Rheological evaluation of deacetylated gellan gum (Gelrite) for pharmaceutical use

Abstract

The general rheological properties of deacetylated gellan gum (Gelrite) were examined using a purpose-built air turbine viscometer, whose independent variable was shear stress. Continuous shear experiments on 1% w/w Gelrite dispersions in water prepared by autoclaving or an elevated heat treatment showed that such systems had similar pseudoplastic flow properties. The heat-treated dispersions were examined in greater detail and showed evidence of limited irreversible shear breakdown and little thixotropy. The dispersions decreased progressively in apparent viscosity with rise in temperature over the range 10–50°C but the activation energy of flow, derived from an Arrhenius-type plot, was low indicating good thermal stability. At low pH Gelrite dispersions were much less viscous. The apparent yield value and viscosity of dispersions increased disproportionately with increasing concentration of the polymer used. Dispersions of Gelrite were found to be sensitive to gelation at much lower concentration of divalent rather than monovalent cations. Creep testing confirmed that a 1% w/w Gelrite dispersion behaved as a pseudoplastic liquid, which was devoid of a static yield value.