Steady-shear viscosity of stirred yogurts with varying ropiness

Stirred yogurt was viewed as a concentrated dispersion of aggregates consis ting of protein particles. The steady-shear behavior of three types of stir red yogurt with varying ropiness was investigated experimentally. To descri be the shear-dependent viscosity, a microrheological model was used which w as developed for weakly aggregating dispersions. This model was capable of successfully describing the steady-state viscosity as a function of shear r ate of the stirred yogurts, the protein concentration of which ranged betwe en 2.0% and 3.9%. The value of the fractal dimensionality d(f), following f rom the model for the aggregates is about 2.24, which was similar to the va lue of d(f) found with other methods. Moreover, realistic values were found for the interaction forces (energies) describing interaction between the a ggregates. The calculated size of the aggregates was close to the size foun d before by applying different experimental techniques. Using this model, t he interpretation of the measured curves suggests that the exopolysaccharid es, produced by the lactic acid bacteria in yogurt, play a significant role in the rheology of stirred yogurt. (C) 1999 The Society of Rheology. [S014 8-6055(99)02306-8].