Characterisation of a high acyl gellan polysaccharide using light scattering and rheological techniques

Two high acyl gellan samples, one obtained from the aerobic fermentation using milk permeate-based medium, and the other, a commercial gellan (LT100, CPKelco), were converted to the sodium form (referred to as Na-31461 and Na-LT100, respectively) using a cation exchange resin. The Na-gellan solutions were characterised using rheological and static light scattering techniques. The shear rate dependence of viscosity master flow curve obtained for Na-LT100 solution gave an exponent value (1−n) of the Cross equation (∼0.76) typical of flexible random coil polymers. A higher exponent value (∼0.82) was obtained for Na-31461 suggesting that the molecules had a lower degree of flexibility than that of Na-LT100. The intrinsic viscosity ([η]) and molar mass (Mw) values for Na-31461 ([η]∼3950 ml/g, Mw∼5.2×105 Da) were approximately twice that of Na-LT100 ([η]∼2360 ml/g; Mw∼2.5×105 Da). The mechanical spectra (G′, G″) of Na-LT100 solution was characteristic of entangled random coil polymers whereas a weak gel characteristic (G′>G″) was obtained for Na-31461 solution. All these suggest that Na-LT100 molecules were single-stranded, while Na-31461 molecules were double-stranded. Both Na-LT100 and Na-31461 formed gels when the Na+ concentration was further increased. The complex moduli of Na-LT100 and Na-31461 superimposed closely at all Na+ concentrations implied that the conformation of Na-gellan molecules (from single to double-stranded to three-dimensional gel network) were highly dependent on Na+ concentration.