Time-dependent viscosity of stirred yogurt. Part I: couette flow Original Research Article

Viscosity data at 5°C are presented for commercial yogurt for a shear rate range 5–700 s−1. Data were obtained using a conventional rotational rheometer. When subjected to a range of constant shear rates, yogurt viscosity demonstrated time-dependent and shear-dependent behaviour. The equilibrium structural parameter employed in the characterisation of the time-dependent nature of the yogurt was found to vary over the shear rate range investigated. At both the initial and equilibrium conditions, the shear rate/shear stress data were fitted to an Ostwald power law model of the form τ=K γ̇n with good correlation (average r2=0.97). Experimental shear stress/time data at constant shear rate were modelled using a structural parameter approach and using the Weltmann model. The experimental shear stress data was best described by the Weltmann model.