Compression is one of the most common texture tests, especially for food products.
Typically a sample is placed on a flat surface and an upper compression platen is lowered into the sample.
Compression testing samples should be smaller than both the surface area of the probe and the test surface on which the sample is placed.
This should be true for the duration of the test as the geometry of the sample changes as it is compressed.
Tests are performed by compression to a given force, a given position or a percentage of the original height of the sample.
Compression with smaller diameter probes, compress locally and may also induce penetration or puncture.
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.
The measurement of succulence (juiciness) is also performed by compression, the sample usually measure in bulk.
Typically a constant force is applied for a given time and the expressible moisture is measured in a graduated cylinder.
Compression platens come in many different sizes and are made from various materials such as stainless steel, aluminium and plastics.
Selection of the proper variant depends upon the product and the test sample's geometry.