Comparative study of viscoelastic properties using virgin yogurt

(W)e describe six different tests used to obtain a consistent set of viscoelastic properties for yogurt. Prior to yield, the shear modulus mu and viscosity eta are measured nondestructively using the speed and damping of elastic waves. Although new to foodstuff's, this technique has been applied to diverse materials from metals to the earth's crust. The resultant shear modulus agrees with mu similar to E/3 for incompressible materials, where the Young's modulus E is obtained from a stress-strain curve in compression. The tensile yield stress tau(o) is measured in compression and tension, with good agreement. The conventional vane and cone/plate rheometers measured a shear stress yield tau(os) similar to tau(o)/root 3 as expected theoretically, but the inferred "apparent" viscosity from the cone/plate rheometer is much larger than the wave measurement due to the finite yield (tau(os) not equal 0). Finally, we inverted an open container of yogurt for 10(6) s much greater than eta/mu and observed no motion. This demonstrates unequivocally that yogurt possesses a finite yield stress rather than a large viscosity. We present a constitutive model with a pre-yield viscosity to describe the damping of the elastic waves and use a simulation code to describe yielding in complex geometry. (C) 1998 The Society of Rheology.