Tuning the structure of protein particles and gels with calcium or sodium ions

The effect of the addition of NaCl or CaCl2 on the structure of protein particles and gels was investigated in detail for aqueous solutions of the globular milk protein β-lactoglobulin at 40g/L and pH 7.0. When heated in the presence of NaCl or at very low CaCl2 concentrations, the proteins form small strand-like particles, but if more than about two Ca(2+) ions per protein are present, larger spherical particles (microgels) are formed, which increase in size with increasing CaCl2 concentration. The effect of the heating temperature was investigated between 62 and 85 °C. At lower heating temperatures, more Ca(2+) ions per protein are needed to drive the formation of microgels. Particle size measurements done with dynamic light scattering suggest that the aggregation occurs via a nucleation and growth process. The nuclei grow either by fusion or by addition of denatured proteins. If more than three Ca(2+) ions per protein are added, particulate gels are formed by random association of the microgels. Similar particulate gels are also formed at high NaCl concentrations (>200 mM), but by a different mechanism. In this case, the randomly aggregated small strands formed at the early stage of the heating process formed dense spherical domains at a later stage of the heating process by microphase separation that randomly associated to form a particulate gel.