Chemistry Reference
In-Depth Information
< pH 4
- Progressive change to un-
ionised carboxylate groups
(COO
-
+ H
+
= COOH)
- Suppression of electrostatic
repulsion between xanthan
side chains
- More flexible chain
- Lower low-shear viscosity
- Lower synergy with
galactomannans
-
-
pH 4
-
-
-
-
-
-
-
> pH 4
- Ionised carboxylate groups
(COO
-
)
- Electrostatic repulsion between
xanthan side chains
- Rigid-rod like chain
- High low-shear viscosity
-
-
-
-
-
Fig. 5.12
Schematic representation of the reversible change in xanthan viscosity below
pH 4.
of a more compact molecular shape, resulting in the observed reduction
in viscosity. Neutralisation would re-ionise the carboxylate groups and
allow the xanthan to return to the original conformation at neutral pH
with the associated recovery of the original viscosity. This is illustrated
schematically in Fig. 5.12.
It is important to understand that between pH 4 and 2, the low shear
viscosity of xanthan and the synergistic interactions with galactoman-
nans will decrease with decreasing pH. The practical implications of
this are that small changes in pH in this range could result in changes in
the suspension power, and hence stability, in some acidic food products
such as dressings and vinaigrettes. However, since the viscosity at higher
shear rates is less sensitive to the pH, it is likely that the mouthfeel of
the products will remain relatively unaffected.
5
.
4
.
2
.
2
The stability of xanthan at low pH
Although it has been shown that the viscosity of xanthan gum is sen-
sitive to pH below 4, the stability with time in acidic conditions is still
excellent. Xanthan is widely used in acidic food products such as salad
dressings and vinaigrettes, where it shows exceptional long-term sta-
bility. This is illustrated in Fig. 5.13 in a model vinaigrette solution at
pH 2.5, which has a virtually constant viscosity at low shear rate during
