Characterization of β-lactoglobulin–sodium alginate interactions in aqueous solutions: A calorimetry, light scattering, electrophoretic mobility and solubility study

The influence of pH (3–7) on the properties of sodium alginate (NaA), β-lactoglobulin (β-Lg) and their mixtures in aqueous solutions was studied at 30 °C using isothermal titration calorimetry (ITC), dynamic light scattering, turbidity, ζ-potential and soluble protein measurements. The electrical charge of isolated β-Lg went from positive to negative as the pH increased from 3 to 7 with the isoelectric point being around pH 4.8. The electrical charge of isolated sodium alginate was negative at all pH values, but was appreciably lower in magnitude from pH 3 to 4 due to partial protonation of carboxyl groups (pKa≈3.5). Light scattering measurements indicated that isolated sodium alginate was completely soluble from pH 3 to 7, but isolated β-Lg formed large complexes (dV>200 nm) that scattered light at pH 4 and 5. When β-Lg and sodium alginate were mixed together at pH 3 and 4 they formed large complexes (dV>1000 nm) due to electrostatic attraction between the oppositely charged molecules. At pH 5, β-Lg and sodium alginate formed fairly soluble complexes due to electrostatic attraction between the anionic polysaccharide and cationic patches on the protein surface. At pH 6 and 7, β-Lg and sodium alginate did not form complexes due to electrostatic repulsion between the similarly charged molecules. The knowledge gained in this study could be used to facilitate the rational design of food ingredients and products with desirable functional properties.