Rheology of guar galactomannan/rice starch mixtures

Biopolymer mixtures are of increasing importance in the design of model foods, for example, in the low fat spread area. Much of this work has been dedicated to mixed gel systems. However, the equally important area of a soft filler phase in a biopolymer solution has been less well studied. For example, in vitro model systems are likely to be useful in studying the nutritional properties of water-soluble non-starch polysaccharides ('dietary fibre') in man. The presence of particulate material is likely to modify the rheological behaviour of guar gum and other biopolymers dispersed in the aqueous phase of digesta. The effect of particulate inclusions, or 'fillers', on the rheological properties of a typical polysaccharide entanglement network solution (guar galactomannan/water) have been studied. On increasing particulate concentration the system becomes more rate dependent at low shear rates and there is evidence of an apparent yield stress associated with this increase in particulate inclusions. In the present paper, this flow behaviour has been described by a yield stress modified Cross equation. The use of such a model can lead to a prediction of the rheological behaviour of similar systems when the proportion of the relative components is known. The present study determines the effect of particulate and composite properties on the apparent zero-shear viscosity and yield stress of the guar galactomannan and rice starch mixtures and extends this towards a more realistic food system.