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.
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.
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.
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).
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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