Dynamic mechanical properties of suspensions of micellar casein particles

Small micellar casein particles, so-called submicelles, were obtained by removing colloidal calcium phosphate from native casein by adding sodium polyphosphate. Aqueous submicelle suspensions were characterized using light scattering and rheology as a function of concentration and temperature. The casein submicelles behave like soft spheres that jam at a critical concentration (C(c)) of about 100 g L(-1). The viscosity does not diverge at C(c), but increases sharply, similarly to that of multiarm star polymers. C(c) increases weakly with increasing temperature, which leads to a strong decrease of the viscosity close to and above C(c). Concentrated submicelle suspensions show strong shear-thinning above a critical shear rate and the shear stress becomes independent of the shear rate. The critical shear rates at different temperatures and concentrations are inversely proportional to the zero-shear viscosity. At much higher shear rates, the shear stress fluctuates strongly in time indicating inhomogeneous flow. The frequency dependence of casein submicelle suspensions is characterized by elastic behavior at high frequencies (concentrations) and viscous behavior at low frequencies (concentrations).