Viscous solutions, networks and the glass transition in high sugar galactomannan and kappa-carrageenan mixtures.
Small-deformation rotational oscillation was used to examine the effect of small additions of galactomannan and kappa-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 state (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 (GG') to the glassy state (GG"). 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 kappa-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.
Kasapis
S
S
Department of Food Science and Nutrition, College of Agriculture, Sultan Qaboos University, PO Box 34, Al-Khod, Oman. stefan@squ.edu.om
Al-Marhoobi
I M
IM
Khan
A J
AJ
D002241:Carbohydrates / Q000737:chemistry; D002351:Carrageenan / Q000737:chemistry*; D005139:Eyeglasses; D005690:Galactose / Q000031:analogs & derivatives; D005947:Glucose / Q000737:chemistry; D008351:Mannans / Q000737:chemistry*; D011134:Polysaccharides / Q000737:chemistry; D012996:Solutions; D014783:Viscosity
D002241:Carbohydrates; D008351:Mannans; D011134:Polysaccharides; D012996:Solutions; C012990:galactomannan; D002351:Carrageenan; D005947:Glucose; D005690:Galactose