Stirred yogurt was viewed as a concentrated dispersion of aggregates consisting of protein particles. The steady-shear behavior of three types of stirred yogurt with varying ropiness was investigated experimentally. To describe the shear-dependent viscosity, a microrheological model was used which was developed for weakly aggregating dispersions. This model was capable of successfully describing the steady-state viscosity as a function of shear rate of the stirred yogurts, the protein concentration of which ranged between 2.0% and 3.9%. The value of the fractal dimensionality df, following from the model for the aggregates is about 2.24, which was similar to the value of df found with other methods. Moreover, realistic values were found for the interaction forces (energies) describing interaction between the aggregates. The calculated size of the aggregates was close to the size found before by applying different experimental techniques. Using this model, the interpretation of the measured curves suggests that the exopolysaccharides, produced by the lactic acid bacteria in yogurt, play a significant role in the rheology of stirred yogurt.

We give a quantitative treatment of the destabilization of three types of milk protein dispersions. For this we consider the proteins as adhesive-hardsphere bio-colloids. If the attractive interactions become strong enough the system passes the percolation threshold and gels. Macroscopic properties of these gels are studied by measuring viscoelasticity and permeability coefficients. These coeficients are related to structural (fractal) properties of the gels which were measured using scattering and confocal scanning laser microscopy (CLSM) techniques. The behaviour of the protein gels can be understood on a qualitative level.