A compression test may be the preferred test method, but where fruit samples vary in size this will immediately reflect in the magnitude of force measured as it is subject to surface area differences and consequently the reproducibility of results will be poor.
In this instance, the fruit will be required to be prepared into pieces of accurate dimensions – usually cubes or cylinders.
Cubes are often obtained with the aid of dual razor blades mounted a set distance apart then used to trim the cylinder after the first 3mm (including skin) are discarded whilst cylinders are usually prepared with the aid of a core borer. Because specimen orientation significantly affects resultant mechanical textural properties it may be preferable to test the pieces with the original skin or rind-side down taken from the same region of the fruit to avoid differences due to anisotropy.
Measuring Homogeneous Fruit
A compression test to, for example, 75% of the original height using a cylinder probe or platen larger than the sample, is typical. Force versus distance values are recorded and used to calculate: the rupture or breaking force (bioyield point) – a measure of the fruit firmness or resistance to rupture, toughness or the area under the force-distance curve up to the point of rupture of the fruits (bioyield area), and slope of the force deformation curve until the 1st inflection point (modulus of elasticity or Young’s modulus) – a measure of stiffness in the region of linear elasticity (~3% strain).
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| Fracture Wedge cutting test |
Cutting tests of cubed or cylindrical samples (with or without skin) using a Blade Set, Craft Knife, Light Knife Blade or Fracture Wedges (as shown left) to a depth of 0.5mm above the base of the test platform, are also valid methods and testing tools but again have the disadvantage of requiring careful sample preparation to make sure that samples are the same in every dimension and tissue orientation. Peak fruit cutting force and energy (area under the curve) are typically recorded. The use of a blade when testing is a useful way to measure ‘bite’ characteristics.
