main assignment file

8/7/2019 main assignment file

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The physical and chemical properties of proteins:

The most important physical property of proteins of relevance to food science is the

ionic property which influences solubility of proteins. At pH values close to the isoelectric point

of a protein, its solubility in water decreases due to hydrophobic interactions and

agglomeration. Presence of salts at concentration up to about 0.2M increases protein solubility

due to salting-in but at higher concentrations of salt protein solubility decreases due to salting

out phenomenon.

The important chemical properties of proteins include denaturation, gelation, Maillard

reaction and hydrolysis. Extremes of pH and temperature as well as addition of certain

chemicals such as urea or guanidine at high concentrations or synthetic detergents denature

proteins. Egg white (88% water and 12% protein) on heating becomes a solid gel due to

denaturation of proteins. Denaturation of a protein makes it more amenable for hydrolysis by

proteolytic enzymes, decreases its solubility while intrinsic viscosity increases. Denaturation of

an enzyme results in loss of its enzymic activity.

Proteins with a high degree of asymmetric structure from gels by immobilizing water

in a physically entrapped form. Two well-known examples of proteinaceous gels in foods are

those of casein and gelatin.

Proteins undergo hydrolysis by the action of acids, alkali and proteolytic enzymes. The

peptide bond is hydrolyzed releasing the amino acids.

Essential Amino Acid:

There are nine essential amino acids which are essential for nutrition of human as well

as animals too. They are given below:-

1. Histidine2. Lysine3. Tryptophan4. Phenyl Alanine5. Methionine6. Threonine7. Leucine8. Iso-leucine9. Valine


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Protein:

Proteins are extremely complex nitrogenous organic compound in which amino acids

are the units of structure. Protein accounts for about 1/6th

of the total body weight, 1/3rd

of

which is found in muscles, 1/5th

in the bones and cartilages, 1/10th

in the skin and the

remainder in other tissues and body fluids.

Functions of proteins:

1. To replace the daily loss of body proteins.2. To provide the amino acids necessary for the formation of tissue proteins during

growth.3. To provide amino acids necessary for the formation of enzymes, antibodies, blood

proteins and certain hormones of protein nature.

4. To provide amino acid for the growth of foetes in pregnancy and for theproduction of milk proteins during lactation.

5. Maintenance of tissues already built.6. As regulatory substances for internal water and acid-base balance.7. For energy.

Chemical composition:

The structural units of proteins are amino acids. Proteins contain the elements C, H, O,

N and with few exceptions, S. Most protein also contains P and some specialized proteins

contain very small amount ofFe, Cu and other inorganic elements.

Structure of proteins:

Proteins are built from 20 or more simpler compound called amino acids, which are

also known as building blocks of proteins. The amino acids contain a basic (amino) and an acid

(carboxyl) group in their molecules. The presence of these acidic and basic groups in the

constituent of amino acids is responsible for the amphoteric nature of proteins, which is veryimportant from the biological point of view, as it prevents a sudden change of pH in body.


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Where NH2 is an amino group and COOH an acid or carboxyl group. In common structure of an

amino acid, R indicates the remainder of the molecule, this may be a H atom or a morecomplex group; its structure accounts for the difference in various amino acids.

Classification of proteins:

Because of the complexity of the protein molecule, they are classified on the basis of

their solubilities and physical properties or on the basis of their nutritive value.

The classification of proteins according to the recommendation of the American physiological

Society and the American Society of Biological Chemists id given below-

1. Simple proteins: proteins which yield only amino acids or their derivatives onhydrolysis.

a) Albumins: Soluble in pure water and coagulable by heat e.g.- egg albumin, serumalbumin (blood).

b) Globulins: Insoluble in pure water, but soluble neutral salt solution e.g. serumglobulin (blood), tuberin (potato).

c) Glutelins: Insoluble in all neutral solvents, but soluble in very dilute acids and alkalis.e.g. wheat.

d) Prolamins: Soluble in 70-8 0% alcohol. e.g. gliadin (wheat), and zein (maize).e) Fibrous proteins: proteins characteristic of the skeletal structures of animals and also

of the external protective tissues. e.g. elastin, keratin.

f) Histones: Soluble in water and in very dilute ammonia.g) Protamins: Strongly basic proteins with low molecular weight, soluble in water, not

coagulable by heat, and on hydrolysis yield large amounts of basic amino acids. e.g.

Salmine from salmon sperm.

2. Conjugated proteins: proteins united to some other molecule than as a salt:-a) Nucleoproteins: Compounds of proteins with nucleic acid.b) Glycoproteins: Protein molecule combined with a carbohydrate group. e.g. mucin.c) Phosphoproteins: Compounds of proteins with P. e.g. caseinogens (milk), ovovitelin

(egg yolk).


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d) Haemoglobins: Protein molecule combined with haem. e.g. haemoglobin of blood.e) Lacithoproteins: Combined with lecithin or related substances3. Derived proteins:a) Primary protein derivatives: Derivatives of the protein molecule apparently formed

through hydrolytic changes which involves only slight alterations:

--Proteins: Insoluble products which apparently result from the incipient action of very

dilute acids or enzymes. e.g. casein (curdled milk), fibrin (coagulated fibrinogen).

--Metaproteins: products resulting from the action of acids alkalis whereby the molecule

is sufficiently altered to from protein soluble in weak acids and alkalis, but insoluble in

neutral solvents

--Coagulated proteins: insoluble proteins which results from the action of heat on

protein solution or of alcohol on the protein. e.g. albumin.

b) Secondary protein derivatives: products of further hydrolytic cleavage of the protein

molecule:

--proteose: Soluble in water, not coagulated by heat, precipitated by saturating their

solutions with ammonium sulphate.

--Peptones: Soluble in water, not coagulated by heat and not precipitated. Theserepresent a further stage of cleavage than the proteose.

-- Peptides: These are compounds containing 2 or more amino acids. An anhydride of 2

amino acid is called a dipeptide. If there is 3 amino acids, then it is called tripeptide. If

more, then it is known as polypeptide. Peptides result from the further hydrolytic

cleavage of peptones.

Classification on the basis of nutritive value:

On the basis of nutritive value, proteins are of3 types-

1. Incomplete proteins: These proteins neither maintain life nor support growth. e.g.most vegetable proteins are incomplete, the glycerin of soybeans is an exception. The

zein of maize and animal protein, gelatin are incomplete.


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2. Complete proteins: These proteins maintain life and provide for normal growth ofthe young when used as the only protein in the diet of experimental animals. For

example- all animal proteins except gelatin.

3. Partially Complete or Incomplete proteins: These proteins maintain life but fail tosupport normal growth. Ex- gliadin of wheat, hordein of barley, and prolamin of rye.

Study of milk proteins, egg proteins, cereal proteins, pulse proteins and meat

proteins:

Milk Protein:

Milk of cow is an important protein source for humans and particularly for children. It

is an aqueous solution of proteins (about 27 g/l), lactose, minerals, certain vitamins, emulsified

fat globules and colloidal dispersion of casein micelles consisting of protein with phosphate,

citrate and calcium. Skim milk or skimmed milk is milk from which fat has been removed. Whey

or serum of milk is obtained by removing casein from skim milk by precipitation at a pH of4.6.

Whey obtained as a by- product in cheese making has a different composition as some of the

casein is solubilized and much of the lactose is converted to lactic acid by the action of bacteria.

The important proteins in whole milk are caseins (about 80% of total protein content) and

whey proteins (20%).

Caseins are a heterogeneous group of associated phosphoproteins which are heat stable (well

beyond 1000

C) and include 1-casein (32% of total protein content), 2 casein (8%),-casein

and the (32%) and -casein (8%). Milk contains about 0.1% of calcium ions and the majority of

caseins exist as large particles called casein micelles.

Whey proteins include -lactoglobulin (12%), -lactabumin (4%), immunoglobulins (37%), and

serum albumin (1%).

Milk and its products, which include cheese, yoghurt, whole milk and skimmed milk powders

caseinates and whey powders, are used extensively in the confectionery and baking industries.

Dried acid precipitated caseins are used for fortification of cereals, meat products and bread.

Caseins precipitated by rennin is used for manufacture of cheese.


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Milk whey powder has high nutritional value as the dried powder contains up to 70% of lactose

.The denatured whole whey proteins obtained by heating to 900

C, known as lactabumin, is

used as nutritional supplement in soups, cereals and snack foods. The native whey proteins are

soluble over a wide range of pH and have desirable functional properties. They form foams and

gels and can act as emulsifiers. In the manufacture of Fetta and Camembert cheese, the whey

proteins are retained in the cheese curd by ultra filtration and hence these have enhanced

nutritional value.

Egg Protein:

Hens egg is a complete food for a growing chick embryo throw not complete for

humans. The structure of egg plays an important role in preventing microbial attack on the egg.

The egg consists of three main parts, the shell, the egg white and the yolk. The outer shell is a

hard porous protective layer made of calcium carbonate. The viscous colorless liquid inside the

shell is egg white. The egg white gel is a poor medium for the growth of microorganisms and is

held firmly in position by its attachment to the inner shell membranes of the egg. It accounts

for about 60% of the total weight of the egg. It is a dilute aqueous solution of proteins (12%) ,

dissolved salts and riboflavin. The main protein is ovalbumin, and another protein called mucinis responsible for the viscosity of the liquid. The third component of egg, the egg yolk, is

suspended in the egg white as a yellow or orange oil-in-emulsion stabilized by lecithin and held

centrally by the chalazae, which are anchored to the thick white gel. It is a rich medium for the

growth of microorganisms. The egg yolk is more concentrated source of nutrient containing

about 33% fat, 15% protein and 60% water, in addition to vitamins and minerals. The fat of egg

is concentrated in the egg yolk and it is rich source of cholesterol.

The egg white proteins known as albumin(about ten of them have been identified) include

ovalbumin, ovomucoids, lysozyme, globulins and avidin. Avidin which is present in small

proportion in the egg, has a strong affinity for the vitamin biotin in human diet and makes it

unavailable to the body. However, avidin is inactivated during cooking. Egg yolk of proteins are

the phosphoprotiens called lipovitellin and lipvitellenin. They also contain the lipid lecithin.

The nutritional value of egg is due to the good amount of proteins, iron, phosphorus, fat,

vitamin A and calcium. It Is also a source of vitamin D, riboflavin, thiamine and biotin.


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Ovalbumin, the major protein in egg white is easily denatured at 60%C to form a unique soft

gel, acceptable as a solid food. However, the delicate gelatinous structure denatured egg is

disrupted by freezing. Thus it is not possible to store boiled eggs in the frozen state for use in

convenience foods.

Cereal proteins:

Of the different cereals such as wheat, rye, barley, oats, sorghum, maize and rice, only

wheat and rye can be used for making bread with a leavened, open, crumb structure which is

mainly due to the unique type of proteins in these. The grains (seeds) of cereals have similar

structure consisting of three main components: (1) The embryo of germ of new plant, (2) The

endosperm which is the store of nutrients for the germinating plant and (3) The protective

layers of seed coat called the bran. The endosperm constitutes to about 80% of the bulk of the

grain and is the source of white flour used for making bread. It consists of cells packed with

starch granules lying in a matrix of proteins, which constitute about 7-15% in the flour. Two

types of proteins are available in endosperm: (1) Cytoplasmic proteins, mostly enzymes, which

are soluble in water and dilute salt solutions and (2) storage proteins of about 80% of the

protein content collectively called as gluten (or prolamins). The gluten proteins are insoluble in

aqueous media and responsible for dough formation on mixing the flour with water. Gluten can

be extracted from wheatflour by washing off the soluble material from the dough. The

remaining mass is tough, viscoelastic and sticky, consisting of about one-third gluten proteins

and the rest, water.

Flours from different varieties of wheat vary in their protein content. Flours that are good for

bread making (they give a good loaf volume) are obtained from spring-sown wheat varieties in

North American continent. The protein content is quite high in this type of flours (12-14%).


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relatively poor in the essential amino acidmethionine, although Indian cuisine includes sesame

seeds, which contain high levels of methionine. Grains (which are themselves deficient in lysine)

are commonly consumed along with pulses to form a complete protein of diet.

Meat proteins:

The edible muscle and flesh of cattle, sheep and pig are described as red meat because

of the colour of beef, lamb or pork in contrast to the light or dark colour of poultry meat. A jointof lean meat from the butchers shop devoid of all external fat contains about 18 to 20%

protein. It consists of a number of muscles, each having its own independent attachment to

skeleton, its own blood supply and nerves. A layer of connective tissues consisting of almost

entirely of the protein collagen covers each muscle. Bloods vessels, adipose (fatty) tissues and

nerves also are embedded in the connective tissue. The composition of the muscle tissue is

different from that of the organs, such as kidneys and liver, which are referred to collectively as

offal.

Muscle tissue contains 55-80% of water. The tenderness and juiciness of meat is also influenced

by the effect of the intermolecular interactions between different meat proteins on the

swelling of meats and its water holding capacity. Protein denaturation on cooking releases

some of this water to make this meat its desirable moist appearance. Water also comes out

joint of meat is cut in the shop giving meat a attractive moist appearance.

The tenderness of meat is also finally determined by cooking. Meat cooked to an internal

temperature of60C is described as rare, whereas at 80C the meat is well cooked and changes


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to a more granular texture, although the softer fibrillar structure is a highly desirable

characteristic.

The connective tissue also contains invisible fat which makes digestion of muscle more difficult

as it coats the muscle fibres with a thin oily film. A much larger amount of fat is stored in fat

depots of the animal mainly under the skin and around the internal organs and therefore, fat is

not a part of lean meat. Beef, lamb and pork contain more of saturated fats while chicken

contains more of polyunsaturated fatty acids (PUFA).

The red colour of meat is due to interaction of myoglobin with oxygen to form oxymyoglobin.

Higher concentrations of of oxygen give meat a desirable bright red colour whereas meat at

anaerobic conditions is purple in colour. Cut meat on aging at room is brown in colour due to

oxidation of myoglobin. When meat is cooked the myoglobin is denatured along with most

other proteins.

Meat contains minerals and vitamins. It is a good source of iron, zinc and vitamin of B group,

particularly niacin. The attractive flavor of meat is largely due to a variety of substance known

as meat extractive that are soluble in water. This include compounds produced during muscular

activity such as lactic acid, those derived from ATP, protein metabolism, amino acid such as

glutamic acid and urea. Meat extractives aid digestion by stimulating the secretion of saliva and

gastric juice.

GEL

FORM

ATION:

Gel formation is a very important process in food chemistry .Not only do the

properties of living cell, both animal and vegetable depend on the gel structure, but in food

preparation the stiffening which occurs during meat and flour cookery, the rigidity of pectin and

starch, gels, the high viscosity of many plant juices, the change that occur in egg cookery and

many other processing operations are a function of gel.

A gel is a remarkable phenomenon, displaying the property of rigidity, sometimes at quite low

concentrations of solute and yet often showing the properties of the solvent practically

unchanged. For example, most gels in water show vapor pressure and electrical conductivity

very close to water. Gel formation occurs in gelatin dispersions in water with as low a

concentration as 1 per cent gelatin and in plasma with a fibrinogen concentration of 0.04 %

.The phenomena displayed by gels are complex are not always the same with different gels.


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THEORIES OF GEL FORMATION :

Many attempts to explain the properties of a gel have been made, and through the years a

number of theories have been proposed and argued back and forth. Three major theories are

now supported by colloid chemists. These theories are

1. Adsorption of solvent

2. Three dimensional network formation

3. Particle orientation.

1. Adsorption Of Solvent :

This theory postulates that adsorption of solvent molecules by the solute particles results, on

cooling, in the formation of larger and larger particles with increasing layers of solute. The

enlarged particles eventually touch or overlap enclosing more solvent, so that the entire system

is immobilized and rigidity occurs. Support of this theory depends on demonstrating that

adsorption of solvent molecules is very extensive and that the adsorption increases with

decreased temperature.

2. Three - dimensional Network :

This theory postulates that the compound capable of gelation is either fibrous in structure or

can react with itself to form a fiber. On cooling the fibers form a three- dimensional network by

reacting either at widely separated intervals on the chain or at relatively small distances. The

bonds established which tie the fibers into the three-dimensional network can be either

primary bonds between functional groups, secondary bonds, such as hydrogen bonds, or

nonlocalized secondary attractive forces such as might occur between alkyl groups.

(a) The first type of bond would yield a network that would possess considerablepermanence. This type could only occur in gels that are not readily dissociated by

cutting or beating, for example. They would capable of swelling and shrinking, within

limits.

(b) A secondary type bond is one of considerably lower strength a primary valance bond,and these gels would consequently be broken by relatively small forces.


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(c) The third type of bond, a nonspecific attraction between portions of the molecules oralong the entire molecule, can explain gelation in a few situations. The properties of

this type of gel would depend on a nice balance of forces-the solute molecules would

attract one another at same spots but would separated by solvent molecules that areattract at others. The result would be gelation if the solvent molecules were too

strongly attracted to the solute, then the solute molecules would have no opportunity

to contact one another and a network would be impossible. Under these conditions a

gel would not form. On the other hand, if the attraction of the solute molecules one

another were too strong, dispersion in the solvent would not occur. A precipitate or an

insoluble compound will be formed rather a gel. This theory can explain gel formation in

systems that are very demanding as far as temperature, concentration, pH, and salt

concentration are concerned.

3. Particle orientation:

This theory postulates that in some systems there is a tendency for the solute and solvent

particles to orient themselves in definite special configurations through the influence of long

range forces such as occur in crystals. Certain protein crystals have the ability to take on or lose

water without distortion of the crystal. They may form structure of this type. Tobacco mosaic

virus has been studied and found to form gel under a large range of concentrations. X-ray

diffraction studies indicate that the gels possess two-dimensional lattice. This is interpreted to

mean that the particles are oriented in one direction.

Denaturation of protein:

Denaturation is a process in which proteins or nucleic acids lose their tertiary structure and

secondary structure by application of some external stress or compound, such as a strong acid orbase, a concentrated inorganic salt, an organic solvent (e.g., alcohol or chloroform), orheat. Ifproteins in a living cell are denatured, this results in disruption of cell activity and possibly cell

death. Denatured proteins can exhibit a wide range of characteristics, from loss of solubility tocommunal aggregation.

This concept is unrelated to denatured alcohol, which is alcohol which has been mixed withadditives to make it unsuitable for human consumption.


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When food is cooked, some of its proteins become denatured. This is why boiled eggs becomehard and cooked meat becomes firm.

A classic example of denaturing in proteins comes from egg whites, which are largely eggalbumins in water. Fresh from the eggs, egg whites are transparent and liquid. Cooking thethermally unstable whites turns them opaque, forming an interconnected solid mass. The same

transformation can be effected with a denaturing chemical. Pouring egg whites into a beaker ofacetone will also turn egg whites opaque and solid. The skin which forms on curdled milk is

another common example of denatured protein. The cold appetizer known as ceviche is preparedby chemically "cooking" raw fish and shellfish in an acidic citrus marinade, without heat.[1]

Although denaturing egg whites is irreversible, in many other cases denaturing is reversible.

Denatured proteins can exhibit a wide range of characteristics, from loss of solubility to communal

aggregation. Communal aggregation is the phenomenon of aggregation of the hydrophobic proteins tocome closer and form the bonding between them, so as to reduce the total area exposed to water.

Proteins are amino acid polymers. A protein is created by ribosomes that "read" RNA that is

encoded by codons in the gene and assemble the requisite amino acid combination from thegenetic instruction, in a process known as translation. The newly created protein strand then

undergoes posttranslational modification, in which additional atoms ormolecules are added, forexample copper, zinc or iron. Once this post-translational modification process has been

completed, the protein begins to fold (spontaneously, and sometimes with enzymatic assistance),curling up on itself so that hydrophobic elements of the protein are buried deep inside the

structure and hydrophilic elements end up on the outside. The final shape of a protein determines

how it interacts with its environment.

When a protein is denatured, the secondary and tertiary structures are altered but the peptidebonds of the primary structure between the amino acids are left intact. Since all structural levels

of the protein determines its function, the protein can no longer perform its function once it hasbeen denatured. This is in contrast to intrinsically unstructured proteins, which are unfolded in

theirnative state, but still functionally active.

Renaturation:

Renaturation is the opposite of denaturation for example in proteins. Basically in proteins if thepolypeptide chain has been broken through denaturation, sometimes it is possible to be renatured

or rebuilt to form the polypeptide chain.


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N-Terminal residue of a peptide:

Very important in the procedure for establishing amino acid sequence are

methods for identifying the terminal amino acid residues. The first useful method for the N-

terminal residue of polypeptides was described by Sanger, who found that the free

unprotonated -amino group of peptides reacts with 2, 4-dinitrofluorobenzene (DNFB) to form

yellow 2, 4-dinitrophenyl derivates. When such a derivative of a peptide, regardless of its

length, is subjected to hydrolysis with 6 N HCl, all the peptide bonds are hydrolyzed, but the

bond between the 2,4-dinitrophenyl group and the -amino group of the N-terminal amino acid

is relatively stable to acid hydrolysis. Consequently, the hydrolyzate of such a dinitrophenyl

peptide contains all the amino acid residues of the peptide chain as free amino acids accept theN-terminal one, which appears as the yellow 2, 4-dinitrophenyl derivative. This labeled residue

can easily be separated from the unsubstituted amino acids and identified by chromatographic

comparison with known dinitrophenyl derivatives of the different amino acids.

C-Terminal residues of a peptide:

The C-terminal amino acid of peptides can be reduced with lithium borohydride to

the corresponding -amino alcohol. If the peptide chain is then completely hydrolyzed, the

hydrolyzate will contain one molecule of an -amino alcohol corresponding to the original C-

terminal amino acid. This can be easily identified by chromatographic methods; all the other

residues will be found as free amino acids. Another important procedure is hydrazinolysis,

which cleaves all the peptide bonds by converting all except the C-terminal amino acid residues

into hydrazides. The C-terminal amino acid appears as a free amino acid, which can be readily

identified chromatographically. The C-terminal amino acid of a peptide can be


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Selectively removed by action of the enzyme carboxy-peptidase, which specifically attacks C-

terminal peptide bonds.

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