Testing Products with a Crust, Skin or Multilayers

Products with a crust, skin or multilayers bring textural variety to a product. The breaking of the skin of an apple when bitten into or the biting through the layers of the perfect pastry product brings a textural sensation that is highly desirable and anticipated. Failure to notice a difference in the layers of such products brings disappointment.

The collection of detail of a thin, brittle, laminated or multi-phased structure is best performed with a small diameter probe or thin, sharp fixture. A larger probe such as a platen, cylinder or sphere will give bulk compressive properties but will not resolve the properties of each layer. It is beneficial to penetrate the sample slowly in most cases to give a larger time gap between fracture or puncture events so they can be more easily identified.

A product with a crust may be a pie or sugar coated chocolate. The properties of a pie crust can easily be assessed with a 5–10mm probe – it is not a strong product (so the larger probe area is useful in increasing the measured force). A sugar coated chocolate is better suited to a needle probe, as the outer layer is so thin. A product with a skin, such as an apple, should be prepared so it has a flat bottom for steadiness. A 2mm probe is ideal here.

The force-time graphs collected from these tests will ordinarily show a force peak as the probe pushes through the crust, then a lower force that plateaus as the probe travels through the bulk (which may be the inside of the pie or the flesh of the apple). 

However, the force measurements of the crust or skin in its “natural habitat” will never be the same as a measurement of a crust or skin sample isolated and on its own. The bulk – pie filling or fruit flesh – will always influence the measured properties as the skin is supported and the filling provides a backward pressure onto the crust or skin. It is usually easier to prepare a sample with the bulk in place (it is not trivial to peel an apple and prepare uniform skin samples), so these measurements still have an important role in quality control.

Top: apple skin penetration by needle probe; chewing gum test with craft knife; typical graph of pears.
Bottom: pie crust penetration test with small cylinder probe; puff pastry with extended craft knife.

Samples made up of multi-layers, such as puff pastry or a chocolate bar, can be penetrated with a thin probe or cut with a craft knife. In this case it is even more important to have a slow test speed so the properties of each layer can be determined.

As usual in texture analysis, these measurements are not useful until an acceptable range of product properties has been ascertained.

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We can design and manufacture probes or fixtures for the TA.XTplus texture analyser that are bespoke to your sample and its specific measurement.

Once your measurement is performed, our expertise in its graphical interpretation is unparalleled. Not only can we develop the most suitable and accurate method for the testing of your sample, but we can also prepare analysis procedures that obtain the desired parameters from your curve and drop them into a spreadsheet or report designed around your requirements.

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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