Extrusion Testing for Texture Analysis and Materials Testing

What is an extrusion test?

There are generally two types of extrusion test in texture analysis: backward (or back) and forward. Of the various extrusion techniques the one favoured in recent years is back extrusion. The sample is contained in a strong cell with a solid base and an open top. A rod with a disc is then forced down into the container until the food flows up (backwards) through the space between the disc and the container walls which is called the annulus.

In forward extrusion, the sample is also placed in a container with an open top. However the base of the container accommodates a disc containing a central hole (or annulus) of varying diameter, chosen depending upon the consistency of the sample. The tightly fitting plunger which acts almost like a piston compresses the sample and causes forward flow through the annulus of the disc. Forward extrusion is suitable for the testing of viscous liquids and sauces, gels, pastes, processed fruit and vegetables but is only suited to products that are homogeneous i.e. without particulates.

One reason that the back extrusion technique is more popular compared to forward extrusion is that you need not perform this test in the containers which are provided by Stable Micro Systems. In many situations it may be disadvantageous to decant your sample from one container into the provided back extrusion containers due to disruption of the sample structure, i.e. breakage of any gel structure during decanting is almost inevitable. In this case, using the same procedure and extrusion discs, you may choose to test your sample in the container in which it was originally deposited or formed. This is certainly acceptable so long as you always test within the same size container, in order to make a comparison.

To view a complete set of extrusion testing probe and fixture options click here

What happens during an extrusion test?

Backward Extrusion

The measurements taken from such a test include:- 

- the maximum force developed, 

- the average force when a plateau features in the force-time (or force-distance) plot (which provides a good index of firmness), 

- the work done during a test, i.e. measuring the area under the curve.

Forward Extrusion

From A to B the sample is deformed and compressed to pack more and more tightly into the diminishing space available under the descending plunger; at this point there is little rupture of the sample.  At approximately the point B the sample is packed solid and liquid may be expressed from it filling the interstices.  At point B or soon afterwards the pack is solid except for small amounts of entrapped air, and the force increases steeply from B to C pressing out more liquid or air in the process.  At point C the sample begins to rupture and flow through the annulus (extrusion hole), and this process continues to point D when the compressing plunger is reversed in direction and the force falls to zero.  Point C gives the force necessary to begin the process of extrusion, and the plateau CD shows the force needed to continue extrusion.  From B to C represents the increasing force being applied to an almost incompressible mixture of solids and liquid.

The shape and magnitude of the compression-extrusion curve is influenced by the elasticity, viscoelasticity, viscosity, and rupture behaviour of the material; sample size, deformation rate, sample temperature, type of test cell; sample test size; and homogeneity of the sample.  With most processed fruits and vegetables and many fresh products the plateau CD is horizontal or nearly so.  The unevenness of the plateau is caused by variations in the firmness or toughness of the particles that are passing through the annulus zone at any particular time (although sometimes this is caused by the release of previously uncompressed air pockets or particulates in the sample).

To see how to perform extrusion tests using a TA.XTplus Texture Analyser to measure spreadability, product consistency, extrudability, sachet/tube removal force, syringeability, etc., watch this video and see typical extrusion tests in action: 

There is a Texture Analysis test for virtually any physical property. Contact Stable Micro Systems today to learn more about our full range of solutions.

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For more information on how to measure texture, please visit the Texture Analysis Properties section on our website.

The TA.XTplus texture analyser is part of a family of texture analysis instruments and equipment from Stable Micro Systems. An extensive portfolio of specialist attachments is available to measure and analyse the textural properties of a huge range of food products. Our technical experts can also custom design instrument fixtures according to individual specifications.

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