Mixed gels - Physical Gels from Biological and Synthetic Polymers - page 312

can be attributed simply to a failure to appreciate the contribu-

tion, together with the assumption that, for example, mechanical properties are simply
additive. However, in the case of xanthan and the galactomannans it could be said that the
change is qualitative rather than just quantitative, since neither component forms true
gels at any practical concentration, whereas they can be formed simply by heating the two
solutions together. In future sections we will continue to use the term, bearing in mind the
above caveats.

' effective concentration '

10.6.2.1 Molecular models for polysaccharide gel interactions
A number of molecular models have been employed to understand the molecular
interactions of such gels, and these are now introduced. All are idealized,

but over the years they have been re

' synergistic '

ned. In later sections we introduce the evidence for

each, and the areas of controversy.
The first is that of Dea et al.( 1972 ) for the interaction of κ -carrageenan and galacto-

mannan. They discovered that segmented κ -carrageenan - which does not gel alone at

any concentration firm, rubbery gels in mixtures with a galactomannan with a

limited amount of galactose substituent, whereas the effect decreased with increasing
galactose content; and that the galactomannans having highest content of galactose
residues showed no interaction at all. From this Dea et al. proposed a picture of a
molecular ribbon that is alternately

- forms

' ' ' ' ' '

hairy and smooth , the hairy regions correspond-

ing to the presence of galactose substituents and the ' smooth ' regions their absence. The

' smooth ' regions are presumed to be able to bind to κ -carrageenan, whereas the ' hairy '

regions cannot ( Figure 10.12 ).
A similar explanation was applied to xanthan

galactomannan (X-GM) mixed gels, as
illustrated in Figure 10.13 , the interaction between xanthan and the galactomannan
occurring only where the galactomannan is not side-chain branched (i.e. it is

-

' smooth ' ).

Subsequently this area has become associated with such molecular diagrams - strictly

speaking they are not models at all and Figure 10.13 represents an alternate description

of the binding of xanthan and galactomannans which no longer requires the galacto-
mannan to be perfectly smooth.

-

C G
Figure 10.12 Model proposed for the interaction between chains of κ -carrageenan (C) and galactomannan (G).
The double helix of κ -carrageenan binds with the non-substituted region of galactomannan. The
zigzag line represents the so-called ' smooth ' region of galactomannan. Reprinted with permission
from Dea et al.( 1972 ) © 1972 Elsevier.