Protein-protein Interation

The protein-protein interaction and water protein interaction play a major role in determining gel strength and quality. It is obvious that by changing particular parameters (both intrinsic and extrinsic) the gel properties can be changed). Therefore it could be discussed first how protein interactions originate. A part from peptide bonds linking up aminoacids, only one type of covalent bond may impact the interaction between proteins: these are the disulphide bonds between two residues (SH group). As aaminoacids and their side groups can be charged (due to protonation + charge, due to proton loss: -charge), obviously the carge intermediation are also potentially important; moreover these can be attractive or repulsive.  H-bridge formation between  a H donating (eg. OH group) and a H- acceptor (eg. C=O group) support protein protein interaction too. Temporary dipole interactions between phenyl group from an aromatic amino acid such as tryptophan, tyrosine or phenilaanine are also relevant, but already considerably weaker. So are the lipophilic interactions between the side changes of a-polar amino acids such as leucine, alanine, iso leucine, etc.

Having these interactions in mind it is clear that for instance, by changins the PH of a protein solution, protein-protein interactions may change considerably and thus conduce to gel formation. A typical example can be found in the fresh cheese (cottage cheese) production of yogurt. In fact a combination of other charge interactions and an increased lipophilic interaction between the casein proteins are at the origin of this gelation.

By denaturing proteins, new disulphide bonds can be produced between proteins, ot disulphide bonds I proteins can be broken, thus facilitating the interaction with other proteins (because the protein chain has increased its flexibility, it can move freely in the 3D space). Tus gels can be induces. A typical example is boiled egg, resulting from gelation of ovoalbumin.

As the concentration of ions (salts) impacts protein protein interactions (the higher the salt  concentration the higher the protein-protein interaction) the concentration of salt can have an impact on gel formation.

Upon protein degradation (during cheese fermentation) in which part of the proteins ae hydrolised, shorter peptide chains are produced, thus lowering protein protein interactions. These results in gel losing its strength: the interior of the cheese becomes liquid.

Another example of Gel is gelatin: it is a special case because it is produced by parcial hydrolisis of collagen, increasing its solubility in water and therefore decreasing protein-protein interaction in binding tissue.

Protein-protein interactions can also be influenced by adding crosslinking agents, thus influencing gel formatin potential.