Chemistry Reference
In-Depth Information
100
10
1
0.1
0.01
0.001
0.01
0.1
1
10
100
Frequency (rad per second)
Fig. 5.3
Elastic modulus for 0.6% solutions of 100% guar (
), 80:20 guar:xanthan (
•
)
and 50:50 guar:xanthan (
◦
).
The primary structure of xanthan gum is shown in Fig. 5.4. It consists
of a cellulosic backbone of β-1,4-linked D-glucose units substituted on
alternate glucose residues with a trisaccharide side chain.
The trisaccharide side chain is composed of two mannose units sep-
arated by a glucuronic acid (Jansson
et al.
, 1975; Melton, 1976). Ap-
proximately half the terminal mannose units are linked to a pyruvate
group, and the non-terminal residue usually carries an acetyl group. The
carboxyl groups on the side chains render the gum molecules anionic.
Xanthan gum has a molecular weight of about 2
10
6
Da.
X-ray diffraction studies on xanthan gum fibres have identified a
right-handed fivefold helix conformation (Moorhouse
et al.
, 1977). In
this conformation, the side chains are aligned with the backbone and
stabilise the overall conformation. In the solution, the side chains wrap
around the cellulose-like backbone, thereby protecting it. For example,
although the xanthan gum has a cellulosic backbone, it is very resistant
to enzymatic degradation by cellulases because, due to the presence of
the side chain, they are unable to access the main β-1,4-linked chain.
The role of the acetate and pyruvate groups in the molecular structure
of xanthan gum and their impact on functionality are probably the most
widely studied aspect of the structure-function relationship. Particular
emphasis has been given to their role in controlling the rheology of the
xanthan gum and their influence on interactions with the galactoman-
nans. Assuming only one acyl group per side chain, the stoichiometric
amounts of acetyl and pyruvate are 5.0 and 8.1%, respectively (Shatwell
et al.
, 1990). However, the levels in commercial xanthan gum are typi-
cally lower than this.
It has been shown in several studies that increasing the pyruvate
content of xanthan increases the viscosity (Sanford
et al
., 1977; Smith
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