Texture analysis in action: Comparing fat characteristics

Many times, over the years we have been asked about the measurement of plasticity.  Usually this is with reference to fats and, when asked to describe this property, customers talk about the ability of the fat to be shaped and moulded, often in regard to its use in pastry making. Pastry making often involves shaping and moulding the dough. Flexible fats, like butter, have a plasticity that allows them to be easily shaped without breaking or tearing the dough. This plasticity enables the dough to be rolled out smoothly and gives it the ability to hold its shape during baking.

However, the measurement of such a property is difficult to pin down and mixed definitions mentioning mouldability, shaping, flexing and spreadability are available, as can be shown from a collection of different search attempts from various websites:

“The plasticity of fat is the capacity to keep its shape yet can be moulded or shaped by applying light pressure. It decides spreadability of fats.”

“Plasticity means the ability of a fat to change properties over a range of temperature. Temperature is an important factor in plasticity of fats.”

“Plasticity: Fats have plasticity - they can be spread and shaped. Plasticity is due to the fatty acids, which have different melting temperatures. Fats with a lot of saturated fatty acids are harder and have less plasticity. Fats with a lot of unsaturated fatty acids are softer and have more plasticity.”

“Plasticity means ability to be spread and shaped. Fats have ‘plasticity’ – we’re able to spread and manipulate them.”

“Plasticity: Most fats that appear to be solid at room temperature actually contain both solid fat crystals and liquid oil. The liquid part is held in a network of small crystals. Because of this unique combination of liquid and solid, the fat can be moulded or pressed into various shapes without breaking. This property of fat is called plasticity.”

Choosing the ‘name’ of a property

In the food industry it is common to have a textural or sensory property for which there is no clear definition.  So, let’s revisit this property and discuss the testing options. Finding the right method that measures the property that you are interested in measuring is the key – what you choose to ‘name’ that property is less important and often the same measurement can result in the property being named differently depending upon the sample that is being tested. 

Plasticity

In our own blog article (written in 2018) we attempted to explain the measurement of plasticity of solid soft foods as:

Plasticity is a property shown by many materials, such as polymers, metals and the majority of foods and is caused by ‘plastic flow’. 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. 

Measuring plasticity with your finger

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.

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. The gripping of fats is likely to be difficult and slippage at the grips is anticipated so this precludes this type of testing for fats. However, a Tortilla/Pastry Burst Rig may offer the possibility to measure tensional properties whilst supporting the sample in a drum configuration and pushing a ball probe through the centre, thereby imitating a finger pushing through a material.

Flexibility

Bend/flexure testing however is likely to show very similar results. So, we tried to test fat samples prepared as repeatably as possible in bars (from its block form) as shown below and bent using the Three-point Bend Rig:

Consistent width and thickness are crucial in sample preparation
for three-point bend testing

Results showed a significant and repeatable difference in the force to bend and the distance at which the fat cracked. Both samples showed a marked drop in force as the bend of the sample started to occur with the refrigerated sample failing (force drops to near zero) quite quickly whilst the room temperature sample continued to flex until the appearance of a crack (the force drops) and before failing completely. It was much more ‘flexible’.

Three-point bend testing of a sample and graphs produced from the
bending of fat slices at room temperature (blue) and refrigerated (red)

Pliability?

Pliability, as a term commonly used in materials science and engineering, refers to the flexibility and ease with which a material can be bent, shaped, or deformed without breaking or cracking. It is not a specific measurement parameter but rather a qualitative observation of a material's behaviour. In the context of fats and table spreads, the term "pliability" is not commonly used as a quantifiable measurement. Instead, specific textural properties such as spreadability, hardness, or consistency are assessed to evaluate the desired characteristics of the product. However, pliability would be quantitatively measured in the same way as shown above.

Fats are referred to as "pliable" because they possess the characteristic of being easily shaped or moulded under moderate pressure. This pliability is a result of the physical properties of fats, particularly their melting point and consistency. Fats consist of molecules called triglycerides, which are composed of fatty acids attached to a glycerol backbone. The specific types of fatty acids present in a fat determine its melting point and texture. Fats that are high in saturated fatty acids tend to have a higher melting point and are more solid at room temperature, such as butter or coconut oil. Fats that are high in unsaturated fatty acids have a lower melting point and are usually liquid at room temperature, such as vegetable oils. The pliability of fats arises from their ability to transition between solid and liquid states within a certain temperature range. When fats are exposed to heat, they melt and become more fluid, making them easier to shape, spread, or blend. When fats are cooled, they solidify and regain their shape. This pliable behaviour allows fats to be used in various culinary applications, such as in baking, cooking, or forming into solid structures like butter sculptures. The pliability of fats is advantageous in food preparation as it facilitates the incorporation of fats into recipes, enhances the texture of baked goods, and allows for easy spreading on bread or toast. The ability of fats to provide pliability contributes to the sensory attributes and mouthfeel of food products. 

Spreadability

Plasticity is a property commonly associated with metals, ceramics, and some polymers. However, if a manufacturer does use the term "plasticity" in reference to fats or spreads, it could be an attempt to describe the ability of the product to change its consistency or texture under different temperature conditions. In this case, it would be more accurate to use the term "consistency" or "spreadability" rather than "plasticity." 

Spreadability refers to the ease with which a fat or spread can be spread over a surface, such as bread or toast. It is an important characteristic for table spreads as it affects the overall eating experience and convenience of using the product. Table spreads are designed to have a desirable consistency that allows them to be easily spreadable at room temperature, even when taken out of the refrigerator. Achieving the right spreadability involves selecting appropriate fats or oils and utilising processes such as hydrogenation or blending to modify the texture and consistency. By controlling the composition and processing techniques, manufacturers aim to create table spreads that are pliable, or easily spreadable, at typical serving temperatures. These spreads have a balance between solidity and fluidity, enabling them to spread smoothly without tearing the bread or toast. While the term "pliability" is not typically used to describe fats and table spreads, the concept of spreadability encompasses the desired pliable texture that makes spreading effortless. It ensures that the table spread maintains its structural integrity while being pliable enough to spread smoothly on various food surfaces. 

We can measure the spreadability of a product easily using the Spreadability Rig.  The material is either deposited and allowed to set up in the lower cone holders in advance of testing, or is filled with a spatula and then the surface levelled.  The material is pressed down only so much as is needed to eliminate air pockets which are visible through the perspex cones, and then the surface is levelled with a flat knife. The product is forced to flow outward at 45° between the male and female cone surfaces during the test, the ease of which indicates the degree of spreadability. Withdrawal of the cone probe from the sample provides information about adhesive characteristics which may be present. However, preparation of the sample into cone shaped holders may not be possible for block form products.

The Spreadability Rig and a typical graph

Indentation Testing

So, let’s return to the idea of testing via penetration of the sample.  Indentation/penetration 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:

Typical indentation graph

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 hand-working 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. 

Person vs. instrument 

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.

Read more of our blog articles here

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

The TA.XTplusC 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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