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 at concentrations <30% w/w for dextran (mol. wt 500 000). Higher mol. wt 'native' dextran solutions demonstrated slight pseudoplasticity at concentrations >1.5% w/w. Dilute and concentrated solution behaviour indicated the conformation of dextran in solution is dependent on both molecular weight and polysaccharide concentration. At low concentrations dextran (mol. wt 500 000) demonstrated properties typical of a 'random coil' polysaccharide. Increased concentration resulted in the polymer chain adopting a more compact coil geometry. Non-Newtonian behaviour observed in 'native' dextran solutions (>1.5% w/w) is attributable to the formation of inter-chain entanglements through polymer size effects and unique branching properties. Dextran (mol. wt 500 000) demonstrated two critical concentrations (c* = 4.7% and c** = 19% w/v); quasi-elastic light scattering measurements verified the coil overlap region (c* similar to 4.6% w/v). Dynamic oscillatory evaluations indicated dextran (mol. wt 500 000) solutions exhibited dilute solution characteristics 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. Calorimetric analysis of ternary dextran:sucrose:water solutions demonstrated that dextran addition modified the frozen system behaviour of sucrose solutions. Increasing dextran concentration effectively increased the onset of melting temperatures (T-g) by as much as 12 degrees C.