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Keywords:

  • Apparent viscosity;
  • galactomannans;
  • hydrocolloids;
  • loss modulus;
  • storage modulus

ABSTRACT

Rheological studies of tragacanth and guar gums dispersions were carried out by means of steady shear (3 to 1000/s) and small amplitude oscillatory shear experiments (0.1 to 10 rad/s) for concentrations up to 8.9 g/L for tragacanth and 7.6 g/L for guar gum at 25C using stress controlled rheometer. The dispersions exhibited shear-thinning characteristics. A simplified Cross model was adequate to fit the shear-dependent viscosity behavior for both gums. Model parameters for guar gum were correlated with concentration employing power functions (with flow index constant) and for tragacanth gum the dependence was linear for flow index, power for time constant and exponential for zero-shear rate viscosity. Mechanical spectra revealed that dispersions behave as dilute systems of coil polymers with trend to entangled systems at the highest concentrations. Loss and storage moduli were correlated with frequency and the resulting parameters were correlated with concentration. Cox–Merz rule holds satisfactorily.

PRACTICAL APPLICATIONS

Guar and tragacanth gums are natural thickeners that have many applications in pharmaceutical, food, paper, textile, oil painting and cosmetic industries, among others. The addition of these hydrocolloids at low concentration to systems can give a great variety of functions such as enhancement of viscosity, improvement of texture, formation of films, encapsulation of flavors, creation of gel-structures, etc. In this work, mathematical relationships are proposed for the variation of apparent viscosity and loss and storage moduli with concentration for both gums.