The influence of compression velocity on strength and structure for gellan gels

Small deformation dynamic oscillation was employed to characterise the structural properties of 0.5% κ-carrageenan gels in the presence of glucose syrup at levels ranging from 65 to 85% in the mixture. Controlled cooling of hot solutions produces rubbery gels, which transform gradually into glasses at subzero temperatures. Increasing amounts of co-solute accelerate the onset of vitrification. At the top of the concentration range employed, the glass transition zone follows from the end of the rubbery region (60°C) to the beginning of the glassy state (−10°C). By now, values of shear moduli approach 108.5 Pa. The vitrification of high solids κ-carrageenan samples was equivalently followed by changing the temperature or the time/frequency of measurement. In doing so, mechanical spectra were obtained at constant temperature intervals and superposed horizontally according to the time–temperature superposition principle. This allowed construction of composite curves covering up to eight decades of frequency and six orders of magnitude of shear moduli. At the lower range of glucose syrup (65 and 70%) the rubbery region is dominant, and the shifting of frequency sweeps can be linearised with the Arrhenius equation. At 75 and 80% co-solute the thermal dependence of shift factors transforms from the Arrhenius predictions at the rubbery plateau to the Williams, Landel and Ferry kinetics in the glass transition region, which allows estimation of parameters of the free volume theory. Finally, at 85% glucose syrup the fit reverts from the WLF behaviour covering the glass transition region to the Arrhenius relationship in the glassy state.