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Xanthan gum (XG) is a carbohydrate polymer with anionic properties that is widely used as a rheology modifier in various applications, including foods and petroleum extraction. The aim was to investigate the effect of Na+, K+, and Ca2+ on the physicochemical properties of XG in an aqueous solution as a function of temperature. Huggins, Kraemer, and Rao models were applied to determine intrinsic viscosity, [η], by fitting the relative viscosity (ηrel) or specific viscosity (ηsp) of XG/water and XG/salt/water solutions. With increasing temperature in water, Rao 1 gave [η] the closest to the Huggins and Kraemer values. In water, [η] was more sensitive to temperature increase (~30% increase in [η], 20–50 °C) compared to salt solutions (~15–25% increase). At a constant temperature, salt counterions screened the XG side-chain-charged groups and decreased [η] by up to 60% over 0.05–100 mM salt. Overall, Ca2+ was much more effective than the monovalent cations in screening charge. As the salt valency and concentration increased, the XG coil radius decreased, making evident the effect of shielding the intramolecular and intermolecular XG anionic charge. The reduction in repulsive forces caused XG structural contraction. Further, higher temperatures led to chain expansion that facilitated increased intermolecular interactions, which worked against the salt effect. 

The anionic hydrocolloid polysaccharide xanthan gum is widely used in the food and petroleum industries (among others) as a viscosity enhancement polymer due to its high viscosity at low concentrations and moderate temperatures. The physical properties of microbial polysaccharide xanthan gum aqueous solutions were investigated using temperature dependent viscosity measurements. Specifically, the effect of thermal history on the solution viscosity was investigated. Heating and cooling cycles were assessed in two ways, by using a “sawtooth” and “triangle” pattern, which essentially differed in the rates of cooling. The sawtooth method used a cooling rate of 2.0 ◦C min􀀀 1 whereas the triangle pattern had a cooling rate of 0.20 ◦C min􀀀 1. The sawtooth cooling rate was controlled by the speed at which the Peltier device could cool the sample, and the triangle rate was governed by the time required to measure the viscosity at each temperature on return to the initial value. Cycles measured using the sawtooth pattern for 16 mg/kg xanthan gum in water showed an 8–10% overall decrease in the viscosity over four complete cycles. Comparatively, at 320 mg/kg the xanthan gum solution showed a 25% decrease in viscosity over four cycles. The observed temperature dependent viscosity variation suggested minor modifications in the physical network structure of xanthan gum. When using a triangle heating/cooling pattern, the overall decrease in the xanthan gum solution viscosity was 5–7% for 16 mg/kg and only 10% change for 320 mg/ kg solutions. The activation energy of viscous flow for the aqueous xanthan gum solutions by either method was ~15.0 kJ/mol under all conditions. The data showed that temperature and heating cycles influence xanthan gum viscosity and thermal history, which depends more strongly on xanthan gum concentration than solution temperature.