Rheological characterization of galactomannans extracted from seeds of Caesalpinia pulcherrima

► Caesalpinia pulcherrima gum was extracted from legume endosperms of C. pulcherrima L. ► Steady shear and dynamic viscoelastic properties were studied. ► The rate of entanglement of the polymer is dependent on concentration of the polymer and exhibits shear thinning nonNewtonian behavior. ► The rheological characteristics of the galactomannan demonstrated the suitability of gum as a viscosity modifier in food industry as well as in pharmaceutical industry. The galactomannan from the seeds of Caesalpinia pulcherrima L. was isolated and purified by precipitation method using alcohol to get C. pulcherrima (CP) gum. CP gum was studied for its physicochemical and rheological properties. The composition of CP gum was found to contain mannose:galactose:glucose:xylose in a proportion of 2.8:1:0.1:0.08, with M/G ratio 2.80. The molecular weight (Mw) for CP gum was obtained to be approximately 2.72×106Da by static light scattering measurements and 2.79×106Da using Mark–Houwink relationship. The intrinsic viscosity by Huggins and Kraemer plots using capillary viscometry was obtained as 12–12.5dl/g. The rheological behavior of aqueous galactomannan solutions was studied at 25°C, using steady-shear and dynamic oscillatory measurements. The various concentrations of CP gum exhibits shear thinning non Newtonian behavior at high shear rate and Newtonian flow at low shear rate. Experimental data in steady shear has been correlated and found a better fitting with the Cross and Carreau models. A graph of the specific viscosity at zero shear rate (ηsp0) against coil overlap parameter (C[η]) was plotted and the slope of the lines in dilute and semi-dilute regions were found to be 1.23 and 4.1 respectively. The critical concentration (C*) was found to be about 3.8/[η]. The linear viscoelastic region for CP gum solutions presented nature as that of macromolecular solutions. At all shear rates and frequencies, ηap and η* had almost similar magnitudes, which shows its reasonable agreement with the Cox–Merz rule. The present investigation shows the suitability of CP gum as a pharmaceutical aid application like viscosity modifier, release retardants, binders.