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.
Why perform an extrusion test?
The majority of tests on the texture analyser are only suited to the measurement of materials that are self-supporting i.e. solid or viscoelastic materials. There is however the possibility to measure non-self-supporting or viscous materials via extrusion methods where the rheological properties of the product affect the flow. In these cases an extrusion test can simulate many real-life situations such as the forces required to dispense/extrude ketchup from its container or toothpaste from its tube.
Extrusion tests have been applied to butter, margarine and other fats in an attempt to measure firmness and spreadability. Other materials commonly tested are fruits, vegetables, gels, and some viscous liquid products. Since extrusion requires that the food flow under pressure, it seems reasonable to use it on food that will flow fairly readily under an applied force and not to use it on those foods that do not flow easily, such as bread, cake, cookies, breakfast cereals, and candy.
Another fixture commonly classified as an extrusion device is the Ottawa cell that is available with many different types of extrusion plates (slots, holes, wires). The size of the orifice or grid selected is dependent upon the product being measured and the textural property of interest. The product is compressed until the structure is disrupted and it extrudes through the outlets in the base plate. The pattern of forces involved in such a test is complex but usually the maximum force required to accomplish extrusion and the total work of extrusion is measured as an index of textural quality.
Properties that can be measured with an extrusion test
Extrusion tests are typically chosen to measure:
Firmness, spreadability, product consistency, extrudability, sachet/tube removal force, syringeability. work of extrusion, cohesion
To understand how these properties are measured visit the Textural Properties page.
Typical probes and fixtures used for extrusion tests
To understand how these fixtures are designed and manufactured visit the Texture Analysis Attachments page.
Items with codes prefixed 'HDP/' must be used with the HDP/90 Heavy Duty Platform.
Items tagged * are Community Registered Designs.