Time–temperature studies of gellan polysaccharide gelation in the presence of low, intermediate and high levels of co-solutes

Abstract

Thermorheological scanning, time–temperature superposition (TTS), and modified Cole–Cole (MCC) analyses are sensitive indicators of temperature-dependent morphological changes during gelation. In the present study, the gelation of gellan polysaccharide in the presence of low, intermediate and high levels of co-solutes (sucrose and corn syrup) was studied using thermorheological scans, TTS, and MCC analyses. Thermorheological scans at 0.15 Hz and isothermal oscillatory frequency scans between 0.15 and 15 Hz were performed over a temperature range of 5–85 °C. The addition of low concentrations of co-solutes (0–50% w/w) had a stabilizing effect on gellan, whereby the sol-gel transition temperature, gel strength and thermal hysteresis increased. Both TTS and MCC analyses failed due to micro- and macrostructure transformations associated with aggregation and junction zone formation. At intermediate co-solute levels (70–75% w/w), aggregation was hampered by the high viscosity of the co-solute medium, giving a bimodal cooling profile which extended over the entire temperature range. In addition, increasing co-solute concentrations within the intermediate range caused the network structure to weaken progressively. TTS and MCC analyses were valid only in regions where the viscoelastic properties were governed by co-solute behavior. Thermal scans of gellan: high co-solute systems during cooling showed a rise in the magnitude of viscoelastic parameters (storage modulus ( G′), loss modulus ( G″)) as co-solute concentrations were raised from 77.5 to 85% (w/w). Upon cooling from 85 to 5 °C and then re-heating, samples were thermally reversible up to about 60 °C, above which a significant rise in G′ was evident. The temperature at which this rise initiated increased with added co-solutes. Isothermal oscillatory frequency scans were performed at co-solute levels of 80 and 85% (w/w). With the exception of scans corresponding to the rise in G′ during heating, at temperatures >60 °C successful superposition of isothermal frequency curves was achieved with both the TTS and MCC analyses. Failure to superpose data during structure formation suggests that temperature-dependent relaxation mechanisms associated with macrostructural changes occurred within the material. It is hypothesized that the gellan forms ‘gel particulates’ or ‘gel islands’ embedded within the co-solute matrix, rather than a continuous network.