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concentration exceeded c
*
(Hollowood
et al
., 2002). Therefore, the hy-
pothesis that binding of the volatile to the thickener or inhibition of
volatile release in some other way causes the decrease in perception
can be rejected. As stated earlier, this does not mean that there are not
specific cases of binding of volatiles or indeed tastants which can be
important, such as in the entrapment of flavour molecules in the amylose
helix.
A second hypothesis is that there is a direct relationship between
the rheology of the solution and perception. In a comprehensive
study, Cook
et al
. (2003) investigated the relationship between var-
ious viscosity-related parameters and taste perception and found the
strongest dependence on the Kokini oral shear stress defined by Equa-
tion 8.1. Of course, a viscosity dependence of perception must result
from the relationship shown in Fig. 8.2, but it is of interest that a param-
eter which may be a somewhat better indication of thickness perception
than, for example, the viscosity at a shear rate of 50 per second gives
the best prediction of perception. Since the hypothesis of a decrease in
volatile release with increasing viscosity can be rejected as discussed
above, then there are two further hypotheses that should considered:
(i) Viscosity reduces the extent and/or rate at which the tastant reaches
the taste receptors on the tongue and palate.
(ii) The signal reaching the brain related to the oral shear stress, when
processed in combination with the signals from the taste and volatile
receptors, influences the total perception.
The second hypothesis was suggested by Cook
et al.
(2002, 2003) to
explain the oral shear stress dependence. This multimodal idea where
the response of the brain to changes in apparently unrelated signals
results in perturbation of the 'perception' of the particular response of
interest is very general. It is a very important and exciting aspect of food
science which is developing rapidly, partly because the increasing ease
with which brain activity can be monitored during food consumption.
Partly because it is less directly related to rheology, this will not be
discussed further in the remainder of this chapter.
In addressing hypothesis 1, it is helpful to have exceptions to the gen-
eral relationship between viscosity and perception as found by Baines
and Morris (1988) and Cook
et al
. (2003). One of these is solutions
thickened by gelatinised starch. In view of the importance of starch as
a thickener, it is surprising that not more work has been carried out
on flavour perception from starch-thickened systems. Hill
et al.
(1995)
related the sensory parameters of a lemon pie filling thickened with
cornflour with a range of dynamic rheological parameters. Although
dynamic rheological data were used, it was clear that when these results
