Define Precipitation or Solubility of Proteins?
Most of the functional properties are dependent on the degree to which the proteins are soluble. The solubility behaviour provides a good index of potential application of proteins. This is so because the degree of insolubility is probably the most practical measure of protein denaturation and aggregation and because proteins that initially exist in a denatured, partially aggregated state, often have an impaired ability to participate effectively in gelation, emulsification and foaming. Note, in general, proteins, which are highly soluble, may be used in applications where emulsification, whipping and film formation are important whereas low solubility may be desired in applications with high protein levels and when limited emulsification or protein-protein interactions are needed. Solubility of proteins is markedly and irreversibly reduced when heating is involved.
However, heat treatments may be unavoidable to achieve other objectives (microbial inactivation, removal of off-flavor, removal of water and others). Assumption that the proteins must have a high initial solubility as a prerequisite for other functional properties is not always correct. It has already been noted that water absorption of a protein ingredient sometimes can be improved by prior denaturation and partial insolubilization. This is in agreement with the fact that the formation of emulsions, foams and gels can involve various degrees of protein unfolding, aggregation and insolubilization. On the other hand, whey proteins and some other proteins must have reasonably high initial solubility, if they are to function well in the emulsification, foams and gels. Soluble caseinates have better thickening and emulsifying properties than isoelectric casein (less soluble). Perhaps the main advantage of insolubility is that it permits rapid and extensive dispersion of protein molecules and particles which leads to a finely dispersed colloidal system with homogeneous macroscopic structure and smooth texture. Also, initial solubility facilitates protein diffusion to air/water and oil/water interface, thus improving their surface activity.