Food BiotechnologyDr. Tarek Elbashiti

Food Biochemistry 3Proteins

Proteins

Complex molecules widely distributed in all food stuffs.

Every Protein has a unique structure and confirmation or shape therefore carry out specific functions.

All proteins are made up of amino acids.

Protein Foods

PLANT SOURCES ANIMAL SOURCES

Amino Acids Contain carbon, hydrogen, oxygen and nitrogen. Some

also contain sulfur Each amino acid is composed of a carbon bonded to four

groups The other three

- a nitrogen group (-NH2) called an amino group (or amine)

- an acid group (-COOH)- a hydrogen (-H) - R group – determines type and name of amino acid

Amino Acid Structure

Peptide Bond All protein amino acids joined

together by peptide bond.

Dipeptide 2 amino acids

joined together Polypeptide several amino acids

joined together.

Protein Structure

Conformation Primary Secondary Tertiary Quaternary

Primary Protein

Specific sequence of amino acid joined by peptide bonds along the protein chain.

If one single amino acid in the chain was changed it could completely alter the structure and function of the protein

Secondary Protein

Regular repeating structures beta pleated sheets alpha helix

stabilized by hydrogen bonds between groups in the main chain of the polypeptide. 

The hydrogen bonds form between the oxygen in the carboxyl group of one amino acid and the hydrogen in the amino group of another amino acid.   

Secondary Structure

Tertiary Structure Three dimensional organization of a

complete protein chain.Spatial arrangement of a protein that

contains regions of a beta pleated sheets and alpha helix.

Tertiary structure is build on the secondary structure of a specific protein.

Tertiary Structure

The intramolecular bonds can include ionic bonds, hydrogen bonds, disulfide bonds

(often called bridges),

and hydrophobic interactions.

Quaternary Structure Two or more polypeptides linked together to form

a single protein.  All types of bonding previously mentioned are

involved with quaternary proteins.  Sometimes these proteins will contain a prosthetic

group, also known as a non-polypeptide structure.  For example

in hemoglobin, the four polypeptides are linked to a heme (haem) group which is not made up of amino acids. 

These proteins that contain a prosthetic group are often referred to as conjugated proteins.

Quaternary Structure

REACTIONS AND PROPERTIES OF PROTEINS

1. AmphotericAble to act as acid or base.Enables them to resist small changes

in pH.Buffering capacity

2. Isoelectric Point The pH at which the protein is

electrically neutral. The overall charge is neutral. At isoelectric point, protein molecules

precipitate as they carry no net charge. pH of isoelectric point differs for each

protein. Important in food processing for cheese

production. Lactic acid is added to milk to bring pH to

isoelectric point of major milk protein (casein) – precipitate and form curd.

3. Water Binding Capacity Water molecules can bind to the

backbone and to polar and side chains of a protein.

Protein may bind varying amounts of water . Protein with many charged and polar

groups bind water rapidly. Proteins with many hydrophobic groups do

not bind much water. Protein close to isoelectric point bind less

water.Presence of bound water helps to

maintain the stability of a protein dispersion.

4. Salting In and Salting Out Some protein cannot be dispersed in

pure water.Readily dispersed in dilute salt solution.Salting In Salt solution increases the dispersibility

of a protein.Brine injected into Ham to increase the

dispersibility of protein.Increases water binding capacity thus

Ham is moister and weight is increased.

Poultry – polyphosphates are added.

Salting Out Occurs at high salt concentrationsSalt competes with protein for water. Insufficient water available to bind to

the protein so protein precipitates.Not normally a problem in food

processing .Contributing factor to deterioration of

food quality during freezing of food.During Freezing

Water is effectively removed as ice crystals

Concentration of liquid water decreases The solute concentration increases.

5. DenaturationChange in structure of protein molecules. The process results in the unfolding of molecules. Factors which contribute to denaturation are

heat, salts, pH mechanical action.

Denaturation is a partially reversible change. For example, when an egg white is whisked it incorporates air to form a foam.

If the foam is left to stand, it will collapse back to form liquid egg white.

6. Coagulation

Coagulation follows denaturation.

Example, When egg white is cooked it

changes colour and becomes firmer or sets.

The heat causes egg proteins to unfold from their coiled state and form a solid stable network.

This change is irreversible.

Another form of coagulation occurs in the production of cheese. Rennin (an enzyme from a calf’s stomach) is added to milk causing the protein casein to clot, producing curds (solid) and whey (liquid).

Other applications of coagulation are:• yogurt production;• thickening of sauces with beaten egg;• binding ingredients together, e.g. fish, cakes,reformed meats;• providing a coating for products, e.g. scotch eggs.

7. Gluten formationTwo proteins, gliadin and glutenin, found in

wheat flour, form gluten when mixed with water.

Gluten is strong, elastic and forms a 3D network in dough. In the production of bread, kneading helps untangle the gluten strands and align them.

Gluten helps give structure to the bread and keeps in the gases that expand during cooking. The amount and type of protein present depends on the flour type and quality.Strong flour contains a maximum of 17% protein, plain flour 10%.

Stages of Gluten Development

Products that require short or non-elastic textures, such as biscuits and cakes, use flours with lower protein contents.

8. Gelation

Gelatine is a protein which is extracted from collagen, present in connective tissue in meat.

When it is mixed with warm water the gelatine protein molecules start to unwind.

Although on cooling a stable network is formed, trapping the liquid.

Gelation is reversible.

9. Hydrolysis of peptides and proteins

Breaking peptide bonds to form small peptide chains

1.Acid digestion using concentrated acids. Used mostly in research not in Food

processing.2.Catalysed by proteolytic enzymes

Example 1 - (ficin, pappin and Bromelin) Used as meat tenderisersExample 2 – Rennet - Used to make curd which can be processed

into cheese.

10. Maillard Browning

Non enzymatic browning.

Reaction responsible for brown color of baked products.

Free carbonyl group of reducing sugar reacts with a free amino group of protein

When heated the result is brown color

11.Enzymes All enzymes are proteins.Important In food processing as they

catalyse various reactions that affect Color Flavour Texture Overall Quality of foods

Reactions may be desirable or undesirable.

Produce unwanted discoloration or off flavors in foods

Functional Roles of Proteins in Foods

Functional Property A characteristic of the protein that

enables it to perform a specific role or function in food.

Dependent on the amino acid composition and sequence as they determine the conformation and properties of the protein.

Whey Protein Solubility (does not precipitate at

isoelectric point)Fortify acidic beverages (sports

drinks)Nutritional fortifier in baked

products.Egg ProteinsThickening Binding Egg yolk – best emulsifying agent Egg White – best foaming agent

Gelatin and Egg White Gelling agentsEgg whites upon heating form firm

gel (boiled egg)Gelatin used to make jelly and

other congealed products. Gels formed when protein molecules

form 3D network due to association of hydrogen bonds.

Protein Foods

Protein Foods used in many foods to control texture due to their ability to : Thicken Gel Emulsify

Such food products must be processed, handled and stored with care to ensure that the proteins retain their functional properties.