Chemistry Reference
In-Depth Information
100
1000000
100 000
10
10 000
1000
1
0.01
0.1
1
10
100
0.01
0.1
1
10
100
Frequency (rad per second)
Frequency (rad per second)
(a)
(b)
Fig. 5.19
(a) Complex viscosity as a function of frequency for 1.2% xanthan (
)and
G
,
2% modified starch (
◦
) in tap water. (b) Mechanical spectrum of 1.2% xanthan (
G
) and 2% modified starch (
G
,
G
) in tap water.
•
◦
nectar-like and honey-like consistencies, but because of the highly pseu-
doplastic nature of the viscosity, very high concentrations are needed to
reach the spoon-thick consistency. The starch products must be used at
higher concentration than the xanthan and, based on this measurement
criterion, are more able to create the spoon-thick consistency.
The consistency categories are based on a single point viscosity mea-
surement and do not reflect the overall texture of the products. The
viscoelastic properties of xanthan and modified starch at concentrations
that give approximately equivalent viscosity at a shear rate of 50 per
second are shown in Fig. 5.19a and b. The dynamic viscosity of the
products is very similar at 50 rad per second, but the xanthan has a
constant slope approaching
1, which is typical of a gel, whereas the
starch tends towards a Newtonian plateau at low frequencies, which is
more typical of an entanglement network. Similarly for the moduli, the
xanthan response is typical of a weak gel with the elastic modulus (
G
)
dominating across the measured frequency range and both moduli show-
ing very limited frequency dependence. The starch has a profile typical
of an entanglement network: both moduli show frequency dependence
with the viscous modulus (
G
) dominating at lower frequencies and
the elastic modulus (
G
) dominating at higher frequencies. The ideal
rheology for swallowing has yet to be established, but perhaps the most
important factor in the choice of the thickener will be the palatability
of the product, after all, if the product is not acceptable from a sensory
standpoint, then patients will not eat the products.
Recent work on the miscibility of solutions thickened with different
hydrocolloids has revealed some interesting differences that could be of
relevance to this application. It has been shown that solutions prepared
with particulate thickeners such as starch mix more easily with water
than do those thickened by polymer solutions such as guar gum and
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