Phase separation and rheology of aqueous starch/galactomannan systems

Phase behaviour (at 25 degrees C and 60 degrees C), phase separation kinetics and rheological properties of aqueous starch/galactomannan systems were studied. Pure, soluble amylopectin and waxy maize starch dispersions with varying degrees of granule disintegration were employed to investigate the effect of starch granules and aggregation of starch polysaccharides. Polysaccharide blends were made by either mixing starch and galactomannan stock solutions (solution-based preparations) or by dry blending starch and galactomannan powders prior to dispersion in water (powder-based preparations). In phase diagrams for soluble amylopectin, the binodal is not symmetrical but displaced towards the axis of amylopectin, i.e. the lower molecular weight polysaccharide. The binodals of the waxy maize starch systems nearly coincide with the axes representing the polysaccharide concentrations, which reflects the influence of the high molecular weight and limited solubility. Near infrared reflection analysis was employed to determine the phase separation kinetics in samples of standard volume. Phase separation in a 5 ml sample was completed after 8 h for most systems, the exception being powder-based systems containing waxy maize starch granules in the swollen state and galactomannan at high concentrations (> 3% starch/ > 0.3% galactomannan). In this latter system the separation of the phases proceeded more slowly than in the systems with a lower degree of supermolecular organisation, such as soluble amylopectin and molecularly dispersed waxy maize starch. Likewise, the rheological properties of the blends were largely determined by the extent of starch granule disintegration and polymer degradation. Rheological characterisation revealed the greater structural rigidity, indicated by the increase in measured G' and G" values, of the powder-based preparations. Marked frequency dependence of the dynamic moduli in the frequency range 0.1 rad s(-1) -10 rad s(-1) was found for all blends under study.