VITRIFICATION OF KAPPA-CARRAGEENAN IN THE PRESENCE OF HIGH-LEVELS OF GLUCOSE SYRUP

We present evidence that co-solute (glucose syrup) can cause massive c hanges in the time/temperature dependence of structures formed by K-ca rrageenan. Modest concentrations of glucose syrup ( < 40%) promote con ventional disorder-to-order transitions of its hydrogels and reinforce the final network strength, as seen by small deformation dynamic osci llation. In contrast, at higher concentrations of co-solute, there is a leap in thermal stability which is associated with a drop in network strength. Differential scanning calorimetry demonstrates that the ris e in network strength is accompanied by more pronounced enthalpic even ts, but the change in enthalpy declines in accordance with the drop in storage modulus at higher levels of co-solute. Cooling of the samples triggers a transformation from low modulus solid-like to high viscosi ty liquid-lice behaviour and at 85% cosolute, shear moduli increase by about five decades. Eventually the storage modulus overtakes the loss modulus completing the progression from the rubbery plateau and glass transition region to the glassy state, in the way reported for amorph ous synthetic polymers. Vitrification was equivalently followed by cha nging the temperature or frequency of measurement thus implementing th e time-temperature superposition principle. The Williams-Landel-Ferry/ free volume theory was operative in the glass transition zone, thus al lowing calculation of useful parameters, e.g. the thermal expansion co efficient, but it gave way to an Arrhenius relationship for the relaxa tion processes in the glassy state. It is proposed that the addition o f co-solute induces a transformation from an enthalpic aggregated stru cture to a partially cross-linked structure of more flexible chains wh ere the entropic contribution to elasticity becomes dominant, thus all owing vitrification of the polysaccharide network. (C) 1998 Elsevier S cience Ltd. All rights reserved.