The effect of the competition for calcium ions between κ-carrageenan and β-lactoglobulin on the rheology and the structure in mixed gels

Abstract

Protein aggregates were formed by heating the globular protein β-lactoglobulin (β-lg) in aqueous solution in the presence of CaCl2 leading to small strands at low CaCl2 concentrations and larger spherical microgels at higher CaCl2 concentrations. The structure of mixtures of protein aggregates and the polysaccharide κ-carrageenan (κ-car) was investigated at pH 7 using confocal laser scanning microscopy. Microphase separation in the form of dense protein domains was observed for mixtures with microgels even at very low κ-car concentrations (0.5 g/L). Gelation of κ-car was induced by cooling in the presence of 10 mM KCl. Addition of microgels strongly increased the gelation rate and the elastic modulus of the κ-car gels, while the effect of adding native proteins or strands was small. The effect could be attributed to the presence of calcium ions used to form the microgels. However, the effect was smaller than in pure κ-car gels if the same amount of calcium ions was added in the form of CaCl2. The effect is explained by the competition between κ-car and β-lg for calcium ions. A weaker effect was observed with native β-lg, which can be correlated with the weaker binding of Ca2+ by native β-lg compared to microgels. Gelation of β-lg in the presence of κ-car was induced by heating mixtures of κ-car and native β-lg. The presence of κ-car facilitated gelation of β-lg at lower CaCl2 concentrations. κ-car was gelled within the β-lg gel by cooling. It was found that the elastic modulus of the mixed gel was close to the sum of that of the κ-car gel with native β-lg and the β-lg gel with uncrosslinked κ-car coils.