Slow dynamics and structure in jammed milk protein suspensions

The dynamic mechanical properties and the structure of dense suspensions of sodium caseinate were investigated using oscillatory shear rheology and confocal laser scanning microscopy, respectively. Caseins are the most abundant milk proteins and form in the absence of calcium phosphate small star-like particles with radii of about 10 nm. The viscosity increases strongly with increasing protein concentration above similar to 80 g L-1 due to jamming of the particles. The viscosity increase is stronger at lower temperatures, caused by a strong decrease in the terminal relaxation time with increasing temperature. Addition of calcium ions introduces an attractive interaction that induces phase separation above a critical calcium concentration. Increasing the CaCl2 concentration leads to an increase in the terminal relaxation time, but to a decrease in the high frequency elastic modulus. The effect of adding CaCl2 is stronger at higher temperatures.