Thermodynamic characterization of acacia gum-beta-lactoglobulin complex coacervation

The interactions of beta-lactoglobulin (BLG) with total acacia gum (TAG) in aqueous solutions have been investigated at pH 4.2 and 25 degrees C. Isothermal titration calorimetry (ITC) has been used to determine the type and magnitude of the energies involved in the complexation process of TAG to BLG. Dynamic light scattering (DLS), electrophoretic mobility (mu(E)), turbidity measurements (tau), and optical microscopy were used as complementary methods on the titration mode to better understand the sum of complicated phenomena at the origin of thermodynamic behavior. Two different binding steps were detected. Thermodynamic parameters indicate a first exothermic step with an association constant K(a1) of (48.4 +/- 3.6) x 10(7) M(-1) that appeared to be mostly enthalpy-driven. A positive heat capacity change was obtained corresponding at the signature for electrostatic interactions. The second binding step, 45 times less affinity (K(a2) = (1.1 +/- 0.1) x 10(7) M(-1)), was largely endothermic and more entropy-driven with a negative value of heat capacity change, indicative of a hydrophobic contribution to the binding process. The population distribution of the different species in solution and their sizes were determined through DLS. Dispersion turbidity of particles markedly increased and reached a maximum at a 0.015 TAG/BLG molar ratio. Largely more numerous coacervates appeared at this molar ratio (0.015) and two different kinds of morphologies were noticed for the large coacervates. Above the TAG/BLG molar ratio of 0.015, dispersions turbidity decreased, which might be due to an excess of negative charges onto particles as revealed by electrophoretic mobility measurements. The results presented in this study should provide information about the thermodynamic mechanisms of TAG/BLG binding processes and will facilitate the application of the formed supramolecular assemblies as functional ingredients in food and nonfood systems.