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Sometimes the sample to be analysed may be of variable configuration or structure from piece to piece, such as breakfast cereal flakes. In the case of fruit testing, for example, whilst the preferred method of testing might be compression, the inherent variability of natural products and the range of sizes of fruit from piece to piece may preclude the option of testing the pieces one at a time. On the other hand a manufactured sample such as a cereal bar, or indeed a natural product such as meat, may have uniformity in its sample shape and size but consist of an entirely variable texture throughout.

These challenges call for a range of tests which deal with the compromise of sample heterogeneity. In some instances, the preferred method of testing (such as compression, penetration or shearing) can be adapted to improve the chance of obtaining a repeatable result. What all of these methods have in common is the fact that by testing a larger number of pieces, or a wider surface area with more testing surfaces, an averaging effect is created which is the result of a representative set of pieces or surfaces.

The primary issue of these types of samples is that they are usually of varying sizes or are of non-homogeneous nature, and therefore make comparisons difficult. They have a high variability from piece to piece within the same batch and require a large sample set to be tested. Puncture or compression tests to rupture are possible, but usually produce results with poor repeatability. To ensure that repeatable results are obtained when testing by compression, it is essential that the dimensions of the test pieces are consistent. To do this, fruit cylinders or cubes would therefore need to be prepared, which might be impractical in terms of time available or ease of testing.

In this instance, it is advisable to take a certain number or certain weight of samples and perform a bulk compression or shearing test. This type of test creates an ‘averaging effect’ and gives the result of a representative number or weight of fruit pieces.

The result is an average of the forces required to compress, shear, puncture or extrude the sample of variable geometry. The maximum force and area under the curve are usually recorded for all of these types of test and taken as an indication of bulk firmness. The area under the curve is usually termed the ‘work of shear/compression/penetration’ – a larger value indicating a firmer sample.

TESTING via COMPRESSION

Bulk Compression

When compression is the preferred testing method, the repeatability relies heavily on the uniformity of the sample in terms of size, shape, configuration and homogeneity. A small change in sample size has an immediate effect on surface area for testing, which will obviously affect the magnitude of the force to be measured, and thereby affect the repeatability.

Bulk compression is where the sample is compressed in three dimensions. It is commonly used for fruit and vegetables, breakfast cereals, etc. where a chosen number of pieces or weight of sample is tested at the same time. Normally, as these products are oddly shaped and therefore have inherent variability, a bulk compression test is the most reliable way of assessing their texture. An averaging effect takes place within one test where the variation in piece to piece size, shape and configuration is accommodated. A standard compression test using a cylinder probe can be used for a certain number of sample pieces such as pellets where sample size is more uniform, but in compression tests on single pieces has proven to be disappointingly unrepeatable.

Errors due to changes in the shape of the sample during compression testing may be reduced by confining the sample in a close-fitting cell and compressing it with a plunger that just fits into the cell with little or no clearance between it and the inner surface of the cell wall; these are bulk compression tests.

Due to its relatively small testing diameter and volume, the Granule Compaction Rig is the recommended solution for granular materials, such as tabletting material, whilst the most common bulk compression device for food products is an Ottawa Cell which is comprised of a square section test cell and loose fitting plunger.

Test samples are typically extruded through an extrusion plate located in the base of the cell. Extrusion plates feature holes, wire, blades or bars, to suit the sample. A watertight base and liquid catchment tray expands the range of products which may be tested using this attachment, such as ‘bowl life’ assessment of breakfast cereals.

TESTING via SHEARING

Kramer Shear Cell

Bulk testing can also be performed with a Kramer Shear Cell, which uses a stationary rectangular box with slots in the base to hold the sample and a moving probe composed of 5 or 10 blades to drive through the test specimen. Pieces are placed into the cell to the same filling volume or by weight, and the blades positioned at a constant position above the sample surface. The blades then move down into the sample, compressing, shearing and extruding the bulk to a point close to, or slightly through, the base of the cell. Because individual peas, beans, berries, and similar multi-particle products can be tested, the apparatus permits the determination of the distribution pattern, as well as of an average resistance for a batch of the test material. Both maximum force readings and areas under the peak (work function) can be determined.

For products with variable texture across their length – for example, meat or cereal bars – results from single blade tests may be highly variable. This is because for one test of a typical cereal bar, the blade might come into contact with a peanut and a chocolate chip, and the next blade test of the same bar might slice through a raisin and an apricot piece; in this case the blade is almost testing different samples. The Kramer Shear Cell method is made on a defined sample quantity. The multiple blades provide a measurement on several positions simultaneously, compensating for local texture deviations. So this method provides an averaging effect and has shown to be more reproducible for such highly variable samples.

The Kramer Shear Cell allows the testing of a larger sample size and therefore can have the advantage of obtaining an averaging effect. While this is advantageous for the repeatable testing of highly non-uniform products, the drawback is that this type of test commonly requires a high force load cell, e.g. 500kg, and therefore lends itself to a TA.HDplus application.
A standard Kramer Shear Cell or Ottawa Cell test would ordinarily produce test results of high force. Reducing the volume of small sample pieces to be tested using a Miniature Kramer Shear/ Ottawa Cell consequently reduces the magnitude of the forces produced during the test, making the rig more suitable for use with the single column or low force Texture Analysers.

TESTING via PENETRATION

Using Penetration Testing

A penetration test could well be considered the most simple of all tests. As the size of the probe surface area is constant this often means that the sample size does not need to be carefully controlled, unlike compression testing. However, when faced with a non-homogeneous product, penetration is highly compromised as a smaller surface area for measurement is also more sensitive to variations in sample structure and low reproducibility and misleading data may be obtained. Results may show a wide variance between maximum and minimum forces, depending on whether the probe meets with, for example, an air pocket, hard particle or internal structure variation.

Multiple Puncture Probe

The use of a Multiple Puncture Probe that penetrates the sample in several regions serves to create an averaging effect and is therefore usually more repeatable. Using several testing pins, attached to the TA.XTplus texture analyser, manufacturers can test non-uniform products containing particulates of different size, shape, structure and levels of hardness, to provide repeatable results.

Such products have non-homogeneous textures due to the presence of, for example, dried fruits, vegetables or nuts, or consist of different layers to provide more interest to the consumer. By penetrating the product in several areas at the same time, the Multiple Puncture Probe produces an averaging effect and is therefore more representative. This probe also allows reduced forces by the removal of a proportion of the pins.

This probe quantifies the firmness and hardness of products such as petfood, jams and preserves containing fruit pieces or ice cream containing particulates. For example, these can be meringue pieces, chocolate chips or fruit chunks. These products can be difficult to measure as the number, size, shape and distribution of particulates is usually random within each container.

Multiple Chip Rig

This rig measures the force obtained by the penetration of a multiple sample of chips. Up to 10 chips can be tested at one time and the rig ensures that each chip is completely penetrated by two 2mm diameter probes per test.

Multiple Pea Rig

This rig measures the force to simultaneously penetrate up to 18 samples. The insert has 18 indentations surrounding a cone which allows the sample to locate easily into the test positions.

Puncture strength and penetration force are measured with 18 2mm cylinder probes and the penetration distance is set so that the probes penetrate completely through the sample. Other applications using this rig include beans and confectionery samples.