The measurement of firmness is of paramount importance to know the proper maturity and ripening stage during growth and storage of fruit.
Firmness can be an indicator of immaturity or overmaturity. Excessive peach firmness, for example, can indicate an immature peach with little free juice. Conversely, an overmature, soft peach can be excessively juicy and prone to bruises.
Where individual fruits are to be tested, penetration testing (using a cylinder or ball probe smaller than the fruit) provides a constant surface area for testing which often reduces the variability of results when compared to compression testing data.
Measuring the skin strength and flesh firmness of whole fruit
A penetration test destructively measures firmness by registering the force required for a Cylinder Probe (generally from 2mm – 8mm in diameter), Magness-Taylor Puncture Probe, or Ball Probe, to penetrate the fruit’s flesh to a chosen distance and is frequently used for testing firmness of a wide variety of fruits. Magness-Taylor probes are commonly used, widely accepted in the field of testing of whole fruit and have historically been the reference measure for firmness in many fruits as this method has shown good correlation with consumer acceptability for firmness. With probes of this size and shape the skin will yield once penetrated and the underlying flesh can also be measured.
Penetration testing has the benefit of not requiring samples to be of the same size and does not require sample preparation. However, orientation of the penetration is important as samples of this nature are anisotropic. The depth of penetration varies according to the fruit size and proximity to e.g. pits, cores and it is sometimes possible to perform tests on both sides of each fruit tested e.g. peaches. Some larger berries are commonly penetrated using a small (e.g. 2-3mm) Cylinder Probe and the maximum force taken as the firmness value.
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| Penetration test of apple using a Needle Probe |
For ease of testing, the fruit is often cut in half and the fruit laid down onto the sample platform cut surface down in order to stabilise the sample for penetration testing of its side. This method may be the only testing option if there is limited availability of sample to test but repeatability may be compromised. To obtain reliable results and reduced variation in this way, special attention should be paid to aspects of size, ripening stage and growing conditions.
This type of test primarily assesses skin strength/toughness and elasticity, yield point and resilience, the ripening and softening profile and the firmness of the underlying flesh. Some researchers prefer to remove a small section of the skin/peel with a razor blade so that penetration can be performed on the flesh only and the test is isolated to the measurement of flesh firmness.
A Penetration Test of Whole Fruit can be applied to:
a: Fruit with pits: where the outer skin covers a soft, fleshy fruit and the fruit surrounds a single, hard stone, or pit, which contains the seed – cherries, apricots, nectarines, peaches, plums, avocado, olives
b: Fruit with cores: where there is a central seed-containing core surrounded by a thick layer of flesh – apples, pears
c: Large fruits without cores: melons – large, juicy fruits with thick skins and many seeds
d: Citrus fruits where a measure of the peel characteristics are required: these possess a thick outer rind and a thin membrane separates the flesh into segments – e.g. oranges, tangerines, grapefruits, kumquats, lemons, limes
e: Starchy fruits: banana – pasty homogeneous starchy fruits with very soft texture; easily mashed to pulp or follow viscous behaviour when squashed
f: Large berries
g: Tropical fruits – papaya, figs, dates, guavas, mangoes, kiwis.
Interpretation of Whole Fruit Penetration Curve
The firmness of ripe and unripe samples can be tested by penetrating a small diameter cylinder probe into the whole fruit (around the equatorial region) and measuring the subsequent force to rupture the skin and further penetrate through the underlying tissue to a chosen distance of e.g. 5mm.
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1: Curves of ripe vs. unripe pears
tested using a 2mm cylinder probe |
The probe proceeds to move down onto the fruit and an initial rapid rise in force is observed. During this stage the sample is deforming under the applied force but there is no puncturing of the tissues.
This stage ends abruptly when the probe punctures through the skin and begins to penetrate into the sample flesh, which event is represented by the sudden change in slope called the “yield point” (or “bioyield point”). The yield point (maximum force) occurs when the probe begins to penetrate into the food, causing irreversible damage.
The third phase of the puncture test, namely the plateau of the force after the yield point, is an indication of the underlying flesh firmness of the fruit (1).
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2: Penetration force/distance curve
and parameters
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Six texture parameters can be calculated from the force-displacement curve (2). The maximum force (Fmax) represents the force required to puncture the fruit skin. Fmax represents the skin strength as is often termed the bioyield point. The probe displacement Dp, expressed in mm, is the value of the probe position at Fmax and indicates the elasticity of the skin.
Stiffness, is the slope of the first part of the curve measured from the beginning of the curve to Fmax. Work of penetration (W1) is the mechanical work needed to reach the rupture point, as is taken as the area under the curve to Fmax. Flesh firmness (Ff) is the average value of the forces measured after skin rupture. W2 is the work measured (area under the curve) after the skin rupture.
