Chemistry Reference
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perception results plotted against the oral shear stress parameter which
Cook
et al
. (2003) suggested was the driver for taste perception.
There is more data scatter from an ideal curve compared to some of
the previous study of Cook
et al.
on HPMC-thickened systems, which
is also included in the Figure. This may be due to the high temperature
used by the sensory panel, since the study by Ferry and colleagues was
required to be relevant to soup products. Nevertheless, it is clear that
perception from the starch-thickened systems at high values of the oral
shear stress is much better than that from the HPMC-thickened solution.
Indeed, when wheat starch and modified waxy maize starch are used
as thickeners, it could be argued that there is no inhibition of taste
even at the highest value of oral shear stress measured. If the mixing
hypothesis is to survive, it would therefore be expected that a starch-
thickened solution would mix better with water than a hydrocolloid
(linear polysaccharide) solution. To investigate this, Ferry
et al
. (2006a)
used the simple approach of hand mixing a coloured high-viscosity
'solution' (380 mPa
s at 50 per second and 25
◦
C) with water. The
differences between the starch and the hydrocolloid-thickened solution
were quite dramatic. The results in Figs. 8.2 and 8.3 would suggest that
for the linear polysaccharide, there should be a change in mixing ability
in the vicinity of the c
*
concentration. Subsequent work by Koliandris
et al
. (2008) with locust bean gum showed that this is indeed the case.
The photographs reproduced in Fig. 8.4 illustrate these two related ideas
showing the difference in mixing between the appearance of the HPMC-
and starch-thickened systems mixed at comparable viscosity and the
change in locust bean gum-thickened systems as the concentration is
increased from 0.5 to 0.6%.
The reason why decreasing mixing efficiency should reduce flavour
perception is clear. For a tastant to be perceived, it has to reach the taste
receptor. This receptor will be coated by a static boundary layer of fluid,
and diffusion will be the only mechanism through which the tastant can
be finally transported to the receptor. Diffusion processes however are
extremely slow. For example, for small molecules or ions such as Na
+
,
the diffusion coefficient in water or in the gel is
·
10
−
9
m
2
/s. It is easy to
show that even after 100 seconds, almost none of the molecular species
will have moved a distance of 1 mm from the surface of a gel sphere
with a diameter of 1 mm if diffusion is the only transport mechanism
(Crank
et al
., 1981). Thus if mixing is poor, there will be regions which
retain the original high viscosity/weak gel consistency of the ingested
product. Between these regions, there will be other regions with a low
concentration of tastant. If some of these low-concentration regions are
at the surface of the fluid boundary layer coating, the taste receptor there
will be a reduction in the driving force for diffusion across this boundary
layer. The simple mixing experiment is of course not directly relevant
to the distance scale of mixing or concentrations of tastants in the
∼
