Relationship between Food Rheology and Perception - Practical Food Rheology - page 176

There are two limitations to predicting an oral perceived viscos-
ity simply from the viscosity curve (viscosity-shear rate relationship).
These are as follows:

(i) There will be other rheological parameters which must be important.
For example, it has been shown (Cutler et al ., 1983; Richardson
et al ., 1989) that weak gels such as xanthan gum solutions were
perceived as being thicker than the measured steady shear viscosity
shows. It was found that if a complex viscosity measured at a
frequency of about 50 rad/s was used, then the perceived thickness
could be predicted correctly (Richardson et al ., 1989). This could
be taken as showing that a dynamic viscosity is more appropriate
for predicting perceived thickness than a steady shear viscosity, but
there is no obvious direct mechanistic reason why this should be the
case. A possible reason for under-predicting the perceived thickness
of xanthan gum solution from shear viscosity measurements is that
there is a stress overshoot when the elastic xanthan solution is
sheared, and this peak stress rather than the equilibrium value is the
relevant one for thickness perception.
(ii) Time effects will be important. Many non-Newtonian liquid foods
are thixotropic, that is to say, the viscosity decreases not only with
increasing shear rate but also with the time of shearing. This clearly
complicates the measured liquid viscosity required to predict per-
ceived thickness. As will be discussed in more detail below, dilution
with saliva may also be important. To obtain the best correlation
between perceived thickness and instrumental measurements, it is
advisable to ask the panel to make the judgement immediately after
the product enters the mouth.

8.3 RHEOLOGY AND FLAVOUR PERCEPTION

There is a long history of studies showing that if the viscosity of a
solution is increased, for example by adding a hydrocolloid, there is a
reduction in perceived flavour or taste (Stone et al ., 1974). The mech-
anism for this is a matter of debate. Perception of flavour comes ini-
tially from a combination of taste which is perceived in the mouth and
volatiles which are perceived in the nose. The reduction in percep-
tion with viscosity is found for a wide variety of thickeners though, as
demonstrated below, starch may be an important exception. This gener-
ality of the effect argues against a specific binding between the volatile
and the thickener being the ubiquitous mechanism, though there have
been several studies where this has been suggested for specific cases. In
a series of particularly valuable studies, Baines and Morris (1987, 1988,