Food Protein Gels
To build a model of protein gelation it helps to start with a model for proteins themselves. For most globular proteins we can start by ignoring the details of sequence and conformation and imagine most globular proteins as fairly dense particles a few nanometers in diameter with a hydrophobic core and a more hydrophilic surface; in effect colloidal particles rather than polymers. Depending on the protein these particles can be more or less water soluble but most gels require them to be able to form at least a stable suspension at a reasonable concentration (~10% protein).
To form a gel (i.e., trigger a sol-gel transition) the dispersion must be partly destabilized so the protein particles, rather than repelling one another, become somewhat “sticky” and aggregate. Different protein dispersions can be destabilized in different ways but common methods include denaturing the protein to properly expose the hydrophobic core (e.g., cooking egg), changing the pH to the isoelectric point to minimize inter-protein electrostatic repulsion (e.g., yogurt) or in some cases adding calcium to bind certain amino acid residues (e.g., forming a tofu gel) is useful. In other cases covalent bonds may form between proteins (e.g., forming gluten while kneading bread).
As destabilization proceeds, the aggregates become larger and larger and the suspension becomes more viscous. Eventually the aggregates will become so large they span the container giving an overall solid-like texture while trapping large amounts of water, i.e., a gel. If the associations between the proteins are too strong or too extensive then they may precipitate as a way of maximizing protein-protein interactions at the expense of the protein-water interactions needed in a gel.
The strength of the gel depends on the amount of material involved in the network and on the strength of the connections between proteins. Globular protein gels of this type typically need more polymer to form a gel than the polymer gels seen in polysaccharides and synthetic polymers (~10% vs 1-2%) and are often opaque as the protein aggregates can scatter light. Anything that disrupts the bonds between protein particles will weaken the gel.