In this guide, we’ll discuss best practices in sample preparation to help you achieve consistent and reliable texture measurement results.
- Samples must be prepared in a reproducible way.
- Sample preparation should be done with very sharp instruments to minimise pre-test deformation.
- Choose representative samples (be aware of segregation particularly for multi particle samples).
- Samples with structural defects should be avoided or if tested high variation in results must be expected.
- Multi-particulate samples are suited to bulk testing to get an averaging effect particularly where the pieces differ in size and shape. If you would like more advice on 'Testing Non-Homogeneous and Non-uniform Products' please request this article by contacting app.support@stablemicrosystems.com
- Semi-solid materials should be tested in their original container, where possible, to avoid structural changes before testing.
- Consider whether the sample has a 'direction'. 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. Manufactured materials, such as films, may have a rolling direction, In these instances, the directional alignment and location of testing will need to be considered.
- If performing a penetration test, avoid penetrating the sample within the 'fracture zone' of another test site.
- In tensile experiments the sample length should be at least twice the width. During crack propagation, the energy needed for fracture is obtained from the elastic energy stored in volumes next to the growing crack. The size of the test piece should be such that this volume is outside the part between the grips holding the specimen.
- 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.
- Minimal handling: Reduce the amount of handling to prevent altering the sample's surface or internal structure. Use tweezers or gloves to handle delicate samples like gels to avoid temperature changes or structural changes.
- Testing schedule: Test all samples within a short timeframe to avoid changes in properties over time due to aging or drying out. For materials sensitive to environmental conditions, test immediately after preparation.
Does temperature need to be controlled?
For many samples, temperature has a strong influence on the rheological and fracture properties, therefore, tests for such samples should be done at a given constant 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. Samples such as gels and fats will require temperature control immediately prior to testing and in some cases temperature control may be necessary during testing (depending upon the sample's sensitivity, the ambient temperature and the duration of the test). Although small temperature fluctuations can have negligible effect, larger fluctuations or temperature sensitive products are best tested in a controlled temperature environment with the use of Temperature Controlled Options. 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. Maintain controlled test conditions, particularly temperature, for comparison purposes.
Results can only be directly compared if the tests are carried out under identical conditions. Care must be exercised to maintain all the tests conditions constant. When reporting the results of texture analysis testing it is necessary to give full details of all the test conditions so that readers of the report can correctly interpret such results.
Size and shape of samples
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/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.
A small difference in dimensions when performing certain tests e.g. compression/cutting tests, can make a large difference in surface area which will have an immediate effect on the likely % variation in results. Larger samples have a lower effect on sample dimension differences. For example, if you are preparing a cube of surface area 10mm x 10mm and instead present for testing a cube of surface area 11mm x 11mm this will result in a cross-sectional area increase of 10%. Subsequently before even testing you could expect to get a 20% higher result for this larger surface area – this is without any + or – error due to sample differences.
Templates, moulds or cutting guides should be developed to standardise sample dimensions. Various sample preparation tools are available to assist in the repeatable preparation of samples such as a Twin Blade Sample Preparation Tool.
Natural vs. formulated/manufactured products
Foods might be classed into two groups, depending on the relative ease with which texture can be controlled:
- Native foods are those foods in which the original structure of the agricultural commodity remains essentially intact. With these foods the food technologist has to take what nature provides in the form of fruit, fish, meat, poultry, vegetables, etc. and can only change the texture by processing methods such as heating, cooling, and size reduction. Usually there is almost no direct control over the composition of these foods, although with some of them it is possible to partially control the composition and texture by breeding, time of harvest, and cultural factors. These products have inherent variability and as such texture measurement results may be subject to large variations. Testing one strawberry on its own and then testing another from the same plant, picked on the same day often give surprisingly different results. For this reason, these types of products are most frequently recommended to be tested in ‘bulk’, i.e. where a certain weight or number of pieces of sample are tested within one test. The resulting test provides an averaging effect.
- Formulated foods are those foods that are processed from a number of ingredients to make a food product that is not found in nature. Many native foods are transformed into ingredients for formulated foods, but in doing so the native plant or animal structure and organisation is usually lost. Examples of this type of commodity are bread, ketchup, ice cream, jellies, mayonnaise, candy, cheese, margarine and sausage. With this class of commodity, it is possible to change the formulation by the number, amount, and quality of ingredients that are used in addition to processing variables, and hence there are more options available to control the texture of the finished product and to develop specified textures and structures not found in native foods.
In summary, when something is manufactured you are measuring the effects of ingredients, processing variables, storage effects, etc. When something is natural – it is what it is and if the variability of the results is high it is not to be confused with the wrong method, settings or measurement failing of your Texture Analyser.
Proper sample preparation is a critical step in texture analysis that significantly impacts the reliability and consistency of your results. By following these best practices, you can minimise variability and ensure that your texture measurements accurately reflect the properties of your material. Consistent sample size, controlled environment, careful handling, and timely testing are essential to achieving reliable data.
This information is also contained within the Education Zone of Exponent Connect software:
Why not request ‘Optimising your texture anlaysis test method - the ultimate guide' to assist your understanding:
Or if you have lost it, request a 167 page (pdf) in-depth manual to be sent to you.
There is a Texture Analysis test for virtually any physical property. Contact Stable Micro Systems today to learn more about our full range of solutions.
For more information on how to measure texture, please visit the Texture Analysis Properties section on our website.
The TA.XTplusC 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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