Biochemistry of Meat Processing

8/2/2019 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 value of theprotein.


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Heat denaturation of protein is a familiar experience in food cookery, but it is

not always accompained by a toughening of the protein.

Denaturation of a protein molecule is believed to be a series of reactions in

which the protein molecule is altered by splitting of some of the links,

particularly some of the hydrogen bonds and sulfur-sulfur links which helpmaintain the three-dimensional shape of the molecule.

The molecular weight of the molecule may not be very much changed in the

first steps of the reactions.

In heat denaturation, as in other types of denaturation, it is believed that

these changes in molecule account for alteration in such properties as

solubility and density.


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Griswold studied a number of factors in beef rounds, commercial and primegrades, cooked by various methods.

Collagen was significantly higher in the raw meat for the commercial gradeover the prime but not for the top round over bottom round.

The loss of collagen on cooking occurred with all the methods and increased

in the internal temperature to which the round was cooked. When pressure cooking was compared, it was found that meat cooked at 10

lbs pressure showed greater losses of collagen than that cooked at 5 lbspressure.

It was higher in meat roasted at 250 F than in that roasted at 300 F or

braised.


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When meat is cooked, oxyhemoglobin and oxymyoglobin(red) and

hemoglobin (purplish red) are denatured.

The ferrous iron in the free porphyrin formed is rapidly oxidised toferric iron of hemin (brown).

Some transformation of oxyhemoglobin, oxymyoglobin, and

myoglobin to met hemoglobin and metmyoglobin (brown) can occur.


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This accounts for some of the change in weight that occurs in a piece ofmeat during cooking.

The drip is composed of water carrying a number of soluble compoundsas well as come coagulable protein and fat.

As cooking proceeds the coagulable protein is denatured by the heat andforms a curd in the pan gravy.

The cut of meat and the amount of fattiness, the method of cooking, theextent to which the piece is cooked all obviously affect the amount of fatin the drip.

There may also be fat degradation products in the drip.

Griswold found that the amount of soluble nitrogen compounds increasedin the drippings during cooking but did not find that free amino nitrogenshowed an increase.


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The change in the flavor that occurs with cooking is one of the mostapparent changes to the man at the table.

Cooked meat likewise has this slightly salty, slightly sweet taste but it alsopossess an aroma that adds greatly to the flavor.

The aroma is composed of low molecular weight, volatile compounds such

as amines, ammonia, hydrogen sulfide, and organic acids.

These compounds probably arise from the cracking of amino acids duringheating.

The reactions may be decarboxylation, deamination, or desulfuring in whicheither the free amino acid or polypeptides react.

The compounds formed vary according to the species, so that the flavor oflamb is recognizable and different from the flavor of beef.


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Fat cells rupture and fat disperses through the meat on

cooking.

The proteins in fat cells of the adipose tissues undergo

denaturation on cooking just as all other proteins in the

meat.

There is a change in the permeability of the cell walls and

fat flows out.


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The B complex vitamins are all more or less sensitive to heating and

thiamin and pantothenic acid are particularly labile.

If cooking is prolonged and if the temperature is high, the destruction of the

vitamins may be appreciable.

There is always some loss.

Niacin and riboflavin are more stable to heat do not disappear as rapidly

during the cooking of meat.

However, the folic acid group is very sensitive to heat, and as much as

90% may disappear.

Ascorbic acid begins to decrease as soon as an animal is slaughtered,

during cooking any that is still present will suffer further diminution.

Cooked meat cannot be counted on to supply more than very small

amounts of ascorbic acid to our diet.


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The reddening does not make the meat harmful or unpalatable.

A red color is produced whenever the meat comes in contact with

nitrites, nitrates, carbon monoxide, or sulfites during preparation sothat hemoglobin react.

This occasionally occurs when nitrites are present in water supplies

in very small amounts.


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This causes an excessive loss of drip and a toughening of themeat.

The meat becomes stringy as the amount of connective tissue fallsand the fat drips out.

Not only are the tenderness and juiciness of the meat diminishedtribute to the flavor are driven off during the excessive cookingperiod, and the meat becomes less and less flavorful.

A decrease in the nutritive value of the protein may also occur.

Overcooking can be the result either of too long a time or too high atemperature.


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The effect of phosphates on the ability of meat to bind water has beeninvestigated in recent years.

Some results appear to indicate that the effect is specific forphosphate, others that it depends on the increased pH which occurs.

Hamm and Grau found that the addition of 1% phosphate to thesodium chloride that is put in meat has a definite effect on the amountof bound water and decreases the loss of drip.

Commercially, phosphate is now added to canned meat as well as to

shrimp. The increased binding of water decreases the shrinkage during

processing and improves tenderness.


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Alexander, J. C. and Elvehjem, C. A., Isolation and

Identification of Nitrogenous Compounds of meat, J. Agr.

Food Chem., 4, 708-711(1956).

Book of Food Chemistry edited by LILLIAN HOAGLAND

MEYER., Professor and Head Dept. of Chemistry.

Book of Food Biochemistry and Food Processing., Editor., Y.

H. Hui.


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Biochemistry of Meat Processing