Phase separation and rheology of aqueous starch/galactomannan systems

Phase behaviour (at 25 degrees C and 60 degrees C), phase separation kineti cs and rheological properties of aqueous starch/galactomannan systems were studied. Pure, soluble amylopectin and waxy maize starch dispersions with v arying degrees of granule disintegration were employed to investigate the e ffect of starch granules and aggregation of starch polysaccharides. Polysac charide blends were made by either mixing starch and galactomannan stock so lutions (solution-based preparations) or by dry blending starch and galacto mannan 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 coinc ide with the axes representing the polysaccharide concentrations, which ref lects the influence of the high molecular weight and limited solubility. Ne ar infrared reflection analysis was employed to determine the phase separat ion kinetics in samples of standard volume. Phase separation in a 5 mi samp le was completed after 8 h for most systems, the exception being powder-bas ed systems containing waxy maize starch granules in the swollen state and g alactomannan at high concentrations ( > 3% starch/ > 0.3% galactomannan). I n this latter system the separation of the phases proceeded more slowly tha n in the systems with a lower degree of supermolecular organisation, such a s soluble amylopectin and molecularly dispersed waxy maize starch. Likewise , the rheological properties of the blends were largely determined by the e xtent of starch granule disintegration and polymer degradation. Rheological characterisation revealed the greater structural rigidity, indicated by th e 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. (C) 1999 El sevier Science Ltd. All rights reserved.