HDBI

Hans-Dieter-Belitz-Institute for Cereal Grain Research


Rheology:
Micro Extension Tests and Fundamental Rheology

1. Micro Extension Test

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 until they disrupt by using a Texture Analyser. Force-distance diagrams are obtained (see below) that are specific for each wheat cultivar. Furthermore, they provide information about the effect of additives and technological methods.

Typical curve of a micro-extension test with wheat gluten
 
Typical Kieffer-curves of a micro-extension test with wheat gluten

 

2. Fundamental Rheology

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. Typical diagrams are shown below.

typical diagam of a creep-recovery test typical diagram of an oscillation test
 
Typical diagam of a creep-recovery test
 
Typical diagram of an oscillation test

Baking Chemical Analysis Flavour Analysis Homepage

© 2016 Hans-Dieter-Belitz-Institute - Webmaster