The puncture test is one of the simplest, fastest, most versatile and flexible principles used for measuring textural properties of many kinds of foods.  It is probably the most frequently used texture test principle.  It is also the oldest test principle described in the literature.  In 1861 Lipowitz published a paper showing how a primitive puncture test was used to measure the strength of gelatin gels.

The TA.XT2 is especially well adapted for the puncture test because of the wide range of punches and supports that are available.

The puncture test measures the force required to push a punch or probe into a food.  It is characterized by three features:
1.   It is a force measurement
2.   Penetration of the punch into the food causes irreversible crushing or flow of the food.  It is a destructive test.
3.   The depth of penetration is usually held constant.

However, there are several other test principles that are somewhat similar to the puncture test and the operator needs to be aware that unless care is exercised, one can easily stray from a "pure" puncture principle into other test principles, or worse, into a hybrid or "mongrel" test that is partly puncture and partly something else.  Two potential problems that may cause deviation from a true puncture test will now be discussed.

1.  Semi-infinite geometry.

A true puncture test assumes what is called "semi-infinite geometry" which means the test specimen is so much larger than the punch that there are no effects from the sides, bottom, edges, or corners of the specimen.  No matter how much the specimen size is increased while using the same size punch, there will be no change in the measured force when semi-infinite geometry is maintained.  A neat hole is evident in the specimen after completion of a true puncture test.  There should be no splitting out at the sides or bottom, or cracking or fracturing of the specimen.  When first establishing a test protocol the operator should look at the specimen after a preliminary test to see whether any of these events have occurred, and if they have, make some changes in the protocol to ensure that they do not occur in the future.  However, if such an event is a rare occurrence in what is a well established and usually successful protocol then that particular test result should be discarded and the protocol may continue to be used.

      As a general rule, the diameter of the test specimen should be at least three times the diameter of the punch to ensure semi-infinite geometry is maintained.  The sample diameter/punch diameter ratio may need to be more than 3 for brittle or fracturable foods.  In my experience I have found that even the most fracturable food can be successfully tested by puncture provided a small enough punch is used.

     The principle of semi-infinite geometry is illustrated on the left hand side of Figure 1 where the sample is so much larger than the punch that side effects and bottom effects will be absent for most foods.  The central section of Figure 1 illustrates failure to achieve semi-infinite geometry; because the sample diameter is approaching the diameter of the punch this will not be a true puncture test.  What do you do if the sample size cannot be increased to three times the punch size?  The answer is shown on the right hand side of Figure 1.  Semi-infinite geometry is restored by reducing the size of the punch to less than one-third the diameter of the sample.  Depending on the product, I have used punches ranging from 0.05 mm up to 50 mm in diameter.

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 2.  Support.
     The plate that supports the specimen being subjected to the puncture test may introduce errors as illustrated in Figure 2.  When the specimen is large the punch will only penetrate a small distance relative to the size of the food and a solid plate is correct (Fig. 2A).  When the specimen is thin, there is a grave risk of compressing the food against the support plate and the test will become a combination of puncture and compression or pure compression(Fig. 2B) which is incorrect.  A support plate that has a hole centered under the punch is needed for thin or small products as shown in Figure 2C.  This allows the punch to penetrate all the way through the specimen and into the hole.  The diameter of this hole should usually be 1.5 to 3 times the diameter of the punch to give adequate support to the specimen while avoiding the introduction of the errors shown in Figures 2D and 2E.  When the hole is almost the same size as the punch (Figure 2D) the test changes from true puncture to a "punch and die" test where a cylinder of the material is cut through the food and pushed into the hole.  When the diameter of the support plate is much larger than the punch diameter (Figure 2E) the sample is likely to be bent and pushed into the hole thus changing the test from pure puncture into a bending test or part bending and part puncture test.

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     By paying attention to the rules enunciated above you will avoid some pitfalls that might occur in this useful and widely used texture test principle.

This article is copywrited by Dr. Malcolm Bourne, and was printed in TTC's The Texture Report newsletter and at this website by permission of the author.