Extensional Flow

Many food processes involve extensional deformation. Pure extensional flow does not involve shearing and sometimes called shear free flow. In extensional flow, molecular orientation is in the direction of the flow field since there are no competing forces to cause rotation. Extensional flow causes maximum stretching of molecules, producing a chain tension that may result in a large resistance to deformation.

Dough processing is an important food process in which extensional flow is significant. Another example for extensional flow is extrusion, which involves a combination of shear and extensional flow. Formation of carbon dioxide gas during fermentation of bread dough involves extensional deformation. The extensograph is an important instrument used to study dough rheology which gives resistance to extension and stretchability.

There are three types of extensional flow: uniaxial, planar, and biaxial (Fig. 2.19). During uniaxial extension, material is stretched in one direction with a size reduction in the other two directions. In planar extension, a material is stretched in the x1 direction with a corresponding decrease in x2 while width in the x3 direction remains unchanged. In biaxial extension the flow produces a radial tensile stress.

There are many test methods that measure the uniaxial extensional properties of dough such as the Brabender Extensograph and the Stable Microsystems Kieffer dough and gluten extensibility rig (Dobraszczyk & Morgenstern, 2003). However, these methods do not give rheological data in units of stress and strain since sample geometry is not defined or measured, and dimensions change nonuni-formly during testing. Therefore, it is impossible to determine stress, strain, modulus, or viscosity by these methods.

The commonly used methods to measure biaxial properties of foods are inflation methods and compression between flat plates using lubricated surfaces (Chatraei, Macosko, & Winter, 1981; Dobraszcyk & Vincent, 1999). The alveograph operates under the principle of biaxial extension.

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