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Compression is one of the most common tests done on food products. Typically a sample is placed on a flat surface and an upper compression platen is lowered into the sample. For a true compression test, the compressed sample is never smaller in diameter than the two compression platens. Test are performed by compression to a given force, a given position or a percentage of the original height of the sample.
TPA or Texture Profile Analysis, is a specialized derivative of compression testing. TPA consists of a 4 step test of compression, relaxation, recompression and then a final relaxation. A set of mathematical equations are then used on the resulting data to arrive at numerical values of such texture attributes as hardness, cohesiveness, springiness and chewiness. Compression testing can also be useful in measuring the adhesive or stickiness of the product. It is common in stickiness measurements to compress to a preset degree, hold the position for a given amount of time, and then pull back up quickly to accentuate the adhesive result. Compression platens come in many different sizes and are made form various materials such as stainless steel, aluminium and plastics. Selection of the proper variant depends upon the product and the test sample’s geometry.
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Penetration testing is very similar to compression testing with one key difference, the probe is typically much smaller than the sample being tested. Also referred to as puncture testing, penetration can be performed on a wide variety of food product and is a very useful test. It is most common that the probe is penetrated into a sample to a given distance and the peak force encountered during the test is measured.
This however can be different when the product has a “skin” such as an apple. Penetration probes come in a multitude of sizes and shape and the selection of the correct one can seem a bit daunting. The are cylindrical probes with sharp edges, radiused edges, and blunt tips. There are also conical shaped probes with sharp to wide angles. FTC can help with the correct selection for your particular product. One must also consider the supporting mechanism for the sample in conducting penetration test. If the sample is quite thin, then the results can be altered as the test can become a penetration / compression test as the probe near the bottom support. In this case a bottom plate with a hole is employed and the test becomes more of a “punch and die” test.
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Shear testing is a very popular test used in food texture analysis. It is common since many foods are first sliced or “sheared” by the front incisors when introduced to the mouth.
There are many different variations on the basic shear testing apparatus such as razor blades, V shaped blades, rounded blades and straight blades. We also offer a wire shear test cell for softer product like cheese and butter. Some versions such as the Warner-Bratzler meat shear design are somewhat standardized toward a particular type of food. Although the description of “shear testing” is commonly given to any test that uses a cutting action to apply a stress to a food sample, the true engineering definition of shear, does not employ a blade but refers to a material being stressed from two opposite directions and then shearing within it’s own structure.
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Extrusion testing has many applications within the food industry for texture assessment. Extrusion can be divided into two categories; forward extrusion and back extrusion. Forward extrusion is when the test sample is placed into a confined container and then forced through an orifice or grid. The foods resistance to this extrusion is then measured. The size of the orifice or grid is usually changeable and the proper selection is contingent upon the product being measured and the textural property of interest. For example, sour cream, when not processed properly, can be grainy. By forcing sour cream through a small orifice, one will either see a smooth force level or a jagged one.
The jagged one happens when the grains alternately become plugged and unplugged in the orifice building then releasing back pressure. Back extrusion is a more common method of extrusion testing. Back extrusion is typically performed with the product being measured is placed in an open top cylinder, and a piston with a diameter smaller than the cylinder, is forced down into the sample. The results is that the product flows between the space created by the piston and the cylinder and the resulting resistive force is measured. The key factor to consider in back extrusion is the gap between the piston and the cylinder. This is called the “annulus” and the smaller the annulus, the greater the resistance to the flow.
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Bulk analysis fixtures designs are among the most common and widely used fixtures in the food industry. The most famous of which is the Kramer Shear Compression Test Cell (KSC). The original design developed by Dr Ahmed Kramer, employs 10 shear blades that move down through a containment box that holds the sample under test. The test combines compression, extrusion and shear and correlates very closely to sensory panel “mouth feel”. Thousands of these test cells are in use every day in the food industry to measure the texture of a wide variety foods. Fruits, vegetables, pastas, ground meats, animal foods, rice and snack food are all typical products that lend themselves to successful texture measurement using bulk analysis designs.
The big advantage of the KSC, is that a large sample of the product can be tested at once. This gives a better average value of texture then testing one small piece of food multiple times and speeds texture assessment. One side effect from testing such an increased sample size is that higher forces are generated as a result. Since competitor systems cannot supply the higher forces, mock up designs like the “Mini Kramer Shear Cell” were developed to try and copy the original. In a sense this defeats one of the big advantages derived from the ability to test larger samples. FTC offers several variations on the KSC such as the CS-2 with 13 thinner shear blades for small particulate such as rice and bread crumbs also models made from stainless steel and Delrin for high acid food like diced tomatoes and salsa.
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Tension testing is not a very common test method used on food products. In most cases it is quite difficult, if not impossible, to consistently grip a food sample during a tension test without causing a false failure point. This is sometimes over come by forming the samples into a “dog bone” shape so that the cross sectional area in the center of the sample, is smaller that the cross sectional area of the part that is being gripped.
Some products such a stick chewing gum, restructured deli meats and cheeses, can be successfully tested for their elasticity with tensile testing and sensitive fixtures. FTC offers several tensile grip designs to accommodate the applications of our customers. It is not uncommon that a food texture system will find more use in the food packaging arena when it comes to tensile testing.
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Breaking or bending tests are commonly used for hard or brittle foods that snap, though some softer products are tested this way. Usually the product is stressed until it breaks and the peak or maximum force is measured but sometimes the amount of movement it will absorb before it gives way is also of interest.
The most common set up for this type of test is referred to as a “3 point bend”. The product is supported by 2 “fulcrums” on either side and a third, center fulcrum comes down to apply the bending force. The distance between the two side fulcrums is adjustable to accommodate different sized samples. FTC offers several variations of the 3 point bend apparatus to alloy testing very fragile to very hard products.
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