Effect of pH on the interactions of sodium caseinate with soy phospholipids in relation to the foaming ability of their mixtures

We report on the effect of pH (from 7.2 to 5.0) at the same ionic strength (I=0.05 M) of the buffered solutions on the binding of the mixed soy phospholipids and a pure phosphatidylcholine with sodium caseinate in an aqueous medium. The binding isotherms and Scatchard plots suggest an increase in both the protein binding affinity and the cooperative character of the binding relative to the phospholipids with increasing pH from 5.0 to 7.2. As this takes place, it was established that phosphatidylcholine determined basically the binding of the phospholipids by the protein. The pH and temperature dependencies of the mixing calorimetry data postulate the prevailing role of electrostatic interactions between the phospholipids and sodium caseinate. Moreover, the concentration dependencies of the enthalpies of the interactions between the protein and the phospholipids normalized to the concentration of the phospholipids suggest some changeover in the aggregation state of the phospholipids in the interior and/or at the protein surface. The marked effect of the protein–phospholipids interactions on the molecular parameters (the weight-average molecular weight, the gyration and hydrodynamic radii) and the thermodynamic affinity for an aqueous medium of the sodium caseinate nanoparticles were determined by a combination of static and dynamic laser light scattering. In addition, the more compact structures of the protein nanoparticles were formed in the complexes with the phospholipids. In response to the complex formation, the more stable and fine foams have been detected in the case of the mixtures of sodium caseinate with the low concentrations of the phospholipids.