Fernando Grasso » 8.Meat quality

Growth

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
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.

Skeletal muscle structure

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.

Skeletal muscle structure

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.

Muscle fiber

Organization of a muscle fiber. From: World of Teaching. Structure and Function of Skeletal Muscle.

Muscle fibers

Fast twitch (also called Type II):

Type IIa fibers (Fast-oxidative glycolytic fibers – Intermediate fibers)

• is aerobic like slow muscle;
• has fewer mitochondria and less myoglobin;
• high resistance to fatigue.

Type IIb fibers (Fast-glycolytic fibers)

• is anaerobic white muscle;
• is even less dense in mitochondria and myoglobin;
• can contract even more quickly;
• poor resistance to fatigue.

Conversion of muscle to meat

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.

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, 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.

pH changes
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).

Conversion of muscle to meat

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.

Carcass evaluation

3 (Average)

• flesh with the exception of the round and shoulder, almost everywhere covered with fat;
• slight deposits of fat in the thoracic cavity.

4 (High)

• flesh covered with fat, but on the round and shoulder still partly visible;
• some distinctive fat deposits in the thoracic cavity.

5 (Very high)

• entire carcass covered with fat;
• heavy deposits in the thoracic cavity.

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.

Meat colour

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).

Meat colour

Myoglobin content

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.

Meat tenderness

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.

Tenderness
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:

• softness to tongue and cheek;
• resistance to tooth pressure;
• ease of fragmentation;
• mealiness (if it breaks up too much in the mouth);
• adhesion (if it clumps together in the mouth);
• residue after chewing.

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.

Meat 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.

Meat tenderness

Connective tissue
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.

Meat tenderness

Biochemical aspects
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.

Meat quality

Smell
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
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
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.

Factors affecting meat quality

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.

Heredity
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.

Sex
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.

Factors affecting meat quality

Age
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.

Muscle location
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.

Nutritional value of meat

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:

• red meat refers to the meat taken from mammals;
• white meat refers to the meat taken from chicken, turkey and rabbit.

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.
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.

Nutritional value of meat

Carbohydrates
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.

Protein
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.

Nutritional value of meat

Fat
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.

Marbling

Different grades of marbling. From: United States Department of Agriculture (USDA), Meat Quality Grades.

Nutritional value of meat

Order of development of the adipose tissue depots

• mesenteric;
• perirenal;
• subcutaneous;
• intermuscular = seam fat;
• intramuscular = interfascicular = 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 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).

Nutritional value of meat

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.