Types of Tests
Texture (Chewiness, Spreadability, Stickiness)
Texture Profile Analysis or TPA
The specimen under test is compressed to a set displacement, normally twice, by suitable probe/platen and the compressive and tensile forces plotted against time. The probe geometry is determined by the material under test but is normally cylindrical and flat of about 35-50mm diameter. The displacements, test speeds and wait time will alter according to the material under test. On the tensile stoke the displacement should be such that the material is pulled away from the probe. In some cases the material may exhibit cohesive failure with material being left on the probe.
The above diagram shows an idealised curve. The annotations have the following meaning.
F1 first peak. F2 maximum compressive force on the first stroke. Sometimes F1 & F2 maybe the same. F3 the maximum tensile force on the first return stroke. F4 the maximum compressive force on the second stroke.
A1 the area under the curve to the maximum compressive force on the first stroke. A2 the area under the curve to return to the preload force on the first stroke. A3 the area under the curve on the first tensile stroke. This should be until the specimen has fully pulled away from the probe. A4 the area under the curve to the maximum compressive force on the second stroke.
L1 the time to the maximum compressive force on the first stroke from the preload. L2 the time to the maximum compressive force on the second stroke from the preload.
W1 the wait time between the first and second strokes.
Note the areas are in arbitrary mm. seconds and does not equate to Joules the unit of work. Some papers show a plot of displacement against force where L2, L1 are reported in mm. The ratio A4/A1 etc. will however be the same for either type of graph. Some calculations appear to be different according to the source i.e. springiness is calculated as L2(in mm) and not L2/L1 The latter now seems to be the preferred method. The ratio L1/L2 (in mm) has been reported as the spring index. There seems to be inconsistence in the units i.e. Fracturability as mN and N/mm^2, Chewiness as N/mm and mN.mm. Others quote results without units which appears the norm when time is used for the X-axis.
| Result |
Calculation & Description for TPA |
| Adhesiveness |
is a measure of the ‘work’ necessary to overcome the adhesion between the surfaces of the specimen and the surface of the probe.
= A3 |
| Chewiness |
is the quantity to simulate the energy required to masticate a semi-solid sample to a steady state ‘of’ swallowing;
equals (Hardness * Cohesiveness * Adhesiveness).
= F2* A4/A1* A3 |
| Cohesiveness |
the ratio of the effort to compress the specimen between the two strokes. It is meant to relate to the ability of the material to hold together.
= A4/A1 |
| Fracturability or Brittleness: |
is the force at which the material fractures. If the force is still raising at the specified displacement F2 may not necessarily be the fracture point. Ether the fracture point has not been reached or the material it to elastic to have a fracture point.
= F1 |
| Gumminess |
is the quantity to simulate the energy required to disintegrate a semi-solid sample to a steady state ‘of’ swallowing;
equals (Hardness * Cohesiveness).
= A4/A1*F2 |
| Hardness |
is the force necessary to attain a given deformation.
= F2 |
| Stickiness or Tackiness |
is the force required to release the probe from the product.
= F3 |
| Resilience |
is a measurement of how the sample recovers from deformation both in terms of speed and forces derived.
= A2/A1 |
| Springiness |
is a measure of how a material recovers after being compressed.
= L2/L1 |
| Stringiness |
is the distance the product is extended on the first tensile stroke before separating from the compression probe. This is not shown on the diagram as its a measurement of displacement, but is the movement corresponding to the area A3. |
Other measurements could be taken i.e. the force on the second stroke F4, its ratio to F1.
|
