In trying to understand the conversion of muscle to meat, muscle has been used previously to loosely refer to meat. However, in strict terms, muscle in the living animal is not the same as meat and meat can only be obtained from animals after they are dead.
Different physical and chemical changes occur in the body of the animal after it is killed or slaughtered. These changes affect the quality of the final product positively or negatively. It is important to understand how these changes occur in order to be able to minimize undesirable outcomes.
This article will bring to your attention procedures and processes accompanying the slaughtering of animals and the transformation of muscle into the meat.
Meat is pure muscle tissue with some connective tissue, inter-, and intra- muscular fat, and to a lesser extent epithelial and nervous tissue. There are three types of muscle:
Skeletal muscle is attached directly or indirectly to bones through tendons or ligaments. This is the main muscle in the body, making up about 35 – 65% of the entire carcass. It is under voluntary control
Cardiac muscle is found only in the heart. Both skeletal and cardiac muscles are striated (appear to have alternating dark and light bands along their length.
Smooth muscle is not striated and is found in blood vessels, the intestine, and the reproductive tract. Both cardiac and smooth muscles are controlled involuntarily
The figures above show a simplified gross and anatomical structure of skeletal muscle. It is important to be familiar with the structure of the muscle in order to properly understand the changes that occur when it is converted to meat.
A whole or intact muscle is made up of a group of muscle bundles, the muscle bundles are made up of many muscle fibers (or myofiber; ‘myo’ means muscle), and each muscle fiber is made up of muscle fibrils (myofibrils).
The myofibrils are made up of muscle filaments (myofilaments), which are arranged in repeating sections throughout the length of the myofibrils. It is this repeated pattern that gives skeletal and cardiac muscles their striated appearance referred to earlier.
The myofilaments are the smallest structural unit of the muscle where contraction and relaxation of the muscle occurs. It is also the point where important transformations occur in the conversion of muscle to meat.
There are two myofilaments and each is made up of two different proteins, which are actin and myosin. Actin is also referred to as a thin filament and myosin, is a thick filament.
The space that both actin and myosin occupy is called a sarcomere, and the two dark structures forming the boundary of the space or sarcomere are called Z-lines.
There are many other proteins that contribute to the integrity of the myofilaments, but the most important are troponin and tropomyosin, which function as regulatory proteins in contraction and relaxation.
Actin and myosin have a strong affinity for each other but are prevented from binding by troponin and tropomyosin working together. Tropomyosin forms a physical barrier between actin and myosin and troponin helps it to maintain this position.
This is the state of the filaments when the muscle is in a relaxed state. Other important metabolites to contraction and relaxation are adenosine triphosphate (ATP), and creatine phosphate (or phosphocreatine).
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Contraction and Relaxation
Nervous signals, called action potentials are sent from the brain to start the contraction of muscles. The signal causes calcium to be released into the sarcomere, which binds to troponin, and this change results in a loss of control over tropomyosin.
Therefore, tropomyosin is moved away from its position between actin and myosin and they bind. The effective formation of the bond is also enhanced by ATP.
Myosin is bound to ATP before calcium is released but removed from it in the presence of calcium and broken down into adenosine diphosphate (ADP) and inorganic phosphate (Pi). The removal of ATP results in a change in the shape of myosin to move it closer to actin.
This bond is an active one where myosin pulls actin filaments (on either side) toward the center of the sarcomere thereby shortening the space. This activity occurs throughout the lengths of the myofibrils, which make up the myofibers, which make up muscle bundles, which make up the whole muscle. Therefore, the whole muscle shortens or contracts.
Contraction is relaxed when action potentials are sent for the removal of calcium and therefore, troponin returns to its original form and brings back tropomyosin between actin and myosin.
In addition, the removal of the calcium results in new ATP being formed from ADP and Pi, and this binds again to myosin thereby moving it away from actin breaking the bond. Therefore, the sarcomere returns to its original size and the whole muscle relaxes.
Creatine phosphate acts as a ready source of Pi for ADP to be converted to ATP when normal ATP sources are exhausted when the muscle is under stressful conditions such as occur after the animal is slaughtered.
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Conversion of Muscle to Meat
The conversion of muscle to meat refers to the physical and chemical changes that occur to the muscle after slaughtering the animal. Such post-mortem changes may result in a wide variation in the quality of meat. Primary aspects of post-mortem changes of immediate concern to the producer are:
Production of meat of good keeping quality, which maintains an attractive appearance.
Realization of a limited degree of degeneration, which improves the texture of the meat.
Conversion of muscle to meat starts mainly with slaughtering, which involves exsanguination or the removal of blood. However, only about 50% of the blood in the animal’s body can be removed.
The following are the changes that have an important bearing on the production of good-quality meat with limited degeneration:
Loss of blood, which carries oxygen to the tissues results in a shortage of oxygen and there is a shift from aerobic to anaerobic metabolism, which leads to a decrease in ATP or energy production.
Reduction in ATP production affects proper contraction and relaxation of the muscle. This may result in very tough meat if not properly handled.
Circulatory system collapses and metabolic wastes and waste products accumulate within the muscle cells. This is because the circulatory system was responsible for the transport of nutrients and wastes. Therefore, most of the blood must be removed as much as possible in order to have good-quality meat.
The shift from aerobic to anaerobic metabolism leads to the production of lactic acid because glycogen (carbohydrate) stored in the muscle is converted to lactic acid in the absence of oxygen. Most of the glycogen would have been broken down in the first 12 hours after slaughter.
Therefore, muscle pH drops, and the normal pH of muscle at slaughter is 7 but the ultimate or final pH of normal meat after 24 hours of slaughter will drop to pH 5.4-5.8. The extent of the drop depends on the amount of glycogen before slaughter and this will have a significant effect on the quality of meat.
If the pH drops rapidly to about 5.0 one hour post-slaughter, the meat will be pale in color, soft, and with a very wet surface (exudative). This is called PSE meat for pale, soft, and exudative because the meat has a very poor water-holding capacity as most of the proteins that bind to water would have been digested or denatured.
This condition usually happens in pigs susceptible to stress. The opposite occurs with animals that have not been fed for a very long time before slaughter, or have had stressful transport/handling.
In this case, most of the muscle glycogen would have been broken down before slaughter and the pH cannot drop to less than 6.0 24 hours post-slaughter.
This produces dark, firm, and dry meat (DFD). In this case, most muscle proteins still remain intact (since there is very little acid to digest or denature them) and remain bound to water.
The typical taste and flavor of the meat are only achieved after a sufficient pH drop. The pH is also important to the storage of meat as the lower the pH the lower the conditions favorable for the growth of harmful bacteria.
Reduction in heat production and dissipation occurs because of the collapse of the circulatory system responsible for temperature control in the muscle. The heat from within the body can no longer be carried to the lungs or other surface areas for dissipation.
Therefore, the denaturation of protein increases rapidly and this can reduce the quality of meat except the temperature can be reduced by, for example, processing the carcass in a cooled room.
Heat dissipation is also reduced in carcasses that are scalded (hot water or fire applied to remove or burn off the hair), which negatively affects the quality of meat.
Rigor mortis is a condition where the muscle becomes very stiff soon after slaughter because actin and myosin filaments remain permanently bound or contracted because of insufficient ATP to relax the muscle (see the section on contraction and relaxation above).
This normally occurs in the muscle post-slaughter but is resolved or removed (gets softer) with time due to enzymatic action by cathepsin enzymes.
However, if the muscle is cooled rapidly in the rigor state, the bond between actin and myosin will not be broken and the resulting meat will be very tough. Rigor is resolved faster in fish than in other meat types.
Loss of protection from bacterial infection occurs post-slaughter because the mechanisms for protection in the living animal are compromised. Such mechanisms are connective tissues, the lymphatic system, and circulating white blood cells.
Therefore, extra care must be exercised to prevent the contamination of meat from spoilage microorganisms during post-slaughter handling and storage.
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