Plasticity Measurement of Soft Solid Foods

Plasticity is a property shown by many materials, such as polymers, metals and the majority of foods. It is the ability of a material to undergo permanent deformation. 

If you press your finger on a spring, it returns to its original shape. If you made that spring out of cheese, it would not spring back as far – it would remain squashed. When stretching a sample, plasticity is seen on the force-distance graph as a change in gradient after the initial linear section (straight line). The point where the gradient changes is known as the ‘yield force’. If this point is reached and more force is applied, the sample will be permanently changed. Before this point, the behaviour is elastic and spring-like.

Plasticity is caused by ‘plastic flow’. This can mean different things depending on the material under question. In a polymer, the long molecules can move past each other. In a metal, a mechanism called a ‘dislocation’ allows the metal atoms to move along their row in the metal lattice. Food materials can have a large variety of structures so the mechanism will depend on the food under question. However, the effect is always very similar.

If you were to gently push your finger into a sample of the food and remove it, if an imprint of your finger was left behind, plastic deformation has taken place. For example, if you were to do this on the cut surface of a thick slice of bread, a gentle push would not leave an imprint – the bread would spring back. A stronger push would, and this is due to plastic flow.

This effect can be measured in many different ways using a Texture Analyser. Firstly, some elongated samples are suitable for tensile testing. For example, if a long piece of liquorice was stretched on a Texture Analyser, it would stretch until breaking. A good measure of plasticity might be the distance the grips have moved between the yield force and breaking, divided by the initial grip separation. This is known as plastic strain to failure. A more plastic liquorice would show a larger value. If the sample was kept in the freezer overnight and tested frozen, it might show a very low plastic strain to failure. You can feel this effect by pulling it between the hands – it would be more likely to show a brittle snap than the room temperature sample. Bend testing would show very similar results.
 
Secondly, indentation testing can be quick and simple. If a conical, spherical or Vickers probe were pushed into the flat surface of a tub of margarine, the graph may look something like this:

If the indenter was pushed into the surface the same depth each time (e.g. 5mm), a more plastic sample would show a deeper indent when the probe is removed, and a corresponding larger plastic depth. If a sample with very low plasticity was measured (such as rubber), there may be almost no indentation left behind, and so a near zero plastic depth.

Using pastry margarine as an example, a method called the ‘finger method’ is often used to determine the sample’s plasticity. This involves using the fingers to manipulate the sample. If a sample is very ‘short’, this can be felt by cracking and an inability to mould it. This is the opposite of ‘plastic’. A good plastic margarine can be bent without breaking and the plasticity evaluated by repeated handworking to check stability, firmness and greasiness. In this context, plasticity is more of a general description of the product, but the above indentation test would still provide valuable information about its true plasticity, and this is likely to correlate with a more ‘plastic’ sample as felt by the finger method. 

As with all tests performed by a person rather than a machine, although the test may be repeatable when performed by the same person each time, this is not practical. It is much more efficient and accurate to use instrumental test equipment, such as a Texture Analyser. The results are stored as numbers on a spreadsheet for judgement against future batches, or for the design of a new type of product as a basis of comparison.

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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