Viscosity of solutions of xanthan/locust bean gum mixtures

Xanthan and locust bean gums are polysaccharides able to produce aqueous so lutions with high viscosity and non-Newtonian behaviour. When these solutio ns are mixed a dramatic increase on viscosity is observed, much greater tha n the combined viscosity of the separated polysaccharide solutions. In this work the influences of different variables on the viscosity of solutions o f mixtures of xanthan/locust bean gum have been studied. Total polysacchari de concentration, xanthan and locust bean ratio on mixture and temperature at which the gum was dissolved (dissolution temperature) for both xanthan a nd locust bean gums have been considered. Under these different operational mixture conditions shear rate and time have also been considered to descri be the rheological behaviour of the solutions studied. The high viscosity i ncrease observed in these mixtures is due to the interaction between xantha n gum and locust bean gum molecules. This interaction takes place between t he side chains of xanthan and the backbone of the locust bean gum. Both xan than molecule conformation in solution - tertiary structure - and locust be an gum structure show great influence on the final viscosity of the solutio n mixtures. Xanthan conformation changes with temperature, going from order ed structures to disordered or chaotic ones. Locust bean gum composition ch anges with dissolution temperature, showing a dissolved galactose/mannose r atio reduction when temperature increases, ie the smooth regions - zones wi thout galactose radicals - are predominantly dissolved. The highest viscosi ty was obtained for the solution mixture with a total polysaccharide concen tration of 1.5 kg m(-3) and a xanthan/locust ratio of 2 : 4 (w/w) and when xanthan gum and locust bean gum were dissolved at 40 degrees C and 80 degre es C, respectively. (C) 1999 Society of Chemical Industry.