Casein precipitation equilibria in the presence of calcium ions and phosphates

Calcium ion induced aggregation and precipitation equilibria of b-casein were studied in the presence or absence of monoand polyphosphates at pH 5.5 and 7.5. We analyze the data by assuming ion binding at a well-defined stoichiometric ratio, leading to protein charge neutralization and formation of slightly soluble complexes. The precipitation curves are in good agreement with the model, showing the expected larger induction region and steeper slope at the lower pH. The addition of phosphates leads to better precipitation of the protein, which can be explained by a mechanism involving the formation of calcium phosphate microcrystals. These crystals provide a substrate for protein adsorption, with subsequent cross-binding of the casein micelles and formation of sturdy aggregates of co-precipitated calcium phosphate and casein. The crystallite formation leads to effective separations in commercial caseinates, which would be impossible with Ca alone. # 2003 Elsevier Science B.V. All rights reserved.