Growth is defined as the normal increase in size of animal tissues or organs. This may be accomplished by the following processes:
Prenatal growth begins with the fertilization of an ovum and ends with birth.
After birth, all animals undergo a period of slow growth which is then followed by a period of rapid, constant growth. As growth of muscle, bone, and vital organs slows, fattening accelerates until finally the mature size is reached.
Prenatal growth is the time of greatest hyperplasia (multiplication or production of new cells). Although muscles continue to grow after birth, the increase in muscle fibers is only about 10 to 20 percent. The majority of the muscle growth after birth is due to hypertrophy (enlargement of existing cells).
As a mature animal ages, it enters senescence where muscles and tissue begin to degenerate without complete replacement or repair. During this period, the number of fibers may decrease and the remaining fibers increase in size to compensate for loss of function originally performed by the atrophied fibers.
Adipose tissue growth
In young animals, fat develops first in areas around the internal organs. Next fat is deposited beneath the skin (subcutaneous), between muscles (intermuscular), and within the muscle (intramuscular). With sufficient nutrient intake, these fat deposits continue to develop into adulthood. Fat cells are numerous and widespread in the connective tissues of normal adult animals. In times of poor nutrition, the energy reserves of adipose tissue can be drawn on to supply energy to other tissues.
Intramuscular fat is the last to be deposited. Visible intramuscular fat in meat is called marbling and is an important quality of meat which improves its juiciness and flavor. Thus it is evident that proper feeding and nutrition play an important role in the end quality of meat.
Bone growth occurs both before and after birth by transformation of connective tissue. In some cases such as growth of limbs, bone is formed by ossification (hardening) of cartilage. Other bones such as those of the skull are formed directly.
Growth = an increased body weight until mature size
Development = change in form and/or function of the animal or tissue, which results from changes in the rate of increase or decrease of individual components of the body or tissue.
Allometry is the study of relative growth, of changes in proportion with increase in size.
In order to compare the relative growth of two components (one of which may be the whole body), they are plotted logarithmically on X and Y axes:
log y = log b + k log x
The slope of the resulting regression is called the allometric growth ratio, often designated as k.
With k = 1, both components are growing at the same rate.
With k < 1, the component represented on the Y axis is growing more slowly than the component on the X axis. With k > 1, the Y axis component is growing faster than the X axis component.
An example of allometric growth is the following:
There are three types of muscle found in an animal carcass: skeletal muscle, smooth muscle, and cardiac muscle.
Smooth muscle is much less abundant. It is primarily found in the walls of arteries, lymph vessels, and in the gastrointestinal and reproductive tracts.
Cardiac muscle is found in the heart of the animal. It has the unique ability to contract rhythmically from early embryonic life until death. Like the skeletal muscle, it has microscopic banding (striations).
Skeletal muscle (also known as voluntary muscle), in the living animal, facilitates movement and supports the weight of the body.
Skeletal muscle structure
Skeletal muscles are divided from one another by a covering of connective tissue called the epimysium. 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.
Skeletal muscle structure. From: National Cancer Institute
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 that represent the smallest units of contraction in living muscle.
Myofibrils are long thin contractile elements inside the cell that give the characteristic striated pattern. Each myofibril consists of two types of protein filaments called thick filaments and thin filaments.
The thick filaments and the thin filaments within myofibrils overlap, and the sections where they overlap and occur together are called sarcomeres.
The sarcomere is the unit of muscle structure between the two Z lines. The sarcomere length changes depending on the contractile state of the muscle.
The backbone of the thin filaments is made up primarily of the protein actin while the largest component of the thick filament is the protein myosin.
Myosin consists of a tail or rod region that forms the backbone of the thick filament and a globular head region that extends from the thick filament and interacts with actin in the thin filament.
When muscle contraction occurs, the thin filaments and the thick filaments slide past each other.
Each myosin filament is composed of two twisted strands called the heavy chains and two other, but each different, pairs of twisted strands, called light chains. These light chains are found in the myosin heads.
Lying in the groove formed by the actin filaments are a series of rod-shaped protein molecules called tropomyosin. Bound to the end of each tropomyosin is a third protein called troponin. Troponin consists as three small bound protein molecules; one molecule is bound to the actin filament, one to the tropomyosin, and the third is available to bind with Ca2+.
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.
Organization of a muscle fiber. From: World of Teaching. Structure and Function of Skeletal Muscle.
The muscle fibers are classified into at least three distinct types, each having a different rate of energy metabolism. The muscle is a mixture of fiber types.
The proportion of each fiber type within a muscle influences the rate of postmortem changes that occur in the muscle.
Slow twitch (also called Type I or red fibers):
Main characteristics of the three types of muscle fibers. From: World of Teaching. Structure and Function of Skeletal Muscle.
Fast twitch (also called Type II):
Type IIa fibers (Fast-oxidative glycolytic fibers – Intermediate fibers)
Type IIb fibers (Fast-glycolytic fibers)
Once the life of an animal ends, the life-sustaining processes slowly cease, causing significant changes in the post mortem (after death) muscle. These changes represent the conversion of muscle to meat.
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, 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.
Normally, after death, muscle becomes more acidic (pH decreases). When an animal is bled after slaughter, 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).
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 post mortem 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 post mortem 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.
The European Community, in order to standardise the evaluation of carcass quality, has set a grid based on development of carcass profiles (in particular the essential parts: round, back, shoulder) and fat cover.
Concerning the conformation, the system uses 6 classes identified with the letters S E U R O P.
U (Very good)
For the fat cover (amount of fat on the outside of the carcass and in the thoracic cavity), the evaluation system defines 5 classes identified with the numbers 1 to 5.
1 (Low) none up to low fat cover
2 (Slight) slight fat cover, flesh visible almost everywhere
5 (Very high)
It is important to distinguish between anatomical sites and systemic locations in the adipose tissue distribution. In fact, the distinction between anatomical sites and systemic locations is important commercially. For example, bovine intramuscular marbling fat sometimes first becomes noticeable in rump and loin muscles, where it adds to their value. The systemic deposition of fat in a carcass influences commercial indices of carcass composition such as the dressing percentage. Intramuscular fat present in meat at the time of cooking is mostly retained within the meat.
There are two major aspects of meat quality:
The visual identification of quality meat is based on colour and marbling. Visual appearance is very important in determining the likelihood of purchase. Bright red in the case of beef and lamb and pink in the case of pork is the desirable colour of lean meat.
Meat colour is important because it is the initial impression a consumer gets of a product. The colour of meat is primarily dependant on the concentration and chemical state of the pigment myoglobin, which is responsible for moving oxygen through the muscle. 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 non-protein 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: 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.
Meat colour reactions
Immediately after cutting, meat colour is quite dark-beef would be a deep purplish-red.
As oxygen from the air comes into contact with the exposed meat surfaces it is absorbed and binds to the iron. The surface of the meat blooms as myoglobin is oxygenated. This pigment, called oxymyoglobin, gives beef its bright cherry red colour. It is the colour consumers associate with freshness.
Myoglobin and oxymyoglobin have the capacity to lose an electron (called oxidation) which turns the pigment to a brown colour and yields metmyoglobin. Thus, myoglobin can change from a dark purple color to a bright red color simply from oxygenation or to a brown color by losing electrons.
The pigments myoglobin, oxymyoglobin and metmyoglobin can be changed from one to the other, depending on the conditions at which the meat is stored.
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).
Highly active muscles also have more myoglobin. Because muscles differ greatly in activity, their oxygen demand varies. Consequently, different myoglobin concentrations are found in the various muscles of the animal.
Myoglobin concentration is also greater in intact males (animals that have not been castrated) of similar age and in muscles located closer to the bones.
Muscle pigment concentration also differs among animal species. For example, beef has considerably more myoglobin than pork or lamb, thus giving it a more intense colour.
The type of packaging used at retail and thus the amount of oxygen to which the meat is exposed, influences the meat’s colour and appeal to the customer.
The amount of fat can also influence meat’s visual appeal.
Marbling is small streaks of fat that are found within the muscle and can be seen in the meat cut.
The marbling will increase the tenderness, juiciness and flavor of the product.
Of the six meat palatability factors (tenderness, juiciness, flavor, aroma, color, texture), tenderness is generally considered the most important palatability factor by the consumer. Tenderness can be attributed to a person’s perception of meat, such as:
Different muscles in the meat animal have different functions.
Some muscles are defined as muscles of locomotion. These muscles are used to move the animal and as a result of this function, they are less tender. The other muscles in the meat animal are called muscles of attachment. They do very little work and as a result of less work they are more tender. They are often called middle meats and they sell for a higher price because of their 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 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 consists of a structureless mass called ground substance plus embedded cells and extracellular fibers. The lubricating fluid found in joints (synovial fluid) is one example of a ground substance. Ground substances generally function as lubricants, intercellular cementing substances, and can also be found in cartilage and bone.
There are two types of extracellular fibers which are connective tissue proper. Tendons are one example of extracellular fibers which are dense and uniformly arranged in bundles. Two well known types of extracellular fibers are collagen and elastin. Collagen is the major component in tendons and ligaments. Elastin is a rubbery, connective tissue protein which is present throughout the body in ligaments and arterial walls as well as a number of organs including muscle.
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. Thus, meat from older animals can become more resistant to tenderisation in cooking.
Males tend to have more connective tissue than females and the muscle that are used more (i.e. the leg muscles) will also have more connective tissue.
Marbling has a strong benefical effect on juiciness and flavor and may also have a positive effect on tenderness; meat which has little marbling may be dry and flavorless; excessive amounts of marbling will not necessarily increase the juiciness and flavor over those cuts of meat with modest marbling.
One of the best procedures for measuring tenderness is determining the shear force.
The first and most widely accepted instrumental measure of meat texture is the Warner–Bratzler Shear (WBS) Instrument.
As the slot moves past the blade, the meat specimen is compressed and the cross-sectional shape changes to conform to the restriction imposed by the triangular opening of the blade until it is eventually sheared into two pieces.
Tenderness is one of or the most discussed features in meat.
It is well recognized that the biochemical post-mortem processes are key-steps for meat tenderization.
During tenderization, proteolysis affects all muscle proteins, including connective tissue, and it is now well established that post-mortem proteolysis of myofibrillar and myofibrillar-associated proteins is responsible for this process. Some non-enzymatic aspects also influence the meat tenderization process, such as temperature, pH and Ca2+ concentration.
If carcasses enter rigor mortis below 10-12 °C, cold shortening of the muscle fibres can occur, causing toughness and preventing tenderisation through ageing.
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.
Another quality identification is smell. The product should have a normal smell. This will be different for each of the species (i.e. beef, pork, chicken), but should vary only slightly within the species.
Flavour is the combined result of the taste and smell senses and, because it is a subjective property, is difficult to evaluate. Each species has its own characteristic flavour.
The flavour of meat can be influenced by the diet of the animal. Grass or forage-fed cattle and sheep tend to produce meat with a more intense flavour than grainfed animals. Grass-feeding increases certain polyunsaturated fatty acid concentrations in the muscle and improves flavour.
Water-holding capacity (WHC) is one of the most important meat properties in processed products. WHC is defined as the ability of meat to retain its water during application of external forces such as heating, cutting, mincing or pressing.
Many of the physical properties of meat, including colour, texture and firmness of raw meat are a result of the water-holding capacity of the meat.
Consumer studies show that tenderness and flavour are the most important characteristics determining the acceptability of meat. However, there is a wide range of other attributes that can potentially influence acceptability of meat.
Many physical properties of meat are greatly influenced by genetic factors. For example, tenderness in beef may be up to 60% inheritable while tenderness in pork is only 30% inheritable.
Livestock producers can make improvements to the end quality of meat by careful selection of livestock breeds and strains within a particular breed.
Meat quality variations among sexes of animals is not fully understood, but is believed to be caused by differing levels of sex hormones circulating in the blood. Although intact males tend to produce leaner carcasses, many other qualities make them objectionable to consumers.
Animal aging causes darkening of meat due to increased levels of myoglobin. Tenderness is also greatly affected by age. Although meat generally becomes less tender as the animal ages, this is not the case during periods of rapid growth when meat actually becomes more tender. This is because the connective tissue, which causes toughness, is diluted during periods of rapid growth. Although age is a factor in meat tenderness, it plays a less important role than factors such as muscle location and animal condition.
Some muscles yield more tender meat than others. Muscles which are free to shorten during rigor mortis are generally less tender. Another factor affecting meat tenderness is the strength and usage of the muscle.
The animal carcass consists of muscle, connective tissue, fat and bone and some 75% water in proportions depending on species, breed, size, age, etc..
The muscle (lean meat) is relatively constant in composition in a given species. Generally, consists of approximately 21 percent protein, 73 percent water, 5 percent fat, and 1 percent ash.
Meats are often classified by the type of animal from which they are taken:
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.
The majority of water in muscle is held within the structure of the muscle and muscle cells. Specifically, within the muscle cell, water is found within the myofibrils, between the myofibrils themselves and between the myofibrils and the cell membrane (sarcolemma), between muscle cells and between muscle bundles (groups of muscle cells).
Another fraction of water that can be found in muscles and in meat is termed entrapped (also referred to as immobilized) water.
Free water is water whose flow from the tissue is unimpeded. Weak surface forces mainly hold this fraction of water in meat.
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.
Meat is an excellent source of protein. 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 is a concentrated source of energy for the body, providing 9 calories per gram. A reduction of the fat content of meat is a major goal in the continued improvement of meat animals.
Fat contributes a lot of taste to meat, particularly those flavours that allow us to recognize one species from another.
Adipose depots range in size from small groups of adipose cells located between muscle fibre bundles (intermuscular and intramuscular fat) to the vast numbers of adipose cells that are located subcutaneously (under the skin) and viscerally (around the guts). In fact, lipids (fats) are found at three sites in the body.
The largest amount by far is in the storage deposits under the skin and around the organs; this constitutes the obvious, visible fat in a piece of meat. This adipose tissue is composed largely of triglycerides contained in proteinaceous cells with relatively little water; clearly this visible fat can be trimmed off the meat during processing, before cooking or at the table.
Small streaks of fat are visible between the bundles of muscle fibres, intermuscular fat, i.e. in the lean part of the meat; this is known as marbling. 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.
There are small amounts of fat within the muscle structure – intra muscular or structural fats – in amounts varying with the tissue.
Different grades of marbling. From: United States Department of Agriculture (USDA), Meat Quality Grades.
Order of development of the adipose tissue depots
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 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), mono-unsaturated (with one double bond), or polyunsaturated (containing several double bonds).
The fatty acid composition of meats is dependent on several factors. In nonruminant animals, 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 ruminant animals, 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.
Species, breed, sex, age, and environment influence the amount, as well as the degree, of unsaturation of the fat (mainly the ratio between unsaturated oleic acid and the saturated palmitic and stearic acids).
Vitamins and minerals
Meat contains a number of essential vitamins and minerals.
It is an excellent source of many of the B vitamins, including thiamine, choline, riboflavin, B6 (pyridoxine), B12, niacin, and folic acid. Some types of meat also contain vitamins A, D, E and K.
Meat 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.