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

Tuesday, 28 April 2015

Measuring snap, crackle and pop...

Selection of artisan breads Once product developers have formulated a winning ‘noisy’ food product, the aim would be to ensure the consistency of this signature quality of the product during manufacture. 

Every brand knows that importance of consumer loyalty lies in the consistency of their product quality, be it flavour, appearance or texture. So the measurement of a product's acoustic signature is key to determining the gold standard product ‘noise’ for the purpose of quality control of all future batches of the product.

This post follows on from our previous blog post entitled When sound is a product's signature and provides a few application examples of the measurement of acoustic emission.


When a force is applied to any material, resulting movements within the structure form sound waves. These sound waves can be monitored as acoustic emissions. The definition of acoustic emission is the transient elastic wave generated by rapid release of energy within a material. Another way to describe this is that when a load is applied to a specimen, energy is stored as strain energy. When an inherent critical point is reached in the sample, there is a sudden redistribution of the energy. At this point some of the strain energy is converted into acoustic energy.

Sounds from foods vary primarily in their loudness. Therefore amplitude is one of the variables that distinguish product texture. This is used in combination with the number of sounds (frequency) produced within a given distance or time. The structure of a food and the mechanical properties of that structure are related to bubbles bursting by their capability to generate the appropriate sound and their ability to dampen or amplify this sound. These acoustical sensations can be produced electronically and be independent of any rheological properties.

Below is shown a typical force versus time curve produced from a back extrusion test of an aerated dessert with the measured textural parameters annotated.

Typical back extrusion test
Results graph showing Force (top curve) and Acoustic 
Energy Emissions (lower curve) against time of a back
extrusion test on an aerated dessert.
The acoustic versus time plot shows the acoustic peaks resulting from a back extrusion test of the same aerated dessert with acoustic emission measured using an Acoustic Envelope Detector attached to the TA.XTplus Texture Analyser:

now let's compare 3 different aerated desserts. The force versus time curves below were produced from back extrusion tests of the 3 desserts and highlights clear textural differences, which are explained in the results section. 
The force versus acoustic versus time curve below demonstrates the ability of Exponent software to show dual axes with a synchronised cursor.

Comparison of three different aerated desserts

What story do the curves tell?
When a 5g surface trigger is attained (i.e. the point at which the lower surface of the back extrusion disc is in full contact with the product) the disc proceeds to penetrate to a depth of 25mm (*or other specified distance). At this point, the probe returns to its original position.  

The positive force area of the curve up to this point is taken as a measurement of consistency - the higher the value the thicker the consistency of the sample. The force troughs and the positive acoustic spikes observed during penetration are a result of the bubbles bursting. The number of force troughs and acoustic spikes correspond to the degree of aeration.  

The area of the negative region of the force curve may be referred to as the 'work of cohesion' - the higher the value the more resistant to withdrawal the sample is which is an indication of the cohesiveness and also consistency of the sample. The acoustic emissions are displayed on the acoustic graph as spikes above the base background noise level of approximately 44dB.

The significant acoustic peaks are identified by the macro as being above a pre-set threshold value, in this case greater than 6dB. The number of significant acoustic peaks and the sound level of the top 5 acoustic peaks provide an indication of aeration. Acoustic data together with force data enables a characteristic product profile to be established.  

Typical Results:   
Mean test results obtained from measuring 3 different types of aerated desserts in triplicate:

The results highlight the following differences; the lemon mousse was the thickest in consistency followed by milk chocolate, and dark chocolate. The samples were of similar cohesiveness. The number of force troughs, and number of significant emissions are both indicators of aeration, and usually correlate with one another. Samples ranked in the following order: lemon, dark chocolate and milk chocolate. The average 5 highest emissions correspond to the power of the acoustic emissions. Results indicated that there were significant differences between the power and the number of acoustic emissions of the lemon mousse compared to the other mousses.
The Sound of Carbonation
Our perception of the carbonation in a beverage is based partly on the sounds of effervescence and popping that we hear when holding a drink in our hand. Make the carbonation sounds louder, or else make the bubbles pop more frequently, and people’s judgments of the carbonation of a beverage go up. If you’re using your Texture Analyser for product testing, why not measure the sound of the ‘pop of the cork’ at the same time as measuring the force to remove the cork, as the noise is also an auditory experience?

If your product is bath bombs or ‘Alka Seltzer’, why not measure the ‘Fizz’ whilst you’re using your Texture Analyser to measure their hardness. The two senses (of hearing and feeling) would appear to make a much more balanced contribution to our judgments of the texture and sensation of products.

Quantifying ‘Squeaky’ foods
Let’s take an unusual product: Halloumi cheese. While many people like the sound of its ‘squeakiness’ when bitten into, traditionally it was apparently judged to be rather unattractive. Although it’s an unusual sensation, it has become an auditory cue and therefore an expectation of this product. It’s due to a phenomenon known as stick-slip. This frictional feature can be measured by a Texture Analyser, so it makes sense to simultaneously measure the acoustic component of this quality.

Whilst much of the work to date has focussed on the measurement of crispness of brittle foods, other examples of products that have both a Physical and Auditory Expectation include:

  • The ‘Spray’ of a can (whilst measuring its actuation force)
  • The ‘Bite’ of an apple (whilst measuring the force to puncture its skin)
  • The ‘Crack’ of a chocolate coating (whilst measuring the force to crack)
  • The ‘Snap’ of spaghetti (whilst measuring the break)
  • The ‘Click’ of a switch (whilst measuring the actuation force)
  • The ‘Snap’ of a pencil (whilst measuring the break strength)  

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.

No-one understands texture analysis like we do!

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Acoustic testing video

Download a published article on methods measuring sound of brittle products

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