Viscoelastic properties of xanthan/galactomannan mixtures: comparison of guar gum with locust bean gum

The rheological behaviour of xanthan/guar gum systems has been investigated and compared to that of xanthan/locust bean gum mixtures using oscillatory shear and creep-recovery measurements. The total polysaccharide concentration was kept constant at 0.5% w/w, the xanthan/galactomannan ratio ranged from 1/99 to 90/10 and three ionic strengths were studied. As for xanthan/locust bean gum mixtures, strong synergistic phenomena were exhibited by xanthan/guar gum systems. Clearly, the addition of xanthan gum even at a very low level to a guar gum solution induced a transition of the system from a macromolecular solution to a structured system displaying gel-like properties. The comparison between three guar gum samples with different molecular weights evidenced a strong effect of the molecular weight: the higher this parameter, the stronger the synergistic interaction. At a low xanthan/galactomannan ratio (<10/90), xanthan/guar gum mixtures and xanthan/locust bean gum mixtures (with comparable galactomannan molecular weights) resulted in similar viscoelastic behaviour. In contrast: at a higher xanthan/galactomannan ratio, a stronger synergism was exhibited with locust bean gum, as expected. The rheological properties were greatly influenced by the presence of electrolyte. In the case of xanthan/guar gum systems, the storage (G') and the loss (G ") moduli were increased when electrolyte was present, in contrast to xanthan/locust bean gum mixtures which exhibited a reverse tendency. These results provide evidence that xanthan gum plays a major role in the rheological behaviour of the xanthan/galactomannan systems even at a low concentration. Overall, similar results were found with xanthan/guar or xanthan/locust bean gum mixtures. However, differences in the mechanism may exist according to the mannose/galactose ratio of the galactomannan and also to the xanthan/galactomannan ratio and the ionic strength. (C) 1997 Elsevier Science Ltd.