1.5.2. Rheometer
A rheometer can measure viscosity in the same way, but it can also
show the elastic properties of a material in a second test in which the
specimen is briefly stressed by sudden shearing at a controlled rate, and the
stress then suddenly ceases. As a rule the test only takes a few seconds and
produces measurements in the form of a curve. The curve rises sharply in
relation to the stress and falls again more or less steeply when the shearing
suddenly stops. The falling end of the curve shows the elastic properties of the
material; it reveals a reversible elastic deformation and an irreversible
plastic deformation (Fig. 63). The measurement is "unsteady" because
of the sudden changes in the flow field of the specimen.
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| Fig. 63: Creep recovery / stress relaxation curves of a gluten or a dough |
A special class of rheometers consists of instruments which enable the
rheological properties of a material to be demonstrated in a single test. To do
so they operate in the dynamic oscillating or vibratory mode: instead of
shearing simply by rotation in one direction they perform an oscillating
measuring deformation in which the amplitude of the oscillations (excursion)
and their frequency (movements within a unit of time) can be controlled (Fig.
64). Viscosity is measured as torque according to the familiar method and shown
as complex viscosity, but the "stiffness" of the material is also
recorded as stored elastic energy. The result of the measurement is a sinusoid
("wavy") curve. A comparison of the curve thus produced with the
controlled deformation curve reveals a phase shift measured in angular units between
0° and 90°. The smaller the phase shift, the "stiffer" and more
elastic is the tested material. The "plastic" component of viscosity,
the energy loss, cannot be measured; it is calculated as an imaginary
component, the difference between complex (total) viscosity and the stored
(elastic) viscosity. "Plastic" viscosity divided by elastic viscosity
denotes the viscoelastic behaviour of the material tested. For reasons of
simplicity the results are usually stated in the form of measured moduli that
have to be converted into viscosity values by calculation. The conversion
factors for a measurement are constant, so that the moduli G* (complex shear
modulus), GI (storage modulus) and GII (loss modulus) stand for the relevant
viscosities (complex or total viscosity, elastic viscosity and plastic
viscosity). Viscoelasticity is calculated as tan delta (tangent delta or loss
angle), the quotient from GII divided by GI. If this is smaller than 1, since G
I is greater than GII, it describes an elastic material; if it is greater, the
material is plastic. The greater the deviation of tan delta from this quotient
1, the more distinctly does the viscoelastic behaviour of the material tend in
one direction of viscoelasticity or the other, i.e. elasticity or plasticity.
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| Fig. 64: DMTA of a starch slurry:
storage (G ) and loss (G ) moduli and tan delta over the time in the course of
heating (Weipert, 1995) |
This highly efficient, sensitive and elegant method of recording and
displaying the "true" rheological properties of foods has made a
great contribution to understanding the specific characteristics and behaviour
of raw materials and foods during processing and ultimately to explaining why
consumers like one product and dislike another. The definite advantages of the
dynamic oscillating method for studying and identifying structural changes in
foods during processing can be documented by DMTA (Dynamic Mechanical Thermal
Analysis). If a starch/water slurry is heated as in an Amylograph test and the
changes in viscosity, elasticity, plasticity and tan delta are measured,
several curves similar to an Amylogram are obtained (Weipert, 1995).
At the beginning of such a test the loss modulus GII was greater than
the storage modulus GI, showing that the starch slurry had the properties of a
liquid at low temperatures (Fig. 64). At higher temperatures, following
increased water absorption and gelatinization of the starch, the situation was
reversed: the storage modulus GI was greater than the loss modulus GII,
indicating that the properties of the starch gel were becoming more solid.
These changes were expressed even more clearly by the course of tan delta,
which was well above 1 at the beginning of the test and well below 1 after
gelatinization of the starch. This means that starch gel has predominately the
elastic properties of a "solid". These observations concerning the
changes in the viscoelastic properties of the starch slurry were accompanied by
measurements of the temperature of the heating medium and of the slurry itself.
It was found that the temperature curve of the slurry (Ts) followed the
temperature curve of the heating medium (Tw) with some delay, but that a slight
rise in the temperature curve of the slurry occurred at the beginning of
gelatinization. This additional delay was caused by the fact that the starch
took the heat energy out of the slurry in order to gelatinize. In a dough the
transformation from a soft, "plastic" mass into a firm crumb in which
the "elastic" properties predominate is even more evident. This shows
that such a test is useful for identifying and demonstrating the changes in the
properties of a flour in the course of processing.
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| Fig. 65: Deformation (strain)
test in the dynamic oscillating mode on wheat flour doughs with extremely
different dough properties |
Measurements with such instruments of fundamental rheology have opened
up new ways and means of analyzing the structure and properties of doughs. By
carrying out a frequency sweep (in which the amplitude remains constant and
only the frequency is changed as required) or an amplitude sweep (in which the
frequency remains constant and the amplitude varies) it is possible to record
the flow properties of a dough at different deformation forces. Both the
viscosity and the elasticity or viscoelasticity of the dough are recorded
synchronously and simultaneously in a single measurement. This is a simple,
quick and elegant way of differentiating between doughs with a firm elastic or
soft and plastic structure (Fig. 65). It has also been observed that at an
extremely low deformation load wheat dough shows a plateau of elastic
behaviour, since its structure is not damaged during this part of the
measurement; the dough does not show the expected structural viscosity as its
viscosity decreases under increasing deformation forces. This observation has
been used to develop a "nondestructive" testing method, in fact one
which scarcely touches the dough, in the form of a "recording baking
test" in which the dough is monitored over the desired length of time at
rising baking temperatures and falling cooling temperatures under conditions
simulating the process in the baker's oven (Weipert, 1987a and 1992). The
viscosity and elasticity curves are related to the curve of an Amylogram, since they show the
gelatinization properties of the starch in interaction with other flour
constituents and additives. But in this case we have a dough of the consistency
usual in bread making, and so they show the properties and interaction of these
two most important components of a flour and a dough in the baking process.
They demonstrate the dough properties resulting from the gluten at the
beginning of the process, in the oven stage and as a final result after baking.
The measurements after cooling show the properties of the baked dough, which only
differs from the crumb of the bread in that the inflation is missing (Fig. 66).
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| Fig. 66: "Recording baking
test" - changes in viscosity (G*), elasticity (storage modulus GI), plasticity
(loss modulus G II), sample height (delta L) and viscoelasticity (tangent
delta) in the course of heating and cooling |
Although still comparatively new, the "recording baking
test" method has already shown its value and potential in a few
publications. Baking trials using flours from wheat varieties with different
dough properties have shown that the viscoelastic properties of the doughs are
preserved into the baked crumb. The baked crumb is doubtless firmer and more
elastic than the dough, but the crumb of a wheat flour with soft dough properties
is softer than that of a wheat flour with firm dough properties. Furthermore,
the method showed the effect of the different dough yields, of ascorbic acid,
various enzyme preparations, emulsifiers and other ingredients on the viscosity
and viscoelastic properties of the dough and the crumb, in respect of extent
and also time and temperature (Fig. 67). The influence of the oven temperature
was shown with an enzyme-active rye flour by carrying out recording baking
tests using slowly and rapidly rising temperature gradients (3.5 °C/min and 7
°C or 17.5 °C/min respectively). It was also possible to simulate the process
of producing bread rolls from frozen dough portions. In the measuring device of
the rheometer a dough was frozen to -18 °C, heated to +100 °C and cooled down
to +30 °C in one cycle during which the changes in viscosity and the
viscoelastic properties were recorded continuously. So far the recording baking
test is the only method by which doughs can be tested rheologically in their
full formulation, including yeast (Weipert, 1987a, 1992, 1995 and 1998b).
 |
| Fig. 67: "Recording baking
test" – viscosity (G*) and viscoelasticity (tangent delta) of doughs from one
wheat flour treated with ascorbic acid, α-amylase and protease |
But despite the versatility of the rheometer, there are limits to its
uses. A rotational rheometer working on the principle of shearing is well able
to show the rheological properties of fluids (in coaxial cylinders) and pasty
substances (by the plate/cone or plate/plate system), but it fails with solids
(Weipert, 1987a, 1992, 1997 and 1998b). On the other hand, a rheometer working
in the compression mode might be unable to show the rheological properties of
fluids, but its measurement range covers pasty substances (such as dough) and
solids of different consistencies (bread crumb, cereal grains) (Weipert, 1997).
In the compression mode the measured moduli are termed E* for the complex
modulus, EI for the stored modulus and E II for the loss modulus. Both dynamic
oscillating measuring principles, the shearing mode and the compression mode,
are equally suitable for expressing the rheological properties of materials,
complex viscosity and elasticity or viscoelasticity. But since they measure the
rheological properties of the specimens in a highly sensitive and precise
manner and represent them simultaneously and synchronously, they require
friction-free suspension of their working parts in air bearings and complex
computer software for control and evaluation. This makes them expensive to buy,
maintain and operate (Weipert, 1993). But the new information acquired through
the measurements justifies their use.
1.6. Outlook for the Future
Whether conventional or fundamental, rheometry will remain an
established and important feature of the production of quality bread and other
baked products. The choice of measuring instruments and methods will depend on
the level and purpose for which they are to be used. Both rheometries, the
conventional and the fundamental, have advantages and disadvantages; an ideal
rheometry would combine the advantages of both. But ultimately it is the task
of man – the cereal expert and the rheologist – to use the instruments and
interpret the results. He has to ensure that Finagle's Law does not apply,
namely that:
- The information we have is not the information we want.
- The information we want is not the information we need.
- And the information we need is not available.
The viscosity and viscoelastic behaviour of doughs and the end
products is and will always be the information we have and the information we
need. And since it is available it offers a guarantee of reliable production
processes and good quality in the end products.
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