Fruit Texture: How to measure properties of fruit leather, fruit films and fruit peel

Fruit Leather

A fruit leather is made by drying a very thin layer of fruit puree or a mixture of fruit juice concentrate and other ingredients on a flat surface in an oven, desiccators or in direct sunlight, to obtain a product with a chewy texture similar to soft leather. 

Almost any type of fruit is suitable for making fruit leathers. Fruit leather is easy to eat, convenient to pack, and makes an ideal popular snack almost anywhere for a mouth-watering sugar boost. When dried, the product is usually pulled from the surface, rolled and consumed. The control of the drying temperature is very important, as very high temperatures may cause case hardening, hindering the outflow of water. Too thin a layer of purée, on the other hand, can make the product brittle and difficult to be pulled from the surface.

Fruit Texture: How to measure firm pastes and dried fruit flakes

Measuring Firm Paste Texture 

Texture is a very important property of fruit pastes since they are often used as fillings for energy bars and other products containing fruits.

Among all textural properties, hardness and stickiness are the most important, since they can dictate the possibility of application onto a product. Usually fruit pastes are applied by lightly heated rollers. If the paste is too hard or sticky it may not be properly applied.

Fruit Texture: How to measure toughness and stickiness of dried fruit

Fresh fruits are in many ways ideal snacks, but suffer from difficulties in distribution and lack of universal seasonal availability.

Convenience, therefore, dictates that frequently some preparation and preservation needs to be carried out to ensure their availability throughout the year, ensure consistency of quality and even enhancement of particular attributes. 

Whilst dried fruits retain a great deal of their nutritive value they often come under criticism for texture as drying can cause considerable loss of textural integrity.

Fruit Texture: How to measure detachment and deseeding force

Measuring Detachment Force

Table grapes are highly perishable, non-climacteric fruits. Their shelf life is shortened by loss of firmness, berry drop, discoloration of the stem, desiccation and fungal rots.

The berry drop is due basically to dry-drop (or abscission) – ethylene in conjunction with falling auxin levels, induces the formation of an abscission zone at the pedicel-berry junction, thus stimulating their drop. Grape varieties susceptible to berry drop present a huge problem for successful storage and marketing. For this reason it is required to predict and control the harvested abscission of grape berries, which is not only of inherent scientific interest, but it also has considerable commercial significance.

Fruit Texture: How to measure flesh firmness of non-homogeneous fruits

Testing fruits of non-homogenous nature or variable texture, such as water melons where there is a high seed content, is not only tricky by any puncture, shearing or compression method, but often results in low reproducibility and misleading data.

In any of these tests, the data may show wide variances between maximum and minimum force resistance, depending upon whether the probe or fixture meets with less or more seeds or variable texture when tested. By penetrating the product in several areas at the same time, the Multiple Puncture Probe produces an averaging effect and is therefore more representative.

Fruit Texture: How to measure flesh firmness of prepared homogeneous fruit samples

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.

Fruit Texture: How to measure the overall firmness of a weight or quantity of fruits

This type of test commonly applies to:

Kramer Shear Cell

a: Soft fruits: drupelets and berries – e.g. cranberries, blueberries, blackberry, raspberry, strawberry, pomegranate arils, grapes
b: Fibrous samples: such as pomegranates, citrus fruits, pineapples which have been prepared into pieces.
c: Pieces of fruit that have been prepared into smaller pieces e.g. apple cubes.

The primary issue of these types of samples is that they are usually of varying sizes or are of non-homogeneous nature and therefore make comparisons difficult. They therefore have a high variability from piece to piece within the same batch and require a large sample set to be tested. Puncture or compression tests to rupture are possible but usually produce results with poor repeatability.

Fruit Texture: How to measure the firmness of whole fruit by compression

Fruit can alternatively undergo a mild non-destructive test where the deformation response of a large cylinder probe or platen is measured to effectively mimic the compression between one’s fingers.

However this has the problem of measurement variability due to differing contact area with the fruit surface which varies because of irregularities in fruit shape and size and results may then be compromised for the convenience of using such a test.