GLASSY-STATE PHENOMENA IN GELLAN SUCROSE CORN SYRUP MIXTURES

The method of dynamic oscillation was employed to investigate changes in the mechanical properties of deacylated gellan gels in the presence of sucrose-corn syrup blends. The effect of sugar, at concentrations between 0 and 50% in the mixture, is to raise gradually the gelling temperature and the mechanical strength of the gellan network. However, there is a sharp increase in thermal stability (formation of a structure at the highest experimentally accessible temperature of 90 degrees C) and substantial reduction in shear modulus development at 60% co-solute in the system which is discussed in terms of a disproportionate conformational ordering and diminishing water-supported, intermolecular junction zones of the gellan strands in a low solvent environment. The dramatic change in physical properties of the gellan-solute blends at 90 degrees C produces a spectacular temperature dependence of the in-phase and out-of-phase viscoelastic components during subsequent cooling. Thus, the high-solids gellan network undergoes transformation from a rubber-like consistency to a glassy state in the manner observed for entangled networks of high molecular weight synthetic polymers. The onset of glass transition is enhanced by increasing amounts of sugars, occurring at about 37 degrees C for the 0.5% gellan plus 85% solute system at an experimental frequency of 10 rad/s, Mechanical spectra of sucrose-corn syrup mixtures at this concentration remain Newtonian at 5 degrees C, the lowest experimentally accessible temperature. According to the time-temperature superposition principle, vitrification of the above gellan sample commences at approximate to 17 Hz at 90 degrees C. Following the empirical procedure developed by Williams, Landel and Ferry, the temperature at which the gellan-solute glass forms (Tg) was found to be close to the vitrification temperature of a sucrose preparation at 15% moisture content (approximate to-25 degrees C).