Physical–chemical comparison of cowʹs milk proteins versus soy proteins in their calcium-binding capacities

In this study, calcium–protein interactions were thermodynamically characterized in cowʹs milk and in (Non-Hydrolysed, NH, or Hydrolysed, H) soy protein dispersions using Isothermal Titration Calorimetry (ITC). Both systems were compared in order to determine the efficiency of soy proteins as a possible vector for further calcium (Ca) supplementation. Regardless of the system (milk, Non-Hydrolysed and Hydrolysed-soy proteins), the thermodynamic signal from Ca–protein interactions was endothermic in nature, and was fitted by a one set of sites model. Whatever the system, μE variations were clearly indicative of Ca–protein interactions of an electrostatic nature. Hence, the exothermic part of the signal due to electrostatic interactions was completely hidden by a strong endothermic signal. Not the Coulomb interactions but instead water molecule release, either from the hydration shells of the calcium ion and/or dehydration of the hydrophobic core of the proteins may be the driving energy source for the Ca binding onto milk or soy proteins. The nature of the Ca–protein interaction was similar in all three systems. Furthermore, as compared to milk protein, containing less Ca bound to the protein, the NH-soy protein system could be a possible protein vector for further Ca supplementation.