Stickiness in Foods - Desirable or unwanted?

When a food is described as ‘sticky’, that may refer to one of several manifestations: adhesion to processing equipment, cohesion of powders, sticking to packaging, and sticking to fingers and parts of the mouth. This last characteristic affects the appreciation of food texture perceived during consumption.

In most food systems, the adhesion force in any one context is a combination of an adhesive force and a cohesive force. A food material is perceived as being sticky when the adhesive force is high and the cohesive force is low. Not all foods possess adhesiveness, and so it is not a property worth measuring in all cases. Adhesion can be either a benefit, drawback, or both depending on the circumstances.

It is generally a positive characteristic when appearing in products such as puddings, confectionery, sweet iced bakery or certain cheeses. However, in some cases stickiness presents a problem, as it can cause a disagreeable mouthfeel, cause problems by adhering to processing equipment (which can interrupt or slow production), or adhere to the materials in which the food is packaged. Sticking of food to packaging results in possible packaging material damage, product loss and disfigurement of the product surface. The extent to which this will generate an adverse consumer reaction will depend on the extent of the sticking and on the type and cost of the product. Both the dairy and baking industries have problems with materials that adhere to processing equipment. With regards to mouthfeel, stickiness is considered an unideal property when present in spaghetti or other pasta products, breadcrumbs and some meat products.

Occasionally, the decision to class stickiness as a good or bad property may depend on social or cultural factors, as in the case of rice, which is preferred sticky in the Eastern world and nonsticky in the Western world.

Stickiness/Work of Adhesion graph

There are two primary methods for measuring food stickiness: probe tests and peel tests. Probe tests are used more frequently. Many researchers measure adhesiveness in foods by performing probe tests and calculating the negative area of a force-time graph, either in Texture Profile Analysis tests with a double cycle, or single cycle tests. The following contains information on how to measure the stickiness of several commonly tested products using a TA.XTplus Texture Analyser.

The common link between all of these measurements (and adhesive tests in general) is that stickiness can be measured as the force upon pulling away from the adhesive substrate, and work of adhesion is measured as the area of this force profile. The graph demonstrates this property.

Spreadability Rig

For a semi-solid product with its stickiness under investigation, the Spreadability Rig provides a simple, repeatable measurement. It comprises of a male and female matched cone pair. The female cone is filled with the sample, which is totally displaced by the male cone during the test, the ease of which indicates the degree of spreadability. Withdrawal of the male cone provides information about the sample’s adhesive properties. This test is useful for any creamy product such as cheese spread, mayonnaise or butter. The graph above shows two cheese spreadability tests: one on a full fat sample and one on a low fat sample.

As previously mentioned, optimum rice stickiness depends on the context. The determination of stickiness in cooked rice is of great importance for the evaluation of quality of this product and for differentiation between varieties. This property can be simply measured by compressing a set number of kernels to a high strain under a cylindrical probe, and withdrawing at a moderately high speed. The use of strain as a target helps to take account of different sample sizes, which is particularly helpful for a natural, non-moulded material.

Cylinder Probe test
using Confectionery Holder

For a sample such as toffee, which relies on a certain amount of stickiness for customer satisfaction, a penetration test can differentiate between the success of alternative processing methods and ascertain the relative stickiness of toffee samples. As toffee samples are often small (but produce large adhesive forces), a thinner probe such as a 6mm cylinder is ideal. The force to withdraw the probe from the product is measured; the higher the force the greater is the measurement of stickiness. It is important to hold the sample down during testing to avoid lifting of the sample when the probe withdraws, ideally using the Confectionery Holder, as illustrated here in the image of the cylinder probe test.
 

Warburtons
Dough Stickiness System

Dough with a high adhesiveness can create serious problems in modern mechanised factories by causing production stoppages, product losses, or contamination. As stickiness is most commonly evident when dough is subjected to shear, tests should ideally be performed immediately after mixing or processing, e.g. after dividing or moulding.

The research, development and engineering team at Warburtons worked with Stable Micro Systems to develop a sample testing box into which dough samples can be placed quickly and with minimum exposure of the cut surface to the atmosphere. A retaining plate is placed on top of the dough, applying slight compression to the sample. A narrow blade is driven through a slot in the retaining plate, to a defined distance.

This action provides bakers with an accurate assessment of the compression peak and compression area of the dough, indicators of its consistency (firmness / softness). As the blade is withdrawn upwards, software calculates the adhesion peak and adhesion area. The higher the peak, and/or larger the area, the stickier the dough.

 

Pasta Firmness/Stickiness Rig

A factor much appreciated for the quality of pasta-type cooked products is a low adhesiveness value. In this case, the greatest difficulty for methods used to determine adhesiveness has lain in the presentation of the sample to the measuring instrument. A repeatable method developed at Stable Micro Systems, for evaluating surface stickiness of cooked pasta/noodles, uses an adhesive test with a Pasta Firmness/Stickiness Rig (left).

This consists of a rectangular aluminium probe attached to the load cell of the Texture Analyser. Samples are centrally aligned under the probe on a raised platform and are retained by a plate with a hole, on top of the strands. The probe then applies a suitable compression force to the sample to achieve a good probe/sample contact before withdrawing at maximum speed to measure the sample stickiness.

The measurement of adhesiveness of cheeses has received considerable attention. It is performed almost exclusively by means of Texture Profile Analysis (TPA) as it is simple to obtain samples with a regular size and shape. However, TPA is not the ideal method for stickiness measurement of cheese, as it is a pressure sensitive adhesive, and TPA does not load to a set force. It loads to a strain. This causes exaggerated stickiness values for harder samples. A standard compression test will provide more reliable stickiness results.

Whichever type of sample you are putting to the test, it is good practice to perform some research into the best adhesive measurement to suit the sample’s characteristics. This will help you to achieve good repeatability and perform a measurement imitative of a real life sticky situation. Stable Micro Systems have a large catalogue of adhesive tests that will help you find your perfect testing solution.

There is a Texture Analysis test for virtually any physical property. Contact Stable Micro Systems today to learn more about our full range of solutions.

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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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