Does muscle tissue contain different types of protein?

Jul 31, 2015

Protein

Muscle tissue contains many different proteins with many different functions.  Meat proteins are grouped in three general classifications: (1) myofibrillar, (2) stromal, and (3) sarcoplasmic. Each class of proteins differs as to the functional properties it contributes to meat.

 Myofibrillar Proteins—Muscle fibers, the muscle cells which are grouped into muscle bundles, are composed of myofibrils. The proteins that comprise the myofibril, including actin and myosin and several more, are collectively called the myofibrillar proteins.  The myofibrillar protein components most important for muscle fiber structure are actin and myosin. They are the most abundant proteins in muscle and are directly involved in the ability of muscle to contract and to relax. 

The orderly arrangement of the protein molecules, actin and myosin and other myofibrillar proteins, forms the myofilaments. When viewed through an electron microscope these filaments form a pattern of cross striations, seen as alternating light and dark bands.  The bands will differ in length depending upon the state of contraction or relaxation of the muscles. During contraction, actin and myosin filaments slide together to form a more complex protein known as actomyosin. Where these filaments overlap, darker bands  occur in the striation pattern. Pre-slaughter and postmortem handling are extremely important in controlling  the state of muscle contraction as it  relates to muscle tenderness. if muscles  are contracted (if the bands overlap)  when the meat is prepared for eating,  it will be less tender.       

Stromal Proteins—Connective tissue is composed of a watery substance into which is dispersed a matrix of stromal- protein fibrils; these stromal proteins are collagen, elastin and reticulin.

  • Collagen—Collagen is the singlemost abundant protein found in theintact body of mammalian species,being present in horns, hooves, bone,skin, tendons, ligaments, fascia, cartilage and muscle. Collagen is a unique and specialized protein which serves a variety of functions. The primary functions of collagen are to provide strength and support and to help form an impervious membrane (as in skin). In meat, collagen is a major factor influencing the tenderness of the muscle after cooking. Collagen is not broken down easily by cooking except with moist—heat cookery methods. Collagen is white, thin and transparent. Microscopically, it appears in a coiled formation which softens and contracts to a short, thick mass when it is heated, and helping give cooked meat a plump appearance. Collagen itself is tough; however, heating (to the appropriate temperature) converts collagen to gelatin which is tender.
  • Elastin—Elastin is found in the walls of the circulatory system as well as in connective tissues throughout the animal body and provide elasticity to those tissues. Elastin is sometimes referred to as "yellow" connective tissue due to its color. The ligamentum nuchae(heavy gristle) present in chuck bladeand rib roasts and steaks, is almostpure elastin. Unlike collagen, elastin is not degraded by moist—heat cookery methods and should be removed from cuts where it exists. Fortunately, muscle tissue from young animals contains relatively little elastin.
  • Reticulin—Reticulin is present in much smaller amounts than either collagen or elastin. It is speculated that reticulin may be a precursor to either collagen and/or elastin as it is more prevalent in younger animals.Older animals can, but do not necessarily, have more connective tissue per unit of muscle than do younger animals.Cross—linkages between the collagen molecules increase (decreasing susceptibility of collagen to heat—induced solubilization) in older animals yielding tougher muscles than those found in muscle from more youthful animals.In general, the less connective tissue a cut of meat contains the more tender it will be.

Sarcoplasmlc Proteins—the sarcoplasmic proteins include hemoglobin and myoglobin pigments and a wide variety of enzymes. 

  • Pigments from hemoglobin and myoglobin help to contribute the red color to muscle. Hemoglobin carries oxygen from the lungs to the tissues— including muscle. Myoglobin is present in muscle and it stores the oxygen transported to the muscle via the blood by hemoglobin until it is utilized in metabolism. The carbon dioxide produced during cell metabolism diffuses out of the cells (including those of the muscle) and is transported as the bicarbonate ion to the lungs where it is exhaled as carbon dioxide. Myoglobin is present in the sarcoplasm, or bathing fluid, of the muscle cell; hemoglobin is the protein found in red blood cells or erythrocytes. In meat, there is a substantial quantity of hemoglobin because not all blood is removed from capillaries, arterioles and venules during slaughter and dressing.

    The color of muscle cans influence the USDA quality grade of beef carcasses.  Color also plays a role in the aesthetic appeal of meat in the market display case. The color range of pinks to reds found in muscle is partially dependent on the amount of myoglobin present and partially due to the chemical state (and free binding-site occupant) of the heme in myoglobin and hemoglobin.       

    Differences in myoglobin concentration are related to species, age and sex of the animal, and type of muscle. Cattle have more myoglobin in their muscles than pigs; mature sheep more than lambs; bulls more than cows; and the constantly operating muscles of the diaphragm have more myoglobin than less frequently used muscles, like the longissimus dorsi. 

    When exposed to the air, the pigments (myoglobin and hemoglobin) on the surface of the muscles are oxygenated.  Oxygenation forms oxymyoglobin, which is bright red. The interior of the muscle will remain purple because oxygen cannot penetrate to the center portion of the muscle. Store-packaged meat  (wrapped in oxygen—permeable plastic  film) is usually bright red, while vacuum-  packaged meat is purplish-red due to  decreased oxygen permeability of the  packaging film. Ground beef is normally packaged for retail sale in oxygen-permeable clear film, allowing the exterior to become bright red. When these packages are opened and the unit is broken apart, the interior of the chub or patty will be dark brown or purple, but that too will brighten up in a few minutes upon exposure to the oxygen in the air. Prolonged exposure to oxygen causes the oxymyoglobin to oxidize and form metmyoglobin, which is an unappealing brownish—red color. While the  palatability of such meat, after cooking,  may be satisfactory, brown muscle will  become rancid and unpalatable more  quickly than will bright—red muscle if  further storage occurs (prior to cooking). 

    In the curing process, myoglobin unites loosely with nitric oxide to form nitrosomyoglobin. Upon cooking, nitrosomyoglobin is converted to nitrosohemochrome, which is characteristically cured—pink in color and which is sensitive to light. For this reason, cured meat (e.g., slices of ham) are often displayed in the meat case with the meat (on its foam tray) turned face down.  Although sensitive to light, nitrosohemochrome is heat—stable. As a result, reheating of cured meat does not further alter its color. In contrast, fresh meat turns brownish—red when cooked because denatured globins, hemichrome and hemochrome are formed.

  • Enzymes which occur naturally in muscle tissue continue to function during the aging of meat. Proteolytic enzymes are those that degrade protein, amylolytic enzymes degrade carbohydrates, and Iypolytic enzymes degrade fats. During aging, proteolytic enzymes break down myofibrillar proteins, thereby contributing to the tenderness of meat.The enzymes (e.g., calcium—activated proteases and cathepsins) responsible for tenderization are sensitive to time and temperature—the longer meat remains at the temperature optimal for enzyme activity, the more complete will be the enzyme degradation of myofibrilla proteins.

Nitrogenous Extractives—another group of substances that are related to proteins (but are not true proteins) are the nitrogenous substances and nucleopeptides. This water—soluble components excite the flow of gastric juices when cooked meat is ingested.  Along with fat, they (especially the nucleotides and nucleosides, and their metabolic by-products—xanthine and hypoxanthine) provide much of the aroma and flavor of meat. Examples of this group of substances are creatine, creatinine, the purines and free amino acids. Greater quantities of nitrogenous extractives are present in muscle from older animals, and they are more abundant in the more active muscles that occur in the less tender cuts. Nitrogenous extractives, in part, are responsible for the so—called “gamey (intense) flavor” of meat from wild animals.         

Muscle tissue contains many different proteins with many different functions.  Meat proteins are grouped in three general classifications: (1) myofibrillar, (2) stromal, and (3) sarcoplasmic. Each class of proteins differs as to the functional properties it contributes to meat.

 Myofibrillar Proteins—Muscle fibers, the muscle cells which are grouped into muscle bundles, are composed of myofibrils. The proteins that comprise the myofibril, including actin and myosin and several more, are collectively called the myofibrillar proteins.  The myofibrillar protein components most important for muscle fiber structure are actin and myosin. They are the most abundant proteins in muscle and are directly involved in the ability of muscle to contract and to relax. 

The orderly arrangement of the protein molecules, actin and myosin and other myofibrillar proteins, forms the myofilaments. When viewed through an electron microscope these filaments form a pattern of cross striations, seen as alternating light and dark bands.  The bands will differ in length depending upon the state of contraction or relaxation of the muscles. During contraction, actin and myosin filaments slide together to form a more complex protein known as actomyosin. Where these filaments overlap, darker bands  occur in the striation pattern. Pre-slaughter and postmortem handling are extremely important in controlling  the state of muscle contraction as it  relates to muscle tenderness. if muscles  are contracted (if the bands overlap)  when the meat is prepared for eating,  it will be less tender.       

Stromal Proteins—Connective tissue is composed of a watery substance into which is dispersed a matrix of stromal- protein fibrils; these stromal proteins are collagen, elastin and reticulin.

  • Collagen—Collagen is the singlemost abundant protein found in theintact body of mammalian species,being present in horns, hooves, bone,skin, tendons, ligaments, fascia, cartilage and muscle. Collagen is a unique and specialized protein which serves a variety of functions. The primary functions of collagen are to provide strength and support and to help form an impervious membrane (as in skin). In meat, collagen is a major factor influencing the tenderness of the muscle after cooking. Collagen is not broken down easily by cooking except with moist—heat cookery methods. Collagen is white, thin and transparent. Microscopically, it appears in a coiled formation which softens and contracts to a short, thick mass when it is heated, and helping give cooked meat a plump appearance. Collagen itself is tough; however, heating (to the appropriate temperature) converts collagen to gelatin which is tender.
  • Elastin—Elastin is found in the walls of the circulatory system as well as in connective tissues throughout the animal body and provide elasticity to those tissues. Elastin is sometimes referred to as "yellow" connective tissue due to its color. The ligamentum nuchae(heavy gristle) present in chuck bladeand rib roasts and steaks, is almostpure elastin. Unlike collagen, elastin is not degraded by moist—heat cookery methods and should be removed from cuts where it exists. Fortunately, muscle tissue from young animals contains relatively little elastin.
  • Reticulin—Reticulin is present in much smaller amounts than either collagen or elastin. It is speculated that reticulin may be a precursor to either collagen and/or elastin as it is more prevalent in younger animals.Older animals can, but do not necessarily, have more connective tissue per unit of muscle than do younger animals.Cross—linkages between the collagen molecules increase (decreasing susceptibility of collagen to heat—induced solubilization) in older animals yielding tougher muscles than those found in muscle from more youthful animals.In general, the less connective tissue a cut of meat contains the more tender it will be.

Sarcoplasmlc Proteins—the sarcoplasmic proteins include hemoglobin and myoglobin pigments and a wide variety of enzymes. 

  • Pigments from hemoglobin and myoglobin help to contribute the red color to muscle. Hemoglobin carries oxygen from the lungs to the tissues— including muscle. Myoglobin is present in muscle and it stores the oxygen transported to the muscle via the blood by hemoglobin until it is utilized in metabolism. The carbon dioxide produced during cell metabolism diffuses out of the cells (including those of the muscle) and is transported as the bicarbonate ion to the lungs where it is exhaled as carbon dioxide. Myoglobin is present in the sarcoplasm, or bathing fluid, of the muscle cell; hemoglobin is the protein found in red blood cells or erythrocytes. In meat, there is a substantial quantity of hemoglobin because not all blood is removed from capillaries, arterioles and venules during slaughter and dressing.

    The color of muscle cans influence the USDA quality grade of beef carcasses.  Color also plays a role in the aesthetic appeal of meat in the market display case. The color range of pinks to reds found in muscle is partially dependent on the amount of myoglobin present and partially due to the chemical state (and free binding-site occupant) of the heme in myoglobin and hemoglobin.       

    Differences in myoglobin concentration are related to species, age and sex of the animal, and type of muscle. Cattle have more myoglobin in their muscles than pigs; mature sheep more than lambs; bulls more than cows; and the constantly operating muscles of the diaphragm have more myoglobin than less frequently used muscles, like the longissimus dorsi. 

    When exposed to the air, the pigments (myoglobin and hemoglobin) on the surface of the muscles are oxygenated.  Oxygenation forms oxymyoglobin, which is bright red. The interior of the muscle will remain purple because oxygen cannot penetrate to the center portion of the muscle. Store-packaged meat  (wrapped in oxygen—permeable plastic  film) is usually bright red, while vacuum-  packaged meat is purplish-red due to  decreased oxygen permeability of the  packaging film. Ground beef is normally packaged for retail sale in oxygen-permeable clear film, allowing the exterior to become bright red. When these packages are opened and the unit is broken apart, the interior of the chub or patty will be dark brown or purple, but that too will brighten up in a few minutes upon exposure to the oxygen in the air. Prolonged exposure to oxygen causes the oxymyoglobin to oxidize and form metmyoglobin, which is an unappealing brownish—red color. While the  palatability of such meat, after cooking,  may be satisfactory, brown muscle will  become rancid and unpalatable more  quickly than will bright—red muscle if  further storage occurs (prior to cooking). 

    In the curing process, myoglobin unites loosely with nitric oxide to form nitrosomyoglobin. Upon cooking, nitrosomyoglobin is converted to nitrosohemochrome, which is characteristically cured—pink in color and which is sensitive to light. For this reason, cured meat (e.g., slices of ham) are often displayed in the meat case with the meat (on its foam tray) turned face down.  Although sensitive to light, nitrosohemochrome is heat—stable. As a result, reheating of cured meat does not further alter its color. In contrast, fresh meat turns brownish—red when cooked because denatured globins, hemichrome and hemochrome are formed.

  • Enzymes which occur naturally in muscle tissue continue to function during the aging of meat. Proteolytic enzymes are those that degrade protein, amylolytic enzymes degrade carbohydrates, and Iypolytic enzymes degrade fats. During aging, proteolytic enzymes break down myofibrillar proteins, thereby contributing to the tenderness of meat.The enzymes (e.g., calcium—activated proteases and cathepsins) responsible for tenderization are sensitive to time and temperature—the longer meat remains at the temperature optimal for enzyme activity, the more complete will be the enzyme degradation of myofibrilla proteins.

Nitrogenous Extractives—another group of substances that are related to proteins (but are not true proteins) are the nitrogenous substances and nucleopeptides. This water—soluble components excite the flow of gastric juices when cooked meat is ingested.  Along with fat, they (especially the nucleotides and nucleosides, and their metabolic by-products—xanthine and hypoxanthine) provide much of the aroma and flavor of meat. Examples of this group of substances are creatine, creatinine, the purines and free amino acids. Greater quantities of nitrogenous extractives are present in muscle from older animals, and they are more abundant in the more active muscles that occur in the less tender cuts. Nitrogenous extractives, in part, are responsible for the so—called “gamey (intense) flavor” of meat from wild animals.         

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