1. Food Biotechnology Dr. Tarek Elbashiti Food Biochemistry 3
Proteins

2. 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.

3. Protein Foods
Plant Sources
Animal Sources

4. 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

5. Amino Acid Structure

7. Peptide Bond All protein amino acids joined together by peptide bond.
Dipeptide
2 amino acids joined together
Polypeptide
several amino acids joined together.

8. Protein Structure
Conformation
Primary
Secondary
Tertiary
Quaternary

9. 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

10. 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.   
regular repeating structures that include beta pleated sheets and alpha helix coils which are 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.  In some proteins, part of the polypeptide will form secondary structures will other parts will not.  Some proteins do not form secondary structures at all while other proteins are almost all secondary structures. 

11. Secondary Structure

12. 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.

13. Tertiary Structure The intramolecular bonds can include ionic bonds,
hydrogen bonds,
disulfide bonds (often called bridges),
and hydrophobic interactions.

15. 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.

16. Quaternary Structure

17. REACTIONS AND PROPERTIES OF PROTEINS
1. Amphoteric
Able to act as acid or base.
Enables them to resist small changes in pH.
Buffering capacity

18. The pH at which the protein is electrically neutral.
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.

19. 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.

20. 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.

21. Occurs at high salt concentrations
Salting Out
Occurs at high salt concentrations
Salt 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.

22. 5. Denaturation
Change 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.

23. 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.

24. Another form of coagulation occurs in the production of cheese
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.

25. 7. Gluten formation
Two 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%.

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

27. 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.

28. 9. Hydrolysis of peptides and proteins
Breaking peptide bonds to form small peptide chains
Acid digestion using concentrated acids.
Used mostly in research not in Food processing.
Catalysed by proteolytic enzymes
Example 1 - (ficin, pappin and Bromelin)
Used as meat tenderisers
Example 2 – Rennet
- Used to make curd which can be processed into cheese.

29. 10. Maillard Browning Non enzymatic browning
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

30. 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

31. 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.

32. Whey Protein
Solubility (does not precipitate at isoelectric point)
Fortify acidic beverages (sports drinks)
Nutritional fortifier in baked products.
Egg Proteins
Thickening
Binding
Egg yolk – best emulsifying agent
Egg White – best foaming agent

33. Egg whites upon heating form firm gel (boiled egg)
Gelatin and Egg White
Gelling agents
Egg 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.

34. 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.