Texture Analysis Professionals Blog

How to measure and analyse the texture of food, cosmetics, pharmaceuticals and adhesives.

Tuesday, 22 January 2019

Fruit Texture: assessing 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.

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:

View fruit and vegetable video

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

TA.XTplus texture analyser with bloom jar 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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 Fruit and Vegetable testing videoDownload a published article covering methods for the testing of fruit and vegetables

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Tuesday, 15 January 2019

Fruit Texture: measuring firm pastes and dried fruit flakes

Firm Pastes
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.

Tuesday, 8 January 2019

Fruit Texture: measuring 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.

Tuesday, 18 December 2018

Fruit Texture: measuring detachment and deseeding force

Farmer picking grapes
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.

Tuesday, 11 December 2018

Fruit Texture: measurement of flesh firmness of non-homogeneous fruits

Multiple puncture test on melon

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.

Tuesday, 4 December 2018

Fruit Texture: measurement of flesh firmness of prepared homogeneous fruit samples

Mango cube
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.

Tuesday, 27 November 2018

Fruit Texture: measurement of the overall firmness of a weight or quantity of fruits

This type of test commonly applies to:
Kramer Shear Cell
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.

Tuesday, 20 November 2018

Fruit Texture: measurement of firmness of whole fruit by compression

Lady eating appleFruit 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.