Chemistry Reference
In-Depth Information
2.0
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
Native
xanthan
HPMC
Modified
waxy maize
Processed
Gelatine
xanthan
starch
0.00
0.20
0.40
0.60
0.80
Viscosity 50 per second (Pa
.
s)
Fig. 8.11
Comparison between shear viscosity at 50 per second and filament break-up
time using the Haake CaBER extensional rheometer (at 20
◦
C) (reference).
shear rate and taking the shear viscosity measured for the starch suspen-
sion at the shear rate applied to calculate
p
. Hence, since the viscosity
of the second immiscible fluid was kept constant, larger
p
values coin-
cide with lower applied shear rates and therefore flow stresses. As soon
as these were below the yield stress, droplet break-up was no longer
observed; the reader should consult the original publication for detail.
Another approach to evaluate the differences in break-up behaviour
between microstructurally and fundamentally different fluids is to pro-
duce filaments of these fluids, for example in the Haake CaBER ex-
tensional rheometer. This instrument allows filaments to be stretched
rapidly to a given height. Following stretching, the filament thickness
at half the stretching height is monitored with a laser micrometer. The
filament may not always be the thinnest at half height, which, however,
is not crucial for this application as the interest lies in the break-up time
as a parameter, which can be obtained from the instrument's software.
Fig. 8.11 contrasts the difference in droplet break-up time for five sys-
tems discussed in the chapter (molten gelatin, modified waxy maize
starch, native xanthan, xanthan processed to give a particulate structure
(Sereno
et al
., 2007) and HPMC). It can be seen that the good mixing
systems cross-linking starch, gelatin and particulate xanthan have much
shorter droplet break-up times than that of the poor mixers HPMC and
native xanthan.
To predict perception, the authors believe that a combination of shear
and extensional parameters are required. This topic is entitled
Practical
Food Rheology
. Most food laboratories are not well equipped to measure
extensional viscosity, which provides considerable experimental and
theoretical challenges. From a practical point of view, it may be possible
to develop a semi-empirical quantitative parameter related to droplet
