Chemistry Reference
In-Depth Information
100
10
1
0.1
0.01
Suspension
Cling
Mouthfeel
Pouring
Pumping
Levelling
0.001
0.001
0.01
0.1
1
10
100
1000
Shear rate (per second)
Fig. 5.1
Comparison of the flow behaviour of thickening agents: 0.3% xanthan gum (
•
),
1% guar gum (
◦
), 1% LBG (
♦
), 1% alginate (
), and 1% cellulose gum (
).
viscosity at low shear rates at much lower concentrations. Behaviour at
low shear rates is important for suspension stability, whereas behaviour
at medium rates can provide information about cling and mouthfeel.
Viscosity at high shear rates can determine the behaviour in processes
such as filling, pouring, pumping and spraying.
The flow behaviour of xanthan gum is a result of intermolecular asso-
ciation among xanthan polymer chains which results in the formation of
a complex network of entangled rod-like molecules. These associations
are driven by the charges that are carried on the pyruvate group of the
terminal mannose and the glucuronic acid unit sandwiched between the
two mannose units of the trisaccharide side chain. This highly ordered
network of entangled stiff molecules gives rise to xanthan solutions hav-
ing the viscoelastic characteristics of a weak gel. This is illustrated in
Fig. 5.2, which shows that the xanthan solution has a dominant elastic
response to frequency in contrast to other polysaccharide thickeners,
such as guar, which have a dominant viscous response. These rheo-
logical characteristics provide suspension stability to finished products,
combined with the ease of filling, pouring and pumping.
Xanthan gum also has the ability to interact with galactomannans
such as guar gum, cassia gum, tara gum and locust bean gum (LBG)
and with structurally similar polysaccharides such as the glucoman-
nan konjac (Kovacs, 1973; Pettitt, 1982; Dalbe, 1992; Urlacher and
Noble, 1999; Sworn, 2000). The specific binding between the extracel-
lular polysaccharide and typical components of plant cell walls, such
as galactomannans and other polysaccharides having a β-1,4-linked
