Mixed gels - Physical Gels from Biological and Synthetic Polymers - page 314

κ -carrageenan mixtures

κ galactomannan

Dea et al.( 1972 ) proposed their model for the junction zone ( Figure 10.12 ) formed by the
interaction between

-carrageenan -

-carrageenan and galactomannan mainly on the basis of optical
rotation measurements, supported by determination of the

κ

by a falling ball
method, syneresis studies, X-ray diffraction and computer model building. They found
thermal hysteresis in the temperature dependence of optical rotation for native (unde-
graded) and so-called segmented

' gel point '

-carrageenan (in which the chain is split chemically to
produce only the helix-forming parts), in both the absence and the presence of galacto-
mannans with various mannose/galactose ratios. It was already known that a solution of
native

κ

κ

-carrageenan, diluted until it would not gel when cooled alone, could gel if mixed
with a galactomannan with M/G = 3.35, but the same was true even for segmented
κ -carrageenan, which would not gel alone at any concentration but would gel with this

galactomannan. They found that both effects decreased with increasing galactose con-
tent, and that the galactomannans having the highest content of galactose residues
(M/G= 1.08) showed no interaction at all.
Structural studies, including X-ray, infrared and enzymatic degradation, and other
chemical analyses had already suggested that any ordered conformation of the galacto-
mannan would be fairly fully extended and alternately rather densely and rather lightly
substituted with galactose residues. From this evidence Dea et al.( 1972 ) proposed their
picture of the

molecular ribbon ( Figure 10.12 ), where ordered
binding occurs only between the carrageenan helix and parts of the galactomannan
backbone that contain contiguous unsubstituted mannose residues.

' hairy ' and ' smooth '

κ konjac glucomannan

Dea ( 1981 ) reported that konjac glucomannan (KGM) induced

-carrageenan -

-carrageenan at a non-
gelling concentration to form a thermoreversible gel. Based on optical rotation measure-
ments, he concluded that KGM gelled with

κ

-carrageenan by inducing double helix
formation. In 1987, Cairns et al.( 1987 )usedX-ray

κ

fibre diffraction to study the interactions

between KGM and κ -carrageenan. The diffraction patterns obtained for a film prepared

from KGM and -carrageenan, mixed at the weight ratio of 1:1 in aqueous solution, was

characteristic of pure

κ

κ

-carrageenan, the unit cell dimensions were unchanged and there
were no perturbations from KGM. They pointed out that if intermolecular binding had
occurred between the two polymers, the mixed junction zones would have given rise to a
diffraction pattern distinct from either of these two pure polymers. Since this did not occur
in this test, they took issue with the model suggested by Dea, and proposed an alternative
model. In this model the KGMmolecules are not directly involved in the three-dimensional
structure, but only distributed within the gel network of -carrageenan.

Williams et al.( 1993 ) used DSC, electron spin resonance (ESR) and small-
deformation rheology to study the interactions between KGM and

κ

κ -carrageenan.

Figure 10.15 shows DSC cooling curves obtained for -carrageenan and KGM mixtures

at a total polysaccharide concentration of 0.6% in 0.05M KCl solutions. In the systems
where carrageenan was in excess, a second peak appeared at 40°C. They concluded that

κ