Viscous solutions, networks and the glass transition in high sugar galactomannan and κ-carrageenan mixtures

Small-deformation rotational oscillation was used to examine the effect of small additions of galactomannan and K-carrageenan on the Vitrification of glucose syrup at a total level of solids of 83%. The method of reduced variables allowed construction of composite curves covering the glass transition and glassy slate (from 10(5) to 10(9.5) Pa) over a wide frequency range (up to 15 orders of magnitude). The combined WLF/free volume framework was employed to determine the rheological glass transition temperature (T-g), fractional free volume and thermal expansion coefficient of the samples. It was found that the WLF-predicted glass transition temperature matched the cross over of experimental modulus traces in the passage from the glass transition (G " > G') to the glassy state (G' > G "). This coincides with the mechanistic transformation from free volume effects to the Arrhenius-type phenomena, thus ascribing physical significance to the rheological T-g. The T-g value of 83% glucose syrup at a scan rate of 2 degrees C min(-1) was - 25.3 degrees C. Replacing, for example, 1% glucose syrup with guar gum shifted the T-g of the mixture to - 19.7 degrees C. Network formation via the K+-supported junction zones of the K-carrageenan chains further increased the T-g to about - 1 degrees C. It appears that the low rates of relaxation processes and diffusion mobility in the presence of a polysaccharide network accelerate the collapse of the free volume thus inducing vitrification of the high sugar/polysaccharide mixture at high temperatures. (C) 2000 Elsevier Science B.V. All rights reserved.