Rheology of Xanthan Gum: Salt, Temperature, and Strain Effects in Oscillatory and Steady Shear Experiments

Abstract

Dynamic oscillatory shear testing has been used to study the rheology of xanthan gum in the moderate concentration regime (500<c<5000 ppm). The dynamic moduli of two industrial grade xanthan gums (a dried powder and a fermentation broth) have been compared in deionized, distilled water (low salt) and in 0.5% NaCl solutions (high salt). The G′(ω) and G″(ω) of the powdered sample increase with the addition of salt for c>2000 ppm but decrease substantially at lower xanthan gum concentrations. The broth sample is relatively insensitive to salt addition (especially for c>2000 ppm), and maintains higher levels of dynamic moduli at low concentrations in high salt, than the powdered sample. However, for c>2000 ppm in high salt, the dynamic properties of the broth sample are lower than those of the powdered sample. The indication is that the broth sample has a higher apparent molecular weight. Detailed studies of the effects of salt, temperature, and high shear on 5000 ppm xanthan gum solutions in dynamic, steady, and transient shear have been conducted. At this concentration, the solution exhibits “gel‐like” behavior and the rheology is controlled by intermolecular associations. When the solution structure is disrupted by high shear or temperature, the recovery of dynamic properties is slow and incomplete in low salt, and fast and complete in high salt environments. The temperature driven order→disorder transition of xanthan has been examined in low and high salt solutions using dynamic measurements. Frequency/temperature superposition has shown that a conformational transition occurs at T>50–60°C at low salt concentrations, and that this transition is pushed to higher temperature (T>80°C) in 0.5% NaCl. A master curve of the dynamic properties covering six decades of frequency has been obtained for xanthan gum in highly saline solutions.