Time-temperature studies of gellan polysaccharide-high sugar mixtures: effect of sodium ions on structure formation

Changes to the viscoelastic storage and loss moduli were measured as a function of temperature and oscillatory frequency for 0.5% (w/w) gellan:80% (w/w) cosolute dispersions with added Na(+) (40 to 160 mM). Isothermal frequency (0.15 to 15 Hz) and thermal scans (at 0.15 Hz) were performed over a decreasing then increasing temperature range of 85 to 5 degrees C and 5 to 85 degrees C, respectively. Moduli were found to increase in magnitude with decreasing temperature and increasing levels of Na(+) during cooling, then remained relatively thermally irreversible upon heating. Isothermal frequency (ITF) data were described using the time-temperature superposition (TTS) principle and the modified Cole-Cole (MCC) analysis. Both TTS and the MCC analyses successfully described the behavior of samples containing 40 mM added Na(+) during cooling and heating, and at the 100-mM Na(+) level during cooling. TTS superposed ITF data over the entire temperature range, whereas successful superposition was restricted to lower temperatures in the MCC analysis, where the viscoelastic response was dominated by the long-range relaxation of gellan chains between junction zones. Failure of both analyses was attributed to the formation of junction zones composed of polymer-polymer associations. It is proposed that the addition of Na(+) promotes the formation of a weakly cross-linked gellan network.