Hans-Dieter-Belitz-Institute for Cereal Grain Research

Rheology:
Micro Extension Tests and Fundamental Rheology

1. Micro Extension Test   [2. Fundamental Rheology]

The results of baking tests are most important for the evaluation of the breadmaking potential of flour and flour improvers. However, these methods are time-consuming and cumbersome. As an alternative rheological methods have been developed that are easier to perform, faster and more reproducible than baking tests. The results of these indirect methods can be used as an indicative for the properties of additives or the baking performance of flour. Only low quantities of flour (10 g) are necessary for these methods and the results are in good accordance with standard-scale baking tests (1000 g of flour). Micro-farinograms provide information about the mixing properties of flour, and micro extensigrams indicate the strength and the extensibility of dough or gluten.
For the characterisation of wheat dough and wheat gluten strands are formed and extended to rupture by using a Texture Analyser. Force-distance diagrams are obtained that are specific for the wheat cultivar. Furthermore, they provide information about the effect of additives and technological methods.
On the microfarinograph a dough is prepared from 10 g of flour at 22 °C. Water is added so that an optimum consistency of 550 Brabender Units is obtained. For the determination of the gluten properties the dough is washed with sodium chloride solution at 22 °C on a Glutomatik (middle). To perform extension tests with dough or gluten a teflon mould (right) is necessary.
Dough or gluten is brought to an ellipsoid shape and pressed into the teflon mould (left). After 40 min of relaxation the strands are removed from the teflon device (middle), fitted into the SMS/Kieffer Dough & Gluten Extensibility Rig and extended with the Texture Analyser until they disrupt (right). Characteristic curves are generated (below).

2. Fundamental Rheology   [1. Micro Extension Test]

For the determination of physical properties of dough and gluten, e.g. viscosity, yield value and elasticity, measurements with a dynamic stress rheometer (below, left) are carried out. For this a piece of dough or gluten is positioned between two plates. By means of the upper plate a force acts on the sample and the reaction of the sample is measured by the lower plate. In contrast to extension tests the applied force usually is so small that the sample won't be destroyed. Depending on the type of force acting on the sample different modes of measurement have to be distinguished. In one mode a shearing force is applied for a certain time, then switched off, and the relaxation of the sample is monitored. These measurements are called creep-recovery tests. It is also possible to apply an oscillating force by either modifying its frequency or amplitude. The phase difference between the acting and the reacting force can be used for the calculation of rheological and physical parameters. This mode is called stress sweep or frequency sweep.
With the microfarinograph a dough is prepared. For the measurements of gluten dough is washed on a Glutomatik. Then spherical dough or gluten pieces are formed and pressed between two metal plates to a defined thickness (right). These devices are frozen prior to the measurement.
The metal plates are removed and the frozen dough or gluten slice is positioned on the lower plate of the rheometer (left). The upper plate is brought to the position of measurement (right), and, after a certain time to allow the temperature to equilibrate, the measurement is started. Typical curves obtained with the dynamic stress rheometer are shown below.

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