W. Kloek

Yielding behaviour of mixtures of xanthan and enzyme-modified galactomannans.

Abstract The rheologicai behaviour as well as the stabilization against sedimentation of particles was studied for aqueous mixtures of xanthan with enzyme modified galactomannans. Both properties depended strongly on the galactomannan used. The shear modulus increased while the strain at yielding and the sedimentation velocity decreased with decreasing galactose content of the galactomannan. Results are compared with those for mixtures of xanthan with locust bean gum.

Small and large deformation behaviour of mixtures of xanthan and enzyme modified galactomannans

Small and large deformation properties of aqueous mixtures of xanthan with enzyme modified galactomannans at low ionic strength are discussed in terms of the theory of rubber elasticity and the structure of the galactomannans. The linear deformation region of the gels is small indicating that the deformation behaviour is dominated primarily by the rigid xanthan molecule. Galactomannans with a lower galactose content have a smaller region of linear behaviour.

Sedimentation of a single particle in systems of xanthan with locust bean gum in relation to the rheological behaviour

Abstract The sedimentation behaviour of single inert particles in aqueous mixtures of xanthan with locust bean gum is semi-quantitatively compared to the rheological behaviour of these materials as determined with various rheometers. It is concluded that for these mixtures the existence of a yield stress combined with a further slow breakdown of the xanthan locust bean gum network may explain the relatively slow sedimentation observed.

Sedimentation in Aqueous Xanthan + Galactomannan Mixtures

Publisher Summary The sedimentation of suspended particles in a liquid dispersion is regulated by the rheological properties of the dispersion at low stresses and low shear rates. This chapter discusses sedimentation in aqueous xanthan + galactomannan mixtures. It presents a comparison of some experimental results for the viscosity of xanthan + galactomannan mixtures as determined by sedimentation and with a rheometer. For that comparison, sedimentation experiments were performed with glass beads of 0.2-5 mm diameter in a glass tube of 18 mm diameter. Sedimentation viscosities were calculated with the Stokes equation corrected for the wall effect and for non-laminar flow. Apparatus viscosity was measured with a Deer Rheometer PDR 81 equipped with concentric cylinders. The result of the comparison is that the apparent viscosity of glycerol + water mixtures, as well as of dispersions of xanthan in 0.1 wt% aqueous NaCl, as determined by sedimentation or in the rheometer, were, at comparable shear stresses, equal within the experimental accuracy. However, for xanthan + galactomannan mixtures, the apparent sedimentation viscosities were considerably higher than those measured in the rheometer at the same stress.