How to Measure Firmness, Softness, Hardness and Ripeness

Abrasion measurement applications using the TA.XTplus Texture Analyser

FIRMNESS/HARDNESS/SOFTNESS are textural properties that are generally on the same property spectrum. A soft product is one that displays a slight resistance to deformation, a firm product describes one that is moderately resistant to deformation and hardness describes a product which displays substantial resistance to deformation. However, it is also found that depending upon the product industry one of these words may be more favourable or pertinent to a particular product.

The terms Firmness, Hardness and Softness are so broadly used that they can be obtained by a variety of different test types. In certain cases, the product has limitations of presenting to the texture analyser for testing. For these reasons, a compromising solution may have to be found to optimise the testing of the required product property.

The test type is chosen according to the size, form, homogeneity etc. of the sample and can therefore be derived from a single piece compression, bulk compression, penetration, extrusion or cutting tests as is most appropriate. Whilst performing one of these tests it is not uncommon to be able to obtain a number of other textural parameters that are also possessed by the product.

Related properties:

• SPREADABILITY
• BITING FORCE/CUTTING FORCE
• CONSISTENCY
• RUPTURE FORCE
• COMPACTION FORCE/COMPRESSIBILITY
• TOUGHNESS


Typical properties that can be obtained from a texture analyser graph:

Firmness, hardness, softness, work of penetration, work of compression
Typical texture analysis graph

Typical Texture Analyser graph with annotated properties illustrating firmness


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Typical Probe/Fixture used for Measurement:

Compression Probes >>
Penetration Probes >>
Multiple Compression/Penetration Probes >>
Back Extrusion Rig >>
Knife Blades of all varieties >>

In certain cases, the product has limitations of presenting to the texture analyser for testing. For these reasons, a compromise solution may have to be found to optimise the testing of the required product property. The choice of method and equipment for measurement will depend upon several factors, such as:

• Is your product self-supporting? (i.e. can it be tested outside of a container?)

• Even if your product is self-supporting, do you want to test it inside its container?

• Is your product repeatable in terms of size and shape?

• Is your product non-homogeneous, i.e. does it have particulates etc.?

• What type of test did you have in mind?

Compression

The physical nature of some products make them more suitable for testing via a compression test. Generally the higher the force to compress (taken at the maximum force, or a suitable distance prior to the maximum force) the firmer the sample. For the repeatable assessment of products using a compression test, the sample must be repeatable in size and shape, as a compression test (by rule) completely encloses the product.

Where sample pieces are not so repeatable in size and shape (or are non-homogeneous), a number of pieces may be taken and tested together in a 'bulk compression' test. This serves to create an averaging effect.

A compression test may be chosen over a penetration test in some cases. One reason would be the presence of particulates/inclusions in a product. Penetration tests generally use small cylinder/ball/cone probes. They have a small surface area for testing and as such are more affected by non-uniform product structures. A compression test (generally using larger cylinder probes) serves to create an averaging affect, due to its larger surface area for testing. A stress relaxation test (measuring springiness, elastic recovery) should be a compression test where the whole sample is held under a compression probe. A penetration test produces a hole in the product and does not therefore allow the product to recover as in a small deformation compression test.

Bulk compression

Where a high force instrument is available, better repeatability may be provided by testing the sample 'in bulk' (i.e. the testing of a number of pieces or chosen weight of sample for each test.) This is especially suitable for those products that are not repeatable in size or shape and/or are non-homogeneous. Bulk compression tests are usually performed using an Ottawa Cell or Kramer Shear Cell.

Penetration

The physical nature of some products make them more suitable for testing via a penetration test. Generally the higher the force to penetrate, the firmer the sample. A penetration test may often be chosen over a compression test for the following reasons:

a) Where the product is not repeatable in size and shape, but has a certain repeatable facet of its surface to which a small penetration probe can repeatedly be applied. For example, in the testing of an apple, apples are not normally perfectly identical in size and shape. By using a penetration probe to test, the apple could have a diameter of e.g. 60-300mm withoutany effect on the repeatability of the test.

b) Where assessment of the internal structure of a product is more desirable. A penetration test, by its nature, probes through a product (producing a hole in its path). Sometimes small cylinder probes/needles are used to assess, for instance, the aerated or laminated structure of a baked product, as they are effectively more sensitive than a test which crushes (compresses) the whole sample.

c) A smaller cylinder probe reduces the measured force if a test exceeds the load cell capacity, e.g. for hard candy, and may be, for some products, the only possible way to test given a force limitation situation.

Rupture Force and Gel Strength/Bloom Strength are characteristics that can be measured during a penetration test.

Extrusion

The physical nature of some products make them more suitable for testing via an extrusion test. This test is obviously limited to those products that can be extruded (i.e. generally fluid products) and is naturally unsuitable for such 'dry' products as e.g. breakfast cereals and bread. Generally the higher the force to extrude, the firmer the sample.

Cutting

The physical nature of some products make them more suitable for testing via a cutting test. Generally the higher the force to cut/shear, the firmer the sample.

Bending

The physical nature of some products make them more suitable for testing via a bending test. Generally the higher the force to bend (usually up to the point of break), the firmer the sample.

The above are only typical examples of firmness/hardness/softness measurement. We can, of course, design and manufacture probes or fixtures that are bespoke to your sample and its specific measurement.

Once your measurement is performed, our expertise in its graphical interpretation is unparalleled – no-one understands texture analysis like we do. Not only can we develop the most suitable and accurate method for the testing of your sample, but we can prepare analysis procedures that obtain the desired parameters from your curve and drop them into a spreadsheet or report designed around your requirements.

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