Chemistry Reference
In-Depth Information
100
10
1
0.1
0.01
1
3
5
7
9
11
pH
Fig. 5.11
The pH sensitivity of 0.3% xanthan in 1% NaCl measured at 0.01 per second
(
♦
), 0.1 per second (
), 1 per second (
), 10 per second (
) and 100 per second (
◦
)
different shear rates (pH adjusted with a solution of citric acid or NaOH).
pH below 4. This effect, however, is completely reversible upon neutral-
isation, indicating that it is due to changes in molecular conformation
rather than degradation. The effect of pH on the viscosity is much less
marked at higher shear rates, which would indicate that mouthfeel would
be affected less than suspension stability (see Fig. 5.11).
The effect of pH on the synergy of xanthan gum with galactomannans
shows a similar trend to the viscosity of the xanthan at low shear rates
(Sworn and Kerdavid, 2009). The gel strength of the xanthan-LBG
mixed system remains relatively unchanged between pH 7 and 4, but as
the pH is decreased below 4, a progressive drop in the gel strength is
observed (see Fig. 5.10).
Xanthan is a charged polymer and so the pH sensitivity of the low
shear viscosity and synergy is to be expected, since changes in the
pH will result in changes to the charge density of the xanthan, which
will, in turn, influence the molecular associations between the xanthan
molecules themselves and between xanthan and other food ingredients.
Reducing the pH of xanthan gum solutions progressively converts the
carboxylate groups from the ionised to the un-ionised form (COO
−
+
H
+
=
COOH), with consequent suppression of electrostatic repulsion
between xanthan side chains (Agoub
et al.
, 2007; Rinaudo and Moroni,
2009). This could reduce the stiffness of the molecule, allowing adoption
