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.

In the quest for optimum ‘bite’ in these products, a blade test mimics the action of biting, thus representing a useful indicator of eventual eating experience. Alternatively the assessment of surface stickiness would be performed as previously mentioned for dried fruits whereby a cylinder probe contacts the sample surface with a chosen force (to achieve a good bond between two surfaces) and the force to separate the probe from the sample surface is measured as stickiness.

Measuring the Properties of Edible Films

Fruit pomace extracts, which contain pectin, celluloses, pigments, and other functional compounds, may also be used as a novel film-formatting material for making edible films and coatings.

Such edible films and coatings would provide additional benefits to traditional edible film-forming materials by providing unique fruit flavour and colour, thus attracting more potential applications.

Mechanical properties of such films can be measured using ASTM D882 method for the measurement of tensile strength and percent elongation at break. Each film (of specified dimensions) is mounted between Tensile Grips (left) with an initial grip separation of 50mm and a tensile test performed at 0.5mm/s. The maximum force (N) is divided by the film cross-sectional area (mm²) to calculate the tensile strength and elongation at break is divided by the initial length of the specimen and multiplied by 100 to calculate the percent elongation at break.

Alternatively, biextensional properties can be measure with the use of a Film Support Rig (below right), which allows the measurement of the resilience of fine films. Prior to performing the test, the sample is placed over a hole in a raised perspex platform. A top plate prevents the sample from slipping during testing. The test is then carried out as the arm of the texture analyser brings a 5mm stainless steel ball probe down into the aperture. The maximum force to rupture the film is recorded and is referred to as the burst strength of the film.
 
The resilience and relaxation properties of the film can also be measured. Resilience can be assessed by depressing the film surface to a chosen distance before retracting the ball probe. The property is calculated using a ratio of the work of compression and work of withdrawal. Similarly, relaxation can be measured with the addition of a hold period within the test to allow the product's recovery to be evaluated. Both these properties broaden the application of the Film Support Rig. Burst strength, resilience and relaxation are important factors in determining the mechanical properties of the product, allowing manufacturers to optimise product structure and formulation.

Fruit Peel

There is little information on post-harvest physico-mechanical properties changes of orange peel and fruit under ambient and refrigerated storage conditions which are helpful to decide handling, packaging, storage, and transportation systems to be adopted and their designs.

However, researchers have reported the use of a peel tensile strength test to evaluate the behaviour of orange peel under applied tensile loads. Clamps were made to hold a section of orange peel for determining peel strength. Peel pieces were carefully dissected from the equator of five randomly selected fruits.

Immediately after peel removal, peel thickness was measured using vernier callipers and peel strips of 15mm (polar) and 60mm (equatorial) were attached via clamps. Strips were subjected to axial tensile loading in an equatorial direction with a crosshead speed of 10mm/min until rupture. Rupture force was taken as the maximum peak force required to rupture the peel. Tensile strength was calculated by dividing the peak rupture force by the cross-sectional area (thickness x width) of the initial specimen. Modulus of elasticity was calculated as the slope of the initial linear portion of a stress-strain curve.

Watch the video below to see a summary of the types of testing possibilities that are available for the measurement of fruit and vegetable texture to provide quality control tools and ultimately, consumer satisfaction:

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For more information on how to measure texture, please visit the Texture Analysis Properties section on our website.

The TA.XTplus texture analyser is part of a family of texture analysis instruments and equipment from Stable Micro Systems. An extensive portfolio of specialist attachments is available to measure and analyse the textural properties of a huge range of food products. Our technical experts can also custom design instrument fixtures according to individual specifications.

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