RHEOLOGICAL PROPERTIES OF DEXTRAN RELATED TO FOOD APPLICATIONS

Chemical, physical and mechanical properties of dextran were evaluated to assess the potential use of this microbial polysaccharide in food applications. Dextran demonstrated high solubility characteristics and promoted low solution viscosities. Newtonian behaviour was observed a t concentrations <30% w/w for dextran (mol. wt 500 000). Higher mol. w t 'native' dextran solutions demonstrated slight pseudoplasticity at c oncentrations >1.5% w/w. Dilute and concentrated solution behaviour in dicated the conformation of dextran in solution is dependent on both m olecular weight and polysaccharide concentration. At low concentration s dextran (mol. wt 500 000) demonstrated properties typical of a 'rand om coil' polysaccharide. Increased concentration resulted in the polym er chain adopting a more compact coil geometry. Non-Newtonian behaviou r observed in 'native' dextran solutions (>1.5% w/w) is attributable t o the formation of inter-chain entanglements through polymer size effe cts and unique branching properties. Dextran (mol. wt 500 000) demonst rated two critical concentrations (c = 4.7% and c** = 19% w/v); quasi -elastic light scattering measurements verified the coil overlap regio n (c similar to 4.6% w/v). Dynamic oscillatory evaluations indicated dextran (mol. wt 500 000) solutions exhibited dilute solution characte ristics at concentrations <20% (w/w) and obeyed the empirical Cox-Merz rule. 'Native' dextran solutions (10-15% w/w) demonstrated mechanical spectra typical of concentrated polysaccharide solutions. Calorimetri c analysis of ternary dextran:sucrose:water solutions demonstrated tha t dextran addition modified the frozen system behaviour of sucrose sol utions. Increasing dextran concentration effectively increased the ons et of melting temperatures (T-g) by as much as 12 degrees C.