy material on the surface of meat.

      Common spoilage bacteria include Pseudomonas, Acinetobacter, and Moraxella. Under anaerobic conditions, such as in canned meats, spoilage can include souring, putrefaction, and gas production. This is a result of anaerobic decomposition of proteins by the bacteria.

Food-poisoning (food poisoning) microorganisms

      Food-poisoning microorganisms can cause health problems by either intoxication or infection. Intoxication occurs when food-poisoning microorganisms produce a toxin that triggers sickness when ingested. Several different kinds of toxins are produced by the various microorganisms. These toxins usually affect the cells lining the intestinal wall, causing vomiting and diarrhea. Microorganisms capable of causing food-poisoning intoxication include Clostridium perfringens (found in temperature-abused cooked meats—i.e., meats that have not been stored, cooked, or reheated at the appropriate temperatures), Staphylococcus aureus (found in cured meats), and Clostridium botulinum (found in canned meats).

      Infection occurs when an organism is ingested by the host, then grows inside the host and causes acute sickness and, in extreme cases, death. Common infectious bacteria capable of causing food poisoning in undercooked or contaminated meats are Salmonella, Escherichia coli, Campylobacter jejuni, and Listeria monocytogenes.

Prevention of microbial contamination

      The initial microorganism load can be the most significant factor affecting the contamination of meat. If meat is never exposed to pathogenic microorganisms (those capable of causing human sickness), then there is no opportunity for food-borne illnesses to occur.

      Several meat-processing plants have begun to utilize a program called the Hazard Analysis and Critical Control Point (HACCP) system to reduce pathogenic contamination. This program identifies the steps in the conversion of livestock to human food where the product is at risk of contamination by microorganisms. Once identified, these points, known as critical control points, are examined to determine how to eliminate the risk of microbial contamination.

Preservation (food preservation) and storage

      Meat preservation helps to control spoilage by inhibiting the growth of microorganisms, slowing enzymatic activity, and preventing the oxidation of fatty acids that promote rancidity. There are many factors affecting the length of time meat products can be stored while maintaining product safety and quality. The physical state of meat plays a role in the number of microorganisms that can grow on meat. For example, grinding meat increases the surface area, releases moisture and nutrients from the muscle fibres, and distributes surface microorganisms throughout the meat. Chemical properties of meat, such as pH and moisture content, affect the ability of microorganisms to grow on meat. Natural protective tissues (fat or skin) can prevent microbial contamination, dehydration, or other detrimental changes. Covering meats with paper or protective plastic films prevents excessive moisture loss and microbial contamination.

Cold storage

       temperature is the most important factor influencing bacterial growth. Pathogenic bacteria do not grow well in temperatures under 3° C (38° F). Therefore, meat should be stored at temperatures that are as cold as possible. Refrigerated (refrigeration) storage is the most common method of meat preservation. The typical refrigerated storage life for fresh meats is 5 to 7 days.

      Freezer storage is an excellent method of meat preservation. It is important to wrap frozen meats closely in packaging that limits air contact with the meat in order to prevent moisture loss during storage. The length of time meats are held at frozen storage also determines product quality. Under typical freezer storage of -18° C (0° F) beef can be stored for 6 to 12 months, lamb for 6 to 9 months, pork for 6 months, and sausage products for 2 months.

freezing

      The rate of freezing is very important in maintaining meat quality. Rapid freezing is superior; if meats are frozen slowly, large ice crystals form in the meat and rupture cell membranes. When this meat is thawed, much of the original moisture found in the meat is lost as purge (juices that flow from the meat). For this reason cryogenic freezing (the use of supercold substances such as liquid nitrogen) or other rapid methods of freezing meats are used at the commercial level to maintain maximal product quality. It is important to note, however, that freezing does not kill most microorganisms; they simply become dormant. When the meat is thawed, the spoilage continues where it left off.

      Thawing meats often can cause more detrimental quality changes than freezing. In contrast to freezing, thawing should be a slow process. Meats are best thawed in the refrigerator with packaging left intact, so that moisture loss is minimized. Placing frozen meats out on a warm countertop or under warm water subjects the meat's outer layers to room temperatures for long periods of time before the meat is ready for cooking (completely thawed). This rapid method provides a conducive environment for the growth of food-borne microorganisms and increases the risk of food poisoning.

Vacuum packaging

      Oxygen is required for many bacteria to grow. For this reason most meats are vacuum-packaged, which extends the storage life under refrigerated conditions to approximately 100 days. In addition, vacuum packaging minimizes the oxidation of unsaturated fatty acids and slows the development of rancid meat.

canning

      The second most common method of meat preservation is canning. Canning involves sealing meat in a container and then heating it to destroy all microorganisms capable of food spoilage. Under normal conditions canned products can safely be stored at room temperature indefinitely. However, certain quality concerns can compel processors or vendors to recommend an optimal “sell by” date.

Drying

      Drying is another common method of meat preservation. Drying removes moisture from meat products so that microorganisms cannot grow. Dry sausages, freeze-dried meats, and jerky products are all examples of dried meats capable of being stored at room temperature without rapid spoilage.

fermentation

      One ancient form of food preservation used in the meat industry is fermentation. Fermentation involves the addition of certain harmless bacteria to meat. These fermenting bacteria produce acid as they grow, lowering the pH of the meat and inhibiting the growth of many pathogenic microorganisms.

Irradiation

      Irradiation, or radurization, is a pasteurization method accomplished by exposing meat to doses of radiation. Radurization is as effective as heat pasteurization in killing food-spoilage microorganisms. Irradiation of meat is accomplished by exposing meat to high-energy ionizing radiation produced either by electron accelerators or by exposure to gamma-radiation-emitting substances such as cobalt-60 or cesium-137. Irradiated products are virtually identical in character to nonirradiated products, but they have significantly lower microbial contamination. Irradiated fresh meat products still require refrigeration and packaging to prevent spoilage, but the refrigerated storage life of these products is greatly extended.

Curing and smoking

      Meat curing and smoking are two of the oldest methods of meat preservation. They not only improve the safety and shelf life of meat products but also enhance the colour and flavour. Smoking of meat decreases the available moisture on the surface of meat products, preventing microbial growth and spoilage. Meat curing, as commonly performed in products such as ham or sausage, involves the addition of mixtures containing salt, nitrite, and other preservatives.

      Salt decreases the moisture in meats available to spoilage microorganisms. Nitrite prevents microorganisms from growing and retards rancidity in meats. Nitrite also produces the pink colour associated with cured products by binding (as nitric oxide) to myoglobin. However, the use of nitrite in meat products is controversial owing to its potential cancer-causing activity.

      Sodium erythorbate or ascorbate is another common curing additive. It not only decreases the risks associated with the use of nitrite but also improves cured meat colour development. Other common additives include alkaline phosphates, which improve the juiciness of meat products by increasing their water-holding ability.

H. Russell Cross

Additional Reading

R. MacRae, R.K. Robinson, and M.J. Sadler (eds.), Encyclopaedia of Food Science, Food Technology, and Nutrition, 8 vol. (1993); and Y.H. Hui (ed.), Encyclopedia of Food Science and Technology, 4 vol. (1992), are general works that cover all aspects of the science of food. P. Fellows, Food Processing Technology: Principles and Practices (1988), is an introductory text.R. Paul Singh Detailed overviews of meat processing include Harold B. Hedrick et al., Principles of Meat Science, 3rd ed. (1994); John R. Romans et al., The Meat We Eat, 13th ed. (1994); and H.R. Cross and A.J. Overby (eds.), Meat Science, Milk Science, and Technology (1988), which includes comparisons of methods of meat production and processing in various countries.Peter J. Bechtel (ed.), Muscle As Food (1986); and A.J. Bailey and N.D. Light, Connective Tissue in Meat and Meat Products (1989), discuss physical and biochemical aspects.Processed-meat science and technology is treated in Herbert W. Ockerman, Sausage and Processed Meat Formulations (1989); and A.M. Pearson and F.W. Tauber, Processed Meats, 2nd ed. (1984). Herbert W. Ockerman and C.L. Hansen, Animal By-Product Processing (1988), covers the production of edible meat products, hides, glue, bone and meat meals, pharmaceutical products, sausage casings, pet foods, and animal waste products.H. Russell Cross

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