Structure and dynamical mechanical properties of suspensions of sodium caseinate

Sodium caseinate is derived from casein which is the major milk protein and forms small star-like aggregates in aqueous solution. The dynamic mechanical properties of dense sodium caseinate suspensions were studied by measuring the shear viscosity and the frequency dependent loss and storage shear modulus. The viscosity is shown to increase strongly above a concentration, C(c), due to jamming of the aggregates. C(c) depends little on the pH or on the ionic strength. The repulsion between the aggregates increases with decreasing temperature leading to jamming at lower concentrations. As a consequence the viscosity decreases strongly with increasing temperature for concentrated suspensions. The frequency dependent shear modulus of concentrated suspensions shows a solid like behaviour at high frequencies. Shear thinning is observed at Peclet numbers larger than unity.