Specific effect of calcium ions on thermal gelation of aqueous micellar casein suspensions

Abstract : Aqueous suspensions of micellar casein (MC) gel when heated above a critical temperature that depends on the pH. The effect of adding CaCl2 and EDTA on thermal gelation was studied in order to assess the effect of increasing or decreasing the amount of bound Ca2+ on the process. The effect of adding NaCl was investigated in order to distinguish the effect of Ca2+ binding from the effect of screening of electrostatic interactions. Shear moduli were measured as a function of the temperature during heating ramps up to 90 °C. Gel formation was characterized by a sharp increase of the storage modulus at a critical temperature (Tc). In most cases, prolonged heating at 90 °C did not cause a significant increase of the gel stiffness. Addition of CaCl2 caused a decrease of Tc, whereas addition of NaCl caused an increase of Tc. Chelation of Ca2+ by EDTA led to a strong increase of Tc even if the fraction of chelated Ca2+ was small and the micelles remained intact. The gel stiffness was found to increase weakly with increasing CaCl2 concentration up to 20 mM. The gels persisted during cooling to 20 °C with an increase of the elastic modulus by a factor between 4 and 6 depending on the CaCl2 concentration. The results demonstrate that Ca2+ plays a crucial role in thermal MC gelation.