How to measure and analyse the texture of food, cosmetics, pharmaceuticals and adhesives.

Tuesday, 9 February 2016

Measuring Sauce Texture: Stickiness, Tailing & Stringiness

Tailing of sauce
There are differences between types of sauces; tomato ketchup has a short character, whilst Asian sauces, like chilli sauce, tend to be more long in character. This textural difference is also highlighted in spoon coating and tailing when deposited.

In some foods, cohesive and adhesive properties greatly influence the performance of food before consumption, and the perception of texture in the mouth. Stickiness is commonly viewed as a negative food attribute, but in certain foods (like sweet sauces and syrups), a level of stickiness is not only tolerable but desirable. In semisolids, the stickiness is caused by sugars, fats, gums, starches or mixes of these ingredients.

The most commonly used test for measuring the range of adhesive properties in foods is the probe test. In this test, a probe applies a force over the sample for a chosen period of time to achieve a good bond between the two surfaces before it is withdrawn until the sample completely separates (failure). A vessel and disc as used in a back extrusion test are also suitable however the disc would be employed to test the surface of the material rather than venture into the sample.

There are three types of failure identified: if the material remains on the surface of the probe, cohesive failure occurs. If separation is clean (no material remains on the probe), adhesive failure occurs. In adhesive failure, the clean separation occurs at the surface and there is no legging or little necking deformation of the sticking material, whereas cohesive failure occurs within the material and leaves residues in both surfaces. 

The cohesive-adhesive failure is the transition between the complete cohesive and complete adhesive failure. For solid and semi-solid materials where the interfacial bonding balances against the internal mechanical strength of the material, either failure mechanisms could be possible, but for many viscous (or viscoelastic) fluid foods, cohesive failure is most likely the dominant mechanism.

Graph obtained by an adhesive test highlighting where two parameters of interest are obtained
Graph obtained by an adhesive test
highlighting where two parameters
of interest are obtained
Using an Exponent software adhesive test, the adhesive force (or stickiness) is taken as the peak force; the work of adhesion is taken as the area under the curve and represents the total work during the withdrawal of the probe. The area from the start of separation to the maximum force is considered the best measure of work of adhesion. The cohesive force can be taken as the area between the maximum force and the point where force drops to near zero (the force will never return to zero if cohesive failure has occurred and residue remains on the probe at the end of the test). The stringiness is the distance between the sample surface and the point where force drops to near zero where the product finally separates from the probe.

A major difference between texture analysis profiles is the way the force decreases after the maximum peak. Products that possess a ‘short’ texture (e.g. jam, chocolate spread) have their forces drop to zero or a value of negligible very quickly. However, products that are ‘stringy’ produce a tailing curve which extends for a considerable distance before returning to zero (or near zero if sample remains on the probe at the point of separation). The distance that this tailing occurs can usually be termed stringiness and the area from the maximum force to this end point can be considered as the work of material stretching.

There is no doubt that the speed of probe separation has a great influence on the adhesive force and the work of separation and once chosen should remain constant for comparison purposes.

Request our article The Thick (and Thin) of it which looks at the texture analysis of semisolid products.

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

TA.XTplus texture analyser with bloom jar 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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Watch our video about texture analysis Replicating Consumer Preferences
 Texture Analysis applications

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