Chemistry Reference
In-Depth Information
Roof of mouth
V
= velocity of tongue (2 cm per second)
F
= normal force (1 N)
R
= radius of plug (2.5 cm)
t
= time (per second)
h
0
= initial plug height (0.2 cm)
m
= consistency index
n
= power law index
R
h
V
(values in brackets are those assumed in
the calculation of oral shear stress)
F
Tongue
n
2
/(
n
+1)
1/
n
(
n
+1)
t
(8.1)
1
F
n
+3
+
τ
=
m
V
n
h
0
(
n
+1)/
n
2
π
m
2
n
+1
R
n
+3
Fig. 8.1
A model mouth geometry and the Kokini oral shear stress. Reproduced from
Cook et al. (2003), with permission from Oxford University Press.
syrups. The viscosity curves (viscosity as a function of shear rate) of
a soup and a glucose syrup of similar perceived viscosity will cross,
and the shear rate where this occurs was considered to be the shear
rate pertinent to thickness perception. The value for this shear rate that
Wood found from his work was 50 per second. In recent work, this
value is often still considered to be the appropriate shear rate for vis-
cosity measurement if sensory perception studies are to be carried out
subsequently.
Since Woods original work, there were many other studies. Shama
and Sherman (Shama and Sherman, 1973) used a similar approach with
a much wider range of foods. They reported a range of shear rates
and shear stresses relevant to perception of viscosity, suggesting that
the mechanism of perception was different for high-viscosity foods
when compared to low-viscosity foods. At high viscosities, perception
is driven by the stress required to shear the food at a shear rate of 10
per second, whereas for low-viscosity foods, the shear rate in response
to a shear stress of 10 Pa drives perception. The shear stress and shear
rate conditions for thickness perception from the Shama and Shermans'
experimental work agree quite well with the results of a modelling
approach developed by Kokini and co-workers (Dickie and Kokini,
1983; Kokini and Cussler, 1983; Kokini, 1985). This considered the
liquid food as sheared between the tongue and the roof of the mouth
(Fig. 8.1). One output from this approach is an oral shear stress defined
by Equation 8.1 (see Fig. 8.1), which it can be argued is an appropriate
parameter to relate to thickness perception. As discussed in the following
text, the Kokini oral shear stress has often been used as a parameter for
sensory studies of taste perception.
