Mixed gels - Physical Gels from Biological and Synthetic Polymers - page 320

weaker (of lower modulus) than for corresponding xanthan LBG mixtures. They also

found that the moduli were dependent on the degree of acetylation of xanthan, in
agreement with Dea and co-workers.
Cairns et al.( 1987 ) and Brownsey et al.( 1988 ) suggested from X-ray diffraction and
creep measurements that the KGM or galactomannan and xanthan intermolecular bind-
ings occurred only when the xanthan helix had been disordered. Addition of 0.5MCaCl 2
raises the denaturation temperature of xanthan so it is in an ordered conformation at high
temperatures, which suggests that gelation does not occur between ordered xanthan and
konjac glucomannan. The proposal that the junction zone is formed only between
disordered xanthan and galactomannans with lower galactose content is supported by
conformational studies using chiroptical methods (Cheetham and Mashimba, 1990 ;
Bresolin et al., 1998 ) and work by Williams et al.( 1991 ).
Bordi et al.( 2002 ) studied the interaction between KGM and xanthan using CD and
suggested that, in the ordered conformation, the side chains of xanthan lie along the chain
axis. Taking into account that the chromophore groups of xanthan are situated in the side
chains, they concluded that the interaction involves this part of the xanthan moiety, in line
with conclusions of Annable et al.( 1994 ).
Williams et al.( 1991 ) studied the interaction between xanthan and KGM by ESR,
DSC and rheology. They reported the mid-point temperature for gelation to be higher in
water than in NaCl solution, and concluded that, in NaCl solution, the interaction
occurred between ordered xanthan chains and KGM, while in water the interaction is
believed to be predominantly between KGM and the disordered xanthan chains, leading
to a more stable gel.
Annable et al.( 1994 ) used DSC and ESRmethods to show that the interaction between
KGM and xanthan occurred in water immediately following xanthan side chain

-

back-
bone association. In the presence of electrolyte, the conformational change of xanthan
shifted to higher temperatures and depended on the nature of the cation. Their results also
showed that the temperature for KGM

-

xanthan interaction was much lower than that for
the xanthan conformational transition, and that electrolyte promoted xanthan self-
association at the expense of KGM

-

xanthan interaction.
Goycoolea et al.( 1995 ) suggested that there was a structural rearrangement after the
initial stage of gel formation in xanthan

-

-
LBG gels. Figure 10.14c shows their proposition. Using X-ray diffraction, they sug-
gested that the initial gelation involves so-called heterotypic junctions between KGM or
LBG (i.e. different from those formed by either individual component) and xanthan or
deacetylated xanthan, but that junctions involving KGM convert to a more compact six-
fold arrangement at a lower temperature. Comparison with DSC traces for xanthan alone
led the authors to conclude that conformational ordering of the xanthan component
completed at 60°C. This was thought to be consistent with the concept of binding to
the xanthan helix, as in the alternative model ( Figure 10.14a ), but inconsistent with the
necessary involvement of disordered xanthan in synergistic gelation, as in Figure 10.14b .
For a deacetylated xanthan, by contrast, they found that gel formation occurs with the
xanthan in the disordered state, which is entirely consistent with Figure 10.14b but
contradicts Figure 10.14a .

- KGM, but this was not observed in xanthan