Dynamic rheological properties of highly concentrated protein-stabilized emulsions

We have measured the dynamic rheological properties of highly concentrated emulsions of soya oil in water, stabilized with sodium caseinate, as a function of oil phase volume fraction (phi greater than or equal to 0.70). Experiments were carried out in oscillatory mode on a controlled stress rheometer, as a function of applied stress and frequency. At low stresses these emulsions behave as a gel whose elastic properties, as a function of phi, follow the empirical model of Princen (J. Colloid Interface Sci. 1986, 112, 427) and are controlled by the Laplace pressure of the oil droplets. At higher stresses, the frequency spectra show a Maxwellian behavior with a single relaxation time. The frequency sweeps can be superimposed at each phi value onto a single master curve scaled against their Maxwellian relaxation time. Strain sweeps, derived from the frequency spectra at these higher stresses are also superimposable using as scaling factor the ratio, reference stress to the applied stress. A frequency-dependent yield stress is also observed, the response varying with the oil volume fraction.