rheologicalcharacterization and modeling of aqueous guar gum...

3rd International Symposium on Food Rheology and Structure

249

AbstractThe rheological properties of aqueous guar gum solutionshave been investigated for different concentrations andtemperatures in a cone-and-plate rheometer in the linearand in the non-linear viscoelastic regime. In the linearviscoelastic regime we performed low amplitude oscillato-ry shear experiments to measure the complex modulus.We adopted the time-temperature and the time-concentra-tion superposition principle to increase the window ofexperimentally accessible frequencies. The time-concen-tration superposition principle allowed for a frequency shiftof approximately one decade towards the high frequencyregime. Preliminary modeling efforts in the framework ofthe theory of linear viscoelasticity have been undertaken togive a theoretical description of the measured data. In thenon-linear viscoelastic regime we found for low shear ratesa monotonous increase of the transient shear viscosity,whereas at higher shear rates a pronounced overshoot inthe transient rheological properties has been detected. Thesteady state values of the shear viscosity and the first nor-mal stress coefficient reveal the characteristic shear-thin-ning behavior. The PTT model of polymer rheology hasbeen adopted to describe the non-linear viscoelastic mate-rial properties.

1 IntroductionGuar is a naturally occurring polysaccharide extrac-ted from the beans of the guar gum plant. This poly-saccharide possesses remarkable rheological pro-perties and consequently it is adopted as a thickenerto control viscoelasticity in food, personal care, andoil recovery industries. Despite of this not much isknown about its structure-rheology relationships andabout its non-linear rheological behavior. Therefore,it seems worthwhile to undertake a rheological cha-racterization of guar gum quantifying its linear andnon-linear rheological properties over an extendedrange of thermodynamic properties such as defor-mation, concentration, and temperature. Preferablynot only shear rheology but also elongational andmixed flow rheology should be considered.

The typical chemical structure of a galac-tomannan chain is displayed in Fig. 1. The chain con-sists of a linear mannan (mannose) backbone of (1Ø 4)-linked b-D-mannopyranosyl units. To the bak-kbone various amounts of single (1 Ø 6)-linked a-D-galactopyranosyl units are attached which solubilizethe polymer through steric effects. For a typical guargum chain the mannose to galactose (Man:Gal) ratiois about 1.55. The degree of galactose substitutions

along the backbone influences the spatial extension,the chain stiffness and the association of the poly-saccharide molecule. Galactose poor regions alongthe backbone are less soluble and can associateintra- as well as inter-molecularly to form partiallycrystalline complexes.

In most applications, several additives suchas cosolvents or salts are also present in solution,besides guar and water. For example, isopropylalcohol is used in the cosmetic industry for hairsprays and shampoos. The presence of these additi-ves can enhance dramatically the viscoelastic andstructural properties of guar gum solutions, sincethey affect the quality of the solvent and thus boththe chain conformation and the degree of intermole-cular interactions. These chain associations areresponsible for the remarkable rheological propertiesof guar gum. Due to the fact that there are only fewnon-linear rheological data of guar gum in the litera-ture and the influence of additives and salts on theconformational characteristics and rheological pro-perties of biopolymers is not well understood, weinvestigate aqueous guar gum solutions without anyadditives in the first place. Nevertheless, this is inter-esting since the non-linear rheological properties ofthese materials have not been measured up to now.We will report on transient non-linear rheologicalmeasurements and on the measurement of the firstnormal stress coefficient.

In the last few years several researchershave begun to undertake research work on guar gumfocusing on the equilibrium chain structure of themolecules, on the effects of various solvents on thechemical constitution, and on its linear rheologicalbehavior. In what follows, we give a brief overview ofthese research works:

In a theoretical study Petkowicz and collabo-rators [2] applied a Monte Carlo method to computethe conformational properties of undisturbed galac-tomannan chains. These authors have determinedseveral unperturbed conformational characteristicsof idealized galactomannan chains such as the cha-racteristic ratio, the persistence length, the averageend-to-end distance, and the radius of gyration as afunction of chain length and the Man:Gal ratio alongthe backbone. The persistence length of galactom-annan chains was found to vary between 85 Å for the2:1 galactomannan chain and 145 Å for the unsub-

RHEOLOGICAL CHARACTERIZATION AND MODELING OF AQUEOUSGUAR GUM SOLUTIONS

Marco Dressler, Peter Fischer, Erich J. Windhab

Swiss Federal Institute of Technology (ETH) Zurich, Laboratory for Food Process EngineeringSchmelzbergstrasse 9, 8092 Zürich, Switzerland

ISFRS 3 Proceed.Lect./II/.qxd 17.01.2003 9:43 Uhr Seite 249

ISFRS_INHALT_201-300 19.01.2003 11:25 Uhr Seite 250

rheologicalcharacterization and modeling of aqueous guar gum...

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