Calcium-induced gelation of whey protein aggregates: Kinetics, structure and rheological properties

Cold gelation of preformed whey protein aggregates was induced by adding CaCl2. The gels were investigated using rheology and confocal laser scanning microscopy in order to study the evolution with time of the elasticity and the microstructure as a function of the protein and calcium concentrations, the size and shape of the aggregates and the temperature. The net charge density of the proteins was fixed, but the effect of adding CaCl2 on the pH was investigated and was related to specific binding of Ca2+ to the proteins. The gelation process was correlated with the effective charge density of the proteins that was reduced by specific binding of Ca2+ to the proteins. Increasing the temperature or the CaCl2 concentration ([CaCl2]) strongly increased the rate of gelation, but did not influence the stiffness or the structure of the gels. The temperature dependence of the gelation time was characterized by an activation energy of 210 kJ/mol independent of the CaCl2 ([CaCl2] = 2–50 mM) and protein concentrations (C = 5–60 g/L). Increasing the protein concentration not only sped up gelation, but also increased the gel stiffness and rendered the gels more homogeneous. The size of fractal aggregates did not influence the gel structure, the gel stiffness and very little the gelation rate. Gelation of fractal aggregates was compared with that of microgels, which formed weaker and more heterogeneous gels at the same protein concentration. It is shown how syneresis can be avoided by a proper choice of [CaCl2], temperature and aggregate size.