1.5. Fundamental Rheometry
The deformation curve of an Extensogram and the stress strain curve
are similar in appearance and take a similar course (Fig. 62). It is hoped that
this fact will lead to greater acceptance of fundamental rheometry in cereal
laboratories. There is no similarity to the Alveogram, since it is a pressure
curve and not a deformation curve. If a ruler is placed behind the expanding
dough bubble so that the increase in the size of the dough bubble can be
measured, the resulting deformation curve made up of the measured points also
shows similarity to the Extensogram.
1.5.1. Viscometer
Fundamental rheometry came into being with the pioneering work of G.
W. Scott Blair – and it is characteristic that the material he used for his
trials was a wheat dough (Schofield and Blair, 1932). This substance, that was
initially a problem to the rheologists because of its "memory"
(meaning the stored energy of its elastic behaviour), subsequently took
rheometry and rheology a great step forward.
There is a relationship between conventional and fundamental rheometry.
They use a similar deformation force, but in fundamental rheometry this is
variable and therefore capable of describing the flow properties of a material
under different loads or stresses in a test with a universal viscometer. The
result is a flow curve, or stress strain curve, in which changes of stress are
recorded over changes in strain. The stress in the curve is either the chosen
deformation force, by which the change in strain is measured, or it is a
measure of the resistance to deformation if the strain is varied under
controlled conditions during the test. In both cases the viscosity is
calculated from the two physical values stress and shear rate, and since the
magnitude of the deformation force (measured area and force) and of the strain
is defined, it is expressed in absolute physical units. These physical units
permit a direct comparison of results from viscometers made by different
manufacturers. Moreover, by calculating the viscosity, a flow curve can be
"redrawn" as a viscosity curve (Fig. 61); the two types of curve
yield the same information, and it is up to the person interpreting the results
to choose the type of curve he prefers. The two types of curve show the flow
properties of the substance tested; in particular they indicate any shear-dependent
or time-dependent anomalies of flow behaviour that may occur in the test. The
viscosity curve of a dough yields very important information on the rheological
properties of the dough under different deformation forces. It shows that the
dough has a yield point, and that its viscosity (consistency) falls as the load
increases. This property is known as structural viscosity or shear-thinning and
is caused and explained by the orientation of the molecules and aggregates in
the flow field. It means that when exposed to only a low deformation force,
such as stretching by the fermentation processes during the resting time, a
dough has a higher viscosity than during transportation through the pipes or
testing with the Farinograph or Extensograph (Fig. 61). This justifiably raises
the question of how to describe the state of a dough at the low deformation
forces in the process with a method that uses high deformation forces (Tanaka,
et al., 1980). In other words: with high deformation forces it is only possible
to determine the mechanical properties of the dough, not its rheological
properties. A comprehensive work by several authors has been published on the
subject of food rheology in general and the advantages of fundamental
rheometry, including dough rheology, in particular. It deals with the
importance of rheology for explaining and improving the quality of foods
(Weipert and Tscheuschner, 1993).
 |
| Fig. 61: Stress strain curves of wheat flour doughs with different dough properties recorded with a rotational viscometer (left) and a viscosity curve showing "yield point" and "shear thinning" (right) |
 |
| Fig. 62: Comparison of recorded curves: Extensogram, Alveogram and
stress-strain curve |
The instruments used in rheometry differ in respect of their measuring
principle, their mode of use and thus the presentation of the results. In
rheometry the flow behaviour of a material is monitored between two parallel
flat plates, in the circular gap between two coaxial cylinders, between two
round parallel plates, between a cone and a plate or finally in a capillary
tube. The rotational viscometers have shown themselves to have various
advantages and are therefore the most widely used. A rotational viscometer
determines the viscosity of a material in a simple test; in the measuring gap
of the instrument the conditions are "stationary" and the flow is
laminar. A relatively simple and therefore inexpensive viscometer makes it
possible to record stress strain curves that reveal certain information about
the dough. The yield point and the viscous properties of a dough can be read
off from the shape and pattern of a stress strain curve and the (automatically)
re-calculated viscosity curve. From the viscosity it is possible to determine
the water absorption or the volume yield of the dough. The shape of the stress
strain curve reveals the properties of the dough: a dough with short properties
has a steep stress strain curve, whereas the curve of a dough with soft, weak
properties is flatter (Fig. 61). And finally the stress strain curve makes it
possible to assign a numerical value to the surface stickiness of a dough
(Weipert, 1987a, 1992, 1998 and 1998b). So it is no wonder that such a method,
which requires very little specimen material (10 g flour), is used successfully
in the breeding of wheat (Fig. 61).
إرسال تعليق