Biochemistry of Meat Processing

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    Meat is defined by FDA as Meat is the properly dressed flesh derivedfrom cattle, swine, sheep or goats sufficiently mature and in goodhealth at the time of slaughter.

    There are a wide variety of meat products that are attractive toconsumers because of their characteristic color, flavor, and texture.

    Although scientific literature on biochemical changes during meatconditioning (ageing) and in some meat products were abundantlyreported during the 1970s and 1980s, little information was availableon the origin of the biochemical changes in other products such ascooked, dry-fermented, and dry-cured meats.

    The need to improve the processing and quality of these meatproducts promoted research in the last decades on endogenousenzyme systems that play important roles in these processes, as hasbeen later demonstrated (Flores and Toldr 1993).

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    1. Muscle

    2. Connective tissue

    Collagen

    Elastin

    Adipose tissue

    Cartilage

    Bone

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    There are 3 types of muscles in animals.

    Striated/voluntary muscle which constitutes meat.

    Smooth/involuntary muscle which is discarded when the animal is dressed.

    And heart muscle.

    Striated muscle is composed of long cylindrical cells, called the muscle fibers,which lie parallel to one another and lengthwise of the muscle.

    Each cell contains a number of nuclei lying close to the outer edge near themembrane or sarcolemma.

    Muscle also contains lipid-phospholipid and cholesterol.

    However determining how much lipid is in the muscle cell and how much in theconnective tissue is difficult.

    Striated muscle is believed to contain about 3% lipid.

    The proteins of muscle cell are numerous and comprise those important incontraction, in the functions of the nucleus, and in the enzymatic reactions of thecell.

    The salts in the muscle cells are the same as those in the most cells of the bodyand are at the same concentration.

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    Potassium is the most common cation, with magnesium and sodium

    following. The anions are acid phosphate, bicarbonate, and sulfate in theorder of their concentration.

    The organic compounds that can be separated from muscle by

    treatment with water and that are not lipid or protein are called

    extractives. These substances appear in the water when meat is stewed, fricasseed,

    or in any way treated with moist heat and in the pan juice when meat is

    roasted or fried.

    Striated muscle has approximately 1% organic extractives and 1% salts. Glycogen is the most abundant extractive in resting muscle, but, as

    pointed out above, the amount decreases after slaughter.

    Several other compounds, also present in small amounts, are anserine,

    carnosine, and carnitine whose structures are shown below.

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    This tissue is composed of a few rather large cells scattered through a matrix

    composed of fibers and amorphous ground substance.

    There are many types of connective tissue; but they all have this common

    structure of few cells and numerous fibers in ground substance.

    The Collagen molecule is a polypeptide chain with a very high percentage of

    glycine residues.

    Beef collagen, which has been studied most extensively contains 19.9% glycine.

    It contains a very high percentage of amino acid residues which have nonpolar

    side chains(glycine, proline, alanine, leucine).

    Polar side chains are those containing hydroxyl groups(hydroxyproline,

    threonine,serine and tyrosine).

    The collagen molecule is coiled and the protofibril is made of parallel molecules

    coiled or twisted together to form a helix.

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    Elastin is the protein that forms the yellow elastic fibers.

    Unlike collagen, it is not hydrolyzed on boiling with water and consequently a tissuethat contains a considerable number of yellow elastic fibers shows little softening or

    dissolving upon cooking.

    Adipose tissue is a specialized type of connective tissue in which the cells ratherthan the intercellular material are most abundant. Fat cells are found scattered ingroups in loose connective tissue.

    Adipose tissue varies in color from very light cream to dark yellow. In general, the

    outer layers of adipose tissue contain fat with higher iodine numbers and lowermelting points than the inner layers.

    Cartilage is also a specialised type of connective tissue, like all other connectivetissue cartilage is composed of cells, fibers, and ground substance. The groundsubstance is a gel composed of chondroitin sulfate, chondromucoid, and albumoid

    in water. Bone is a specialized connective tissue like cartilage, it is composed of cells located

    in lacunae, fibers, and ground substance in which tiny crystals of salts aredeposited. The ground substance is composed of the proteins ossomucoid andossoalbuminoid surrounding small crystals of salts.

    Food gelatin is manufactured from fresh bones by boiling them in water, a processconverting collagen to gelatins.

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    When an animal dies, the skeletal muscles stiffen in rigor mortisand remain inthis condition for a period after which they soften and become flexible again.

    The stiffness that develops when muscles pass into the rigor is the result of

    changes in the proteins.

    Living muscle fibers contain protein in a soft, pliable gel.

    During rigor this gel stiffens, but when rigor passes, the muscle again becomessoft and pliable.

    Finally during cooking, another change called rigor calorisoccurs and the proteinsstiffen again.

    In a living muscle changes occur in the proteins actin and myosin when a musclecontracts.

    The energy for contraction and its accompanying heat production comes

    principally from these high energy phosphate bonds. Resynthesis of ATP in the living cell is at the expense of glycogen which passes

    through a long oxidized by way of the citric acid cycle to CO2 and small bursts ofenergy are released as each carbon and hydrogen are oxidized.

    2(C6H11O5)n + 5[0] 4n CH3COCOOH + H2O

    Glycogen Pyruvic acid

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    The series of chemical reactions occuring after slaughter produces enough heat to

    cause a rise in the temperature of the meat. The average body temperature is 99.7 F in cattle, but shortly after death, the

    internal temperature of a round beef may rise to 103 F.

    The fresh meat cools very slowly even in a refrigerator because of the continuingproduction of heat.

    Meat that develops a relatively high pH is difficult to cure properly because thepickling salts do not penetrate the meat in a normal rate.

    Ripening or Aging:

    After passing of rigor mortis, meat becomes progressively more tender, juicier, andmore flavorful.

    The speed with which this ripening or aging occurs depends on the time thecarcass is kept and the temperature.

    Changes occur quite rapidly at room temperature but more slowly at refrigeratortemperatures.

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    A recent study shows that aging beef at elevated temperatures with high humidity,

    with air velocity 5 to 20 lineal ft/min and with ultraviolet radiation to controlmicrobes for 2 or 3 days produces beef equal in quality to that aged in arefrigerator 12 to 14 days.

    Aging caused an increase in the free amino acid nitrogen.

    The nitrogen in drippings of freshly slaughtered meat was present mostly as non

    protein nitrogen with a large proportion of it as amino nitrogen. After the beef had aged for two weeks, drippings showed an increase in the

    amount of NPN.

    The amino acids histidine, leucine, tyrosine, glutamic acid, and lysine were presentin a bound form.

    Histidine accounted for as much as 26% of the NPN in the extracts.

    It was concluded that part of the bound histidine is present as carnosine.

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    The principal pigment present in muscle cells is myoglobin, a red conjugated proteinclosely related to hemoglobin of the red blood.

    Nitric oxide Denat.Globin Pink

    OxymyoglobinProt. Fe++ Porph.

    O2Bright red

    Nitric Oxide MyoglobinProt. Fe++ Porph.

    NOPink

    HeatDenat.

    MyoglobinProt. Fe++ Porph.

    H2OPurple Red

    MetmyoglobinProt. Fe+++ Porph.

    Brown

    HemeProt. + Fe++

    Porph.

    HeminFe+++ Porph.

    Brown

    denaturation

    oxidation

    oxidation

    reduction

    O2

    Probable Reactions of Myoglobin*

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    The chemical and physical changes occur on cooking are numerous andalthough some studies have been made on this problem the reactions arenot all known or understood.

    They are:

    1) Denaturation of the protein,

    2) Hydrolysis of collagen to gelatin,

    3) Color change,

    4) Formation of grid,

    5) Development of brown flavor,6) Rupture of fat cells and dispersion of fat through the meat, and

    7) Decrease in vitamins and possibly decrease in the nutritive