Chemistry Reference
In-Depth Information
the acetate group by alkali hydrolysis. These results appear to question
the conventional thinking on xanthan gum structure and synergy and
suggest that the pyruvate group, and perhaps the acetate/pyruvate ratio,
plays a more important role than previously thought.
The structure of the galactomannan also has an influence on the syn-
ergy with xanthan gum. The degree/strength of associations is increased
as galactose substitution is decreased. For example, LBG, which has
a galactose content of 17-26%, forms self-supporting gels with xan-
than gum, whereas guar gum, which has a galactose content of 33-40%,
forms weak gel networks, resulting in a synergistic increase in viscosity.
The molecular weight of the galactomannans is also known to influence
their interactions with xanthan gum. The lower the molecular weight,
the weaker the interactions seen with xanthan gum (Schorsch
et al.
,
1997; Sworn, 2009).
5.3
THE CONFORMATIONAL STATES OF
XANTHAN GUM
Solutions of xanthan gum undergo a conformational transition during
heating, which is associated with the change from a rigid, ordered state at
low temperature to a more flexible, disordered state at high temperatures.
This conformational change was first observed as a sigmoidal change
in viscosity (Jeanes
et al.
, 1961). This is illustrated in Fig. 5.5 using a
0.5% solution of xanthan gum in 0.1% NaCl. The temperature of the
conformational transition increases with increasing ionic strength of the
solution as shown for NaCl concentration in Fig. 5.6.
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
20
40
60
80
100
Temperature (°C)
Fig. 5.5
The conformational transition of 0.5% xanthan gum solution in 0.10% NaCl
measured by viscosity.
