RHEOLOGICAL CHARACTERIZATION AND MODELING OF AQUEOUS GUAR GUM SOLUTIONS

Transcription

1 3 rd International Symposium on Food Rheology and Structure RHEOLOGICAL CHARACTERIZATION AND MODELING OF AQUEOUS GUAR GUM SOLUTIONS Marco Dressler, Peter Fischer, Erich J. Windhab Swiss Federal Institute of Technology (ETH) Zurich, Laboratory for Food Process Engineering Schmelzbergstrasse 9, 8092 Zürich, Switzerland Abstract The rheological properties of aqueous guar gum solutions have been investigated for different concentrations and temperatures in a cone-and-plate rheometer in the linear and in the non-linear viscoelastic regime. In the linear viscoelastic regime we performed low amplitude oscillatory shear experiments to measure the complex modulus. We adopted the time-temperature and the time-concentration superposition principle to increase the window of experimentally accessible frequencies. The time-concentration superposition principle allowed for a frequency shift of approximately one decade towards the high frequency regime. Preliminary modeling efforts in the framework of the theory of linear viscoelasticity have been undertaken to give a theoretical description of the measured data. In the non-linear viscoelastic regime we found for low shear rates a monotonous increase of the transient shear viscosity, whereas at higher shear rates a pronounced overshoot in the transient rheological properties has been detected. The steady state values of the shear viscosity and the first normal stress coefficient reveal the characteristic shear-thinning behavior. The PTT model of polymer rheology has been adopted to describe the non-linear viscoelastic material properties. 1 Introduction Guar is a naturally occurring polysaccharide extracted from the beans of the guar gum plant. This polysaccharide possesses remarkable rheological properties and consequently it is adopted as a thickener to control viscoelasticity in food, personal care, and oil recovery industries. Despite of this not much is known about its structure-rheology relationships and about its non-linear rheological behavior. Therefore, it seems worthwhile to undertake a rheological characterization of guar gum quantifying its linear and non-linear rheological properties over an extended range of thermodynamic properties such as deformation, concentration, and temperature. Preferably not only shear rheology but also elongational and mixed flow rheology should be considered. The typical chemical structure of a galactomannan chain is displayed in Fig. 1. The chain consists of a linear mannan (mannose) backbone of (1 Ø 4)-linked b-d-mannopyranosyl units. To the bakkbone various amounts of single (1 Ø 6)-linked a-dgalactopyranosyl units are attached which solubilize the polymer through steric effects. For a typical guar gum chain the mannose to galactose (Man:Gal) ratio is about The degree of galactose substitutions along the backbone influences the spatial extension, the chain stiffness and the association of the polysaccharide molecule. Galactose poor regions along the backbone are less soluble and can associate intra- as well as inter-molecularly to form partially crystalline complexes. In most applications, several additives such as cosolvents or salts are also present in solution, besides guar and water. For example, isopropyl alcohol is used in the cosmetic industry for hair sprays and shampoos. The presence of these additives can enhance dramatically the viscoelastic and structural properties of guar gum solutions, since they affect the quality of the solvent and thus both the chain conformation and the degree of intermolecular interactions. These chain associations are responsible for the remarkable rheological properties of guar gum. Due to the fact that there are only few non-linear rheological data of guar gum in the literature and the influence of additives and salts on the conformational characteristics and rheological properties of biopolymers is not well understood, we investigate aqueous guar gum solutions without any additives in the first place. Nevertheless, this is interesting since the non-linear rheological properties of these materials have not been measured up to now. We will report on transient non-linear rheological measurements and on the measurement of the first normal stress coefficient. In the last few years several researchers have begun to undertake research work on guar gum focusing on the equilibrium chain structure of the molecules, on the effects of various solvents on the chemical constitution, and on its linear rheological behavior. In what follows, we give a brief overview of these research works: In a theoretical study Petkowicz and collaborators [2] applied a Monte Carlo method to compute the conformational properties of undisturbed galactomannan chains. These authors have determined several unperturbed conformational characteristics of idealized galactomannan chains such as the characteristic ratio, the persistence length, the average end-to-end distance, and the radius of gyration as a function of chain length and the Man:Gal ratio along the backbone. The persistence length of galactomannan chains was found to vary between 85 Å for the 2:1 galactomannan chain and 145 Å for the unsub- 249

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