Viscosity of guar gum and xanthan/guar gum mixture solutions JA Casas, 1 AF Mohedano 1 and F Garcı ´a-Ochoa 2 * 1 Area de Ingenierı´a Quı´mica, DepartamentoQuı´mica-Fı´sica Aplicada,Facultadde Ciencias,Universidad Auto ´ noma, 28049-Madrid, Spain 2 Departamento Ingenierı´a Quı´mica, Facultad CC Quı´micas, UniversidadComplutense, 28040-Madrid, Spain Abstract: The viscosity of diluted guar gum solutions and the viscosity of xanthan and guar gum mixture solutions have been studied. Guar gum solutions showed pseudoplastic behaviour. Apparent viscosity increased with gum concentration and decreased with the temperature at which viscosity was measured. A maximum in the plot of viscosity versus increasing dissolution temperature was observed at 60 °C. This behaviour was related to differences in molecular structure of the polymers solved at different temperatures. Mixtures of xanthan and guar gum showed a higher combined viscosity than that occurring in each separate gum. This synergistic interaction was affected by the gum ratio in the mixture and dissolution temperature of both gums. The effect of polysaccharide concentration (1.0, 1.5 and 2.0 kg m À3 ), xanthan/guar gum ratio (1/5, 4/2, 3/3, 4/2 and 5/1) and dissolution temperature (25, 40, 60 and 80 °C for both gums) on the viscosity of solutions of mixtures were studied. The highest viscosities were observed when 2.0 kg m À3 gum concentration was used together with a ratio of xanthan/ guar gum of 3/3 (w/w) and dissolution temperature of 40 and 80 °C for xanthan and guar gum, respectively. # 2000 Society of Chemical Industry Keywords: guar gum; xanthan gum; polysaccharide mixture viscosity INTRODUCTION Guar gum is a galactomannan obtained from the endosperm of the Cyamopsis tetragonolobus seed. 1 This polysaccharide is formed by galactose and mannose molecules. The principal backbone is a chain of (1±4)- b-D-mannopyranosyl units, with single (1±6) a-D- galactopyranosyl units linked to the principal chain. 2,3 The mannose to galactose ratio is 1.2 to 1.8. 1,4 This ratio can change depending on temperature at solu- tion, 4,5 as happens with locust bean gum. 6 The distribution of galactose rami®cations is not constant, there are zones without rami®cations (smooth region) and others with one molecule of galactose linked to every mannose unit (hairy region). 3,6±8 The interac- tions among galactomannan molecules, with them- selves or with other polysaccharides, are enhanced by the presence of smooth regions. 8±11 The average molecular weight of guar gum can vary, depending on the polysaccharide chain-length, from 440 000 to 650 000 Da. 4,12 Low quantities of guar gum in aqueous solution can confer high viscosity, which changes depending on the shear rate, showing a shear- thinning or pseudoplastic behaviour. Guar gum is usually employed as a thickener mainly in the food, textile and paper industries. 1,13 Xanthan gum is a biopolysaccharide synthesized by Xanthomonas sp 14 which is used as a thickener in many industries, such as the pharmaceutical, cosmetic and food industries, and also in enhanced oil recovery. 15 Xanthan interacts with galactomannan to form mix- tures of high viscosity at low polysaccharide con- centrations. The main variables for xanthan and galactomannan interactions are the molecular xanthan conformation and the galactomannan structure in solution. The xanthan structure in solution changes with the temperature of solution. 16±18 At low dissolu- tion temperatures ie lower than 40 °C, xanthan shows an ordered conformation, while at higher temperatures xanthan shifts to a disordered structure. 17,18 Interac- tion of xanthan with galactomannan has been widely studied. 19,20 Techniques such as polarization micro- scopy and X-ray diffraction have been used. 21,22 Both xanthan molecule conformations have strong interac- tions with galactomannan molecules, although these interactions are greater when the xanthan molecules have an ordered conformation. 11,23 This effect has been observed mainly with xanthan/locust bean gum mixtures. The higher viscosity values were reached using xanthan solved at 40 °C. 11 Galactomannans are formed by several fractions of polymers with different mannose/galactose ratios. At low temperatures, mol- ecules with low molecular weight and low mannose/ galactose ratio are mainly dissolved. At high tempera- tures galactomannan fractions with high molecular weight, greater mannose/galactose relation and, there- fore, zones without galactose radicals are solved. (Received 7 April 2000; accepted 18 May 2000) * Correspondence to: F Garcı ´a-Ochoa, Dpto Ingenierı ´a Quı ´mica, Facultad CC Quı ´micas, Universidad Complutense, 28040-Madrid, Spain E-mail: fgochoa@eucmos.sim.ucm.es # 2000 Society of Chemical Industry. J Sci Food Agric 0022±5142/2000/$30.00 1722 Journal of the Science of Food and Agriculture J Sci Food Agric 80:1722±1727 (online: 2000)