For reliable and reproducible results from fracture testing, some basic rules must be taken into consideration. Biological materials are notoriously variable in their morphology and structure. Variation can occur not only at any or all of the hierarchical levels from the molecular, through to material and structural level, but also within the entire test specimen and even between specimens.
Storage, preparation and environment can give rise to discrepancies as can differences in experimental techniques and conditions. When carrying out replicate tests the important rule is to minimise these variables. This reduces scatter in the results and enables a reliable comparison between varieties of products. The main sources of variation are listed below.
Specimen morphology:
Always choose test specimens with similar structure. Natural materials like meats, fruit and vegetables are extremely variable in their material and structural properties which greatly influence the texture. For products that are reconstituted, processed, baked, etc. differences in preparation conditions can produce large structural variations.
Size and shape:
Individual test specimens have to be of comparable size and shape. Specimens that are too small yield different results from larger ones (this is called “size effect”). Always choose the larger specimens to avoid these discrepancies as above a certain critical size the size effect is negligible. If the replicates are of a variety of sizes, the results must be standardised.
Shape is much more critical for reliable results. This determines the distribution of stresses within the specimen and hence its fracture properties. In most processed materials (e.g.: biscuits, pasta, tablets or pellets, extruded snacks etc.) the size and shape is quite consistent and can be controlled easily, but in most natural materials like meat and fruit it is best to eliminate this variable by cutting reproducible geometrically shaped test specimens such as cylinders or cubes.
Orientation and anisotropy:
Some materials can be anisotropic, i.e. their mechanical properties can vary according to the direction they are loaded in. Meat is a classic example with its highly orientated fibres. It is much easier to fracture meat in between the fibres than through the fibres, hence the perception of texture in these two directions is extremely different.
Such a property applies to most fruit and vegetables and some processed and shaped products, e.g. chocolate bars. Orientation of the test specimen must be considered and such a variation eliminated in replicate tests.
Humidity:
The moisture content within materials has a major influence on its mechanical and fracture properties and hence its texture. Fleshy plant material, e.g. fruit flesh, tends to lose about 5% of its moisture every minute. If the test takes several minutes to complete, the material at the end of the test will have a very different mechanical behaviour than at the start of the test.
Such discrepancies must be minimised by either reducing the exposure to air, sealing the specimen loosely in Clingfilm or testing them in a constant humidity environment. Rapid moisture loss (or in some cases uptake) affects most materials such as fruit and vegetables, meat and fish, bakery, cereals, pet foods, pasta, etc.
Temperature:
Like humidity, temperature plays an important part in affecting the mechanical properties of food and other products. Even minor variation in ambient temperature affects the stiffness of plant and animal tissues. Larger fluctuations affect the glassiness (brittleness) of materials such as pasta, bakery and snack foods.
Although small temperature fluctuations have negligible effect, larger fluctuations or temperature sensitive products are best tested in a controlled temperature environment. Temperature control in frozen products is critical as this affects the size of the ice crystals and hence the degree of mechanical damage to the material. Smallest changes of temperature can produce large variations in the results.
Test sample size:
Morphological variation in biological or biologically originated material is nevertheless inevitable. Statistically the larger the test sample, (the more the replicates) the more reliable the result. A sample size of less than 5 is not recommended.
Speed of testing:
For almost all such materials the speed at which the specimen is deformed (strain rate) affects its fracture properties. If a material is deformed quickly, generally the strength and toughness values are unrealistically high. This is because more energy has to be supplied and a large proportion of this is the residual strain energy. Slow deformation enables energy to be more realistically partitioned.
Most materials are strain rate dependent at higher strain rates and independent at lower strain rates. Material scientists recommend slow strain rates which yield lower and more realistic fracture values. Brittle materials such as crisps, some biscuits, some cereals, boiled sweets etc. are strain rate independent.
Any deformation rate can apply to these tests. The softer the material the more strain rate dependent it is and it is advisable to use a slow deformation rate for such tests.
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.No-one understands texture analysis like we do!
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