Chemistry Reference
In-Depth Information
80
60
40
20
0
0
2
4
6
8
10
12
14
Time at room temperature (weeks)
Fig. 5.13
Variation with time of the low shear viscosity of xanthan gum in a model
vinaigrette (0.3% xanthan, pH 2.5, 1% NaCl).
the 13 weeks storage at room temperature. This translates into good
stability throughout the shelf life of the product.
5.4.3
Xanthan and proteins
The incompatibility of polysaccharides with milk proteins is well
known. It is the result of depletion flocculation, is dependent on the
concentration of polysaccharide and milk protein and often results in
phase separation. Xanthan gum is no exception (Hemar
et al.
, 2001).
For example, when low concentrations of xanthan gum (0.01%-
0.1%) are added to milk and stored at 5
◦
C for 3 days, phase sepa-
ration occurs (see Fig. 5.14). Using rheological measurements, it can
be shown that the upper phase is enriched with the polysaccharide evi-
denced by the typical pseudoplastic flow characteristics of xanthan gum,
whereas the Newtonian flow of the lower phase indicates that it is en-
riched with the protein (see Fig. 5.15). A simple phase diagram of milk
content (protein) and xanthan concentration identifies two principal re-
gions (see Fig. 5.16). The continuous line separating the two regions is
called the binodal curve. In the monophasic (stable) region, below the
binodal curve, the two polymers coexist in one single phase. There is
thermodynamic compatibility. In the biphasic (unstable) region, above
the binodal, the system separates spontaneously into two phases. There
is thermodynamic incompatibility. At high levels of xanthan, a single
phase is seen. Flocculation may occur in this region, but the phase sep-
aration is not visible due to the stabilising effects of the viscosity from
the xanthan gum.
