Chemistry Reference
In-Depth Information
110
100
90
80
70
60
50
0.00
0.10
0.20
0.30
0.40
NaCl (%)
Fig. 5.6
The midpoint temperature of the thermal transition of 0.5% xanthan gum as a
function of NaCl concentration as measured in Fig. 7.5.
Xanthan gum has been described as having three conforma-
tional states (Holzwarth and Prestridge, 1977; Milas and Rinaudo,
1984):
(i) Native, ordered form
- Never heated
(ii) Denatured, ordered form
- Heated to transition temperature or above, then cooled
(iii) Disorderd form
- Present at high temperature or low salt concentrations, or both
The native, ordered form (i) is the typical form of the xanthan gum
in the broth at the end of fermentation which has not been subjected
to a heat treatment. It has been shown to have lower relative viscosity
and intrinsic viscosity compared with that of the denatured, ordered
form (ii), but the same molecular weight, suggesting a more compact
conformation. During the production of commercial xanthan gum, the
fermentation broth undergoes a thermal treatment to kill the bacteria
prior to the precipitation of the gum. This means that solutions prepared
with commercial xanthan gum can be considered to be predominantly in
the denatured, ordered form (ii). The impact of the conformational tran-
sition on the rheological properties of xanthan gum in relation to food
ingredients and processes will be discussed in detail in the subsequent
sections.
