Viscoelasticity in the Food Industry

The vast majority of food materials show a combination of viscous and elastic behaviour although many show much more of one than the other.

There are some exceptions – hard crackers are generally completely elastic, whereas oil and runny honey usually show no elastic behaviour. Viscoelastic testing is best used as a comparative measure as many food products have an unusual geometry, so conventional viscoelastic equations cannot be used to find fundamental parameters.

If a cracker is not completely elastic when bitten, it may have become stale. The degree of viscous behaviour can be measured to study this effect. A sample so brittle is not easily clamped without fracturing, so tensile testing is not an option, but equally it is not a suitable shape for compression. 

Measuring Viscoelasticity

The best option is bend testing. The Three Point Bend Rig can be used on a Texture Analyser to quickly measure cracker viscoelasticity. The loading arm moves down towards the cracker, and once a trigger force is reached it is loaded to a set distance (e.g. 0.5mm). The arm stops and holds this distance for a set time (e.g. 30 seconds). The distance must be small for such a brittle sample to ensure it does not break during the initial load. Crackers that have been left out in the air for longer are likely to be softer, and so will show a larger force recovery than freshly opened crackers when loaded to the same distance.

Also suitable for bend testing, cookies are sold in every viscoelastic state from hard baked (elastic) to so soft they are almost dough (viscous). Cookie manufacturers have to ensure their products are consistent all over the world, and the same from batch to batch. The ingredients and processing will have a large influence on this, as will the cooking time and temperature. As with many other food products, it is vital to have a reliable, quick and easy quality control process to monitor these properties. Stable Micro Systems’ Exponent software makes this very simple with batches saved by name and date, automated test procedures and instant data analysis.

Tensile tests are useful for fibrous or elastomeric materials. This type of test simulates the way you pull a bread product, liquorice, mozzarella cheese or beef jerky with the teeth rather than chewing it. Liquorice, for example, is often sold in long cables that are pulled with the teeth until breakage. It is much more satisfying to have a piece of liquorice that stretches and warps over a few seconds. This property can be measured using a tensile test. For such an elongated sample, it is not possible to cut a recess into the centre to form a dog-bone shape. This means the ends must be clamped carefully to prevent the sample breaking at the tensile grips. Self-tightening roller grips are ideal for this purpose. The spring loaded cross-hatched rollers tighten around the sample, so even if the liquorice contracts when a force is applied, it will still be held fast.

Cheese viscoelasticity has been studied extensively over the past few decades and its textural properties can be as important as flavour, heavily influencing the sensory score given by cheese graders. Cheese is easily cut into cuboid samples that are ideal for compression testing, using both the relaxation and creep methods. There are projects built into the Exponent software that have all necessary test settings saved and ready to be used automatically. Cheese viscoelasticity is readily affected by the test temperature, ripening time and protein fibre orientation (for a stretched cheese like mozzarella). The temperature of a cheese compression is controlled simply using a Peltier Cabinet attached to a Texture Analyser. Additionally, if melted cheese is of interest, the cheese extensibility rig provides a method to study the extension and strength of cheese strands pulled from a mass of melted cheese.

Some materials cannot be formed into a convenient geometry for any of the above types of testing, such as grains and some fruit and vegetables. In this case, it is acceptable to compress these irregular shapes between a flat probe and the instrument base for comparative testing. The initial height of each sample is recorded as strain height, and some normalisation is possible by testing to a set strain height each time.

Most food materials appear to have viscoelastic behaviour when loaded in different ways. There have been countless publications on experimental procedures to give insight into the rheological modelling of these materials to characterise them and predict their behaviour under specific physical conditions. These viscoelastic models contain different combinations of springs and dashpots, showing complex behaviour that can represent different food materials. There have been many publications on the subject of modelling viscoelastic foods, particularly if they can be formed into uniform, measurable samples (e.g. cuboids of cheese or apple).

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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