Chemistry Reference
In-Depth Information
100
10
1
0.1
0.01
0.001
0.001
0.01
0.1
1
10
Frequency (rad per second)
G
,
G
) and 0.6% guar gum
Fig. 5.2
Viscoelastic profile of 0.6% xanthan gum (
(G
•
, G
◦
).
backbone, may suggest a part in the host-pathogen relationship. It has
been suggested that the extracellular polysaccharide could represent a
'molecular holdfast' to recognise the site at which the bacteria are finally
to attach themselves. The simplest of such functions would be to locate
the bacteria at the plant cell surface in a single layer. A more elaborate
possible function of this type, suggested by the selectivity of binding,
in vitro
, could be to identify a particular area of the surface of a par-
ticular type of plant cell by means of the characteristic polysaccharide
composition of its wall (Morris
et al.
, 1977).
The interaction with galactomannans results in either a synergistic
increase in viscosity in the case of guar gum, or the formation of strong
self-supporting gels as seen with LBG. With LBG, xanthan is able
to form a very elastic gel with optimum strength at a ratio of 60:40
Xanthan:LBG. At low gum concentration, this synergy can be used
to increase the thickening impact of xanthan or LBG and create fluid
systems with gel-like viscoelastic properties.
With guar gum, an increase in viscosity can be seen beyond the value
predicted for a simple mixture. The optimum synergy occurs at a ratio
of 80:20 guar:xanthan. The addition of xanthan gum to guar gum signif-
icantly increases the viscoelasticity of the solutions. This is illustrated
in Fig. 5.3: as the xanthan content in the mixture is increased, the elastic
modulus is increased and becomes less dependent on frequency, indicat-
ing a more gel-like rheology. This is particularly relevant to suspension
stability in finished products.
