Evolution of microstructure and rheology in mixed polysaccharide systems

Synergistic biopolymer blends composed of xanthan and enzymatically modified guar galactomannan are investigated in terms of their time-dependent properties. In particular, a side-chain cleaving enzyme, a-galactosidase, is used to cleave off galactose sugar units from guar to produce modified galactomannans with varying galactose contents of 25.2 and 16.2%. Laser scanning confocal microscopy and dynamic rheology are used to monitor the properties of each of these two modified guar gum in solution as well as in blends with xanthan as they are allowed to age over a period of 3 weeks. Our results indicate that solutions of guar with a higher galactose (25.2%) content undergo no rheological change over the period of observation and show a constant gel elastic modulus (G') in blends with xanthan. Confocal images of the solutions and the blends also indicate that the systems are stable over a period of 3 weeks. In contrast, guar gum with a lower galactose content (16.2%) forms interchain associations in solution, developing aggregates that convert it from a macromolecular solution to a gel. This is reflected in its dynamic moduli which increase significantly with time and show a transition from frequency-dependent behavior with G (viscous modulus) > G'(elastic modulus) to a ftequency-independent character with G' > G. This process of association and phase separation is directly observed in confocal images of the modified guar as well as in its blend, though not to the same extent in the latter. The presence of a second component thus seems to retard the association process. Interestingly, the blend moduli remain unchanged in magnitude and show gellike features even though the mode of association and concomitant microstructure changes.