Rheology and Microstructure of Mixed Biopolymer Gels
Abstract: The viscoelastic behaviour of biopolymer mixtures showing synergism was investigated under different physicochemical conditions using small deformation oscillatory measurements. In addition, the rheological properties of binary polysaccharide mixtures were related to the microstructure as observed using transmission electron microscopy.Viscoelastic measurements showed that addition of locust bean gum to xanthan or .kappa.-carrageenan strongly influences the viscoelastic properties of the mixture. The effect of locust bean gum on the viscoelastic properties of the mixture was dependent on the degree of side-chain substitution of the galactomannan, ionic environment, temperature and the composition of the mixture. Locust bean gums, with mannose to galactose ratios of ~3 (lbg3) and ~5 (lbg5), have been evaluated. For the mixture of xanthan-locust bean gum, viscoelastic measurements showed that, when the polymers were mixed at room temperature, a gel is formed. Heat treatment of the mixture led to an increase in G' for mixtures containing lbg5. Addition of salt decreased the synergism between xanthan and locust bean gum. Locust bean gum did not influence the supermolecular structure of xanthan. The synergism observed for mixtures of .kappa.-carrageenan-locust bean gum showed a dependence on the ionic environment. The greatest increase in G' was observed under conditions promoting extensive self-association of .kappa.-carrageenan, that is in the presence of K+. Under ionic conditions (Na+ or Ca2+), where the tendency for .kappa.-carrageenan to self-associate is low, the synergistic effect of locust bean gum was reduced. Locust bean gum influenced the supermolecular structure of .kappa.-carrageenan by hindering extensive self-association of the .kappa.-carrageenan.
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