Protein summary for Food Physics (FPH20306)

Chapter 5: Proteins
The role of proteins in food
1. Nutritional value: Source of amino acids
2. Thickening agent: Increased viscosity to influence its sensory perceptions Functional
3. Structuring: emulsions, foams, gels ingredient
Functional ingredient: protein is added to give the system a certain physical property

The physical properties of the protein are mainly determined by the structure of the protein
- The structure is determined by physical conditions
o Temperature
o pH
o ionic strength
o presence of surfactants
o presence of reacting agents (breaking S-S bridges)
o quality of the solvent

Protein structures
4 levels of structure in proteins;
1. Primary structure
o Describes the sequence of amino acids and the location of the S-S bridges
o Chemical structure of the protein
o Formed by covalent chemical bonds (peptide bonds)
o Difficult to take apart, reactants are needed that can break S-S bridges or enzymes

2. Secondary structure
o The local arrangement of chain segments that are close to each other
o These chain segments can arrange themselves to form an α-helix or β-sheet
o α-helix chain segments assume a 3D helix structure in order to create more space
for large side groups
o β-sheet: two or more chain parts arrange themselves in a stretched, plate-like
manner
o both structures are stabilized by a large number of hydrogen bonds within the chain
o secondary structure is stable (less stable than the primary structure because
covalent bonds are stronger than hydrogen bonds)

3. Tertiary structure
o 3D arrangement of α-helices, β-sheets and loops in space
o Stabilized by
 hydrogen bonds
 hydrophobic interactions
 Van der Waals interactions

4. Quaternary structure
o Arrangement of tertiary structures in space

*NOTE: secondary + tertiary structures = CONFORMATION of the protein

, Classification of proteins based on conformation
Distinguish two types of proteins (in order of decreasing complexity)

1. Globular proteins
2. Random coil proteins

Globular proteins
- Rich in secondary and tertiary structures
- Folded into a compact, spherical shape with hydrophobic amino acid residues in the core
and hydrophilic residues with polar groups and charges located on the outer layer creating a
hydrophilic shell
- Because they are compact they show very little swelling in water and therefore when
dissolved as monomers give only a small increase in viscosity
o Not very effective thickening agents
- Examples: Ovalbumin, β-lactoglobulin




- Globular proteins of high molecule weights often fold themselves into chains of several
spheres
- To make a stable solution of a globular protein, the individual molecules must be dispersed
in the solvent
- A solution of a globular protein is a colloidal system and can therefore the stability can be
described using the DLVO theory
- The interaction potential of two dissolved globular proteins is given by;
𝑉𝑇𝑂𝑇 = 𝑉𝐴𝑇𝑇 + 𝑉𝑅𝐸𝑃

 For native globular proteins attractive forces are mainly determined by Van
der Waals interactions
 Other contributions to attractive forces are from hydrophobic interactions,
hydrogen bonding, Sulphur and salt bridges
 Repulsion is mainly determined by electrostatic repulsion and Born
repulsion
 Electrostatic repulsion is a result of the charges in the outer “shell” of the
globular protein
- The strength of the electrostatic repulsion depends on the surface potential of the globular
proteins and the Debye length (the thickness of the diffuse double layer)
o The surface potential mainly determined by the pH of solution
o Debye length is determined by the salt concentration/ionic strength