Extrusion testing has many applications within the food, and other industries, for texture assessment. This method is relevant to semi-solids or viscous liquids, where the rheological properties of the product affect the flow.
Extrusion can be divided into two categories; forward extrusion and back extrusion. Forward extrusion is when the test sample is placed into a confined container and then forced through an orifice or grid. The foods resistance to this extrusion is then measured. The size of the orifice or grid selected is dependent upon the product being measured and the textural property of interest.
Back extrusion is typically performed with the product being measured is placed in an open top cylinder, and a piston with a diameter smaller than the cylinder, is forced down into the sample. The result is that the product flows back between the space created by the piston and the cylinder.
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For example, sour cream, when not processed properly, can be grainy. By forcing sour cream through a small orifice – using forward extrusion- one will either see a smooth force level or a jagged one. The jagged one happens when the grains alternately become plugged and unplugged in the orifice building then releasing back pressure.
Back extrusion is a more common method of extrusion testing. The key factor to consider in back extrusion is the gap between the piston and the cylinder. This is called the “annulus” and the smaller the annulus, the greater the resistance to the flow.
For consistency in testing of viscous liquids, once decanted, samples should be left to recover structure in a controlled environment.