RHEOLOGY
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Fundamental and empirical measurements
Bread dough rheology Cereal scientists have developed a range of test instruments for measuring the important properties of wheat and flour. However, no single instrument is available to inform the baker fully about the settings required for optimal dough mixing. This article explores the range of fundamental and empirical
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It is often stated by bakers that the single most important process in bread making is to mix the dough to its optimal quality. If this is achieved, the downstream processing stages will work smoothly and the baked bread will be of consistently high quality. This means that the dough rheology must be optimal at the end of mixing. To achieve this, firstly it is essential that techniques are available to measure dough rheology and, secondly be able to determine critical settings such as flour water absorption and mixing time. If only it was this simple. Measurement of dough rheology has challenged the milling and baking sectors for around a century. Fundamental rheology methods Instrumental methods for measurement of rheological properties of materials can be divided into two broad classes: + Fundamental tests that measure the inherent properties of the material and do not depend on the geometry and shape of the sample, the conditions of loading or the type of apparatus used. Typically, the properties measured, to name just two include relaxation time and shear modulus. + Empirical tests or imitative tests are those where the mass of sample, geometry and speed of test
will decide the magnitude of the parameters measured. Typical examples include texture profile analysis which uses a compression force to measure parameters such as hardness and springiness. Bread dough is a complex multi-phase material, and, like other structured foods, it displays both viscous and elastic properties, otherwise referred to as a viscoelastic material. Viscoelastic foods lend themselves well to dynamic fundamental tests performed on rheometers. The frequency sweep test in which the frequency of the applied stress or strain frequency is varied is a typical example of a dynamic test. Different types of dynamic tests can be used to study the viscoelastic properties of dough systems. These include creep recovery, stress relaxation and dynamic oscillatory tests. In a creep test a very small but defined shear stress is applied until shear strain increases at a constant rate. Once this constant rate is reached, the applied shear stress is removed and the material is allowed to reach an equilibrium state. There are two primary pieces of information generated by a creep test: + Zero Shear Viscosity – This is the viscosity of the material under very low shear conditions. It is difficult to measure because of the extremely
B R E A D D O U G H R H E O L O G Y M E A S U R E M E N T S – F U N DA M E N TA L A N D E M P I R I C A L
methods for measuring dough rheology and what they can be used for.
