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.

Viscoelasticity in the Materials Industry

Many industrial materials have viscoelastic properties depending on their constituents, processing conditions and the conditions during use.

They can be very obviously viscoelastic when held in the hands, or this behaviour may not be apparent until heated up to 1000°C and put under a large stress. The relaxation time also varies a large amount. Whether this viscoelasticity is beneficial or not depends on the material’s type and intended use.

Memory foam mattresses are becoming very popular due to their ability to mould to the shape of the sleeper’s body, relieving pressure on painful joints. They have a layer of temperature-dependent viscoelastic material - when a load is applied (a person), the material relaxes away to take on the load’s shape. They then show slow springback when the load is removed.

Viscoelasticity in the Pharmaceutical and Cosmetics Industries

Many pharmaceutical and cosmetic products have viscoelastic properties that can be measured using a Texture Analyser.

Pharmaceutical tablets show viscoelastic stress-strain behaviour when the powder compact is being loaded and unloaded, and this behaviour continues once the load has been fully removed. This can cause problems for tablet manufacturers such as capping, when either the upper or lower part of the tablet separates from the main body when ejected from the press or during the handling process. 

Viscoelastic properties are functions of the compression conditions as well as the formulation. It is important to investigate every instance of capping to ensure the problem is not repeated (which is very costly) and to measure the viscoelastic properties of new formulations before they reach the mass manufacturing stage. 

Viscoelasticity in the Food Industry

The vast majority of food materials show a combination of viscous and elastic behaviour although many show much more of one than the other.

There are some exceptions – hard crackers are generally completely elastic, whereas oil and runny honey usually show no elastic behaviour. Viscoelastic testing is best used as a comparative measure as many food products have an unusual geometry, so conventional viscoelastic equations cannot be used to find fundamental parameters.

If a cracker is not completely elastic when bitten, it may have become stale. The degree of viscous behaviour can be measured to study this effect. A sample so brittle is not easily clamped without fracturing, so tensile testing is not an option, but equally it is not a suitable shape for compression. 

Measuring Viscoelasticity using a Texture Analyser

What is viscoelasticity?
 
When a force is applied to a material, it causes the material to deform. This deformation can take one of two extremes – pure elastic deformation (e.g. a spring) or pure viscous flow (e.g. oil). 

The force causes the elastic material to instantly deform by a set amount, and the viscous material to flow for the whole time the force is applied. When the force is removed, the elastic material instantly returns to its original state, whereas the viscous material does not recover at all.

Elastic behaviour is usually caused by the bonds stretching between atoms (which is instant). Viscosity is caused by atoms or molecules moving past each other, which takes time but is also not easily reversed, unlike elasticity.

Measure Bending Force

BENDING FORCE: the physical nature of some products make them more suitable for testing via a bending test. 

Generally the higher the force to bend (usually up to the point of break) the firmer the sample. 

The force and distance references at the point of break provide a measure of BREAK STRENGTH by this action.

STIFFNESS is associated with a material that cannot easily be bent and therefore has a high degree of hardness and usually lacks flexibility.

FLEXIBILITY is the opposite of this and is a characteristic of a product that can be bent, flexed or deformed to a much higher degree of deformation before failing. 

Measure Compressibility

The COMPACTABILITY/COMPRESSIBILITY of a product is its ability to be compacted or compressed, usually to a large degree of deformation.  

This can be a required property of, for example, the formation of cosmetic powder tablets or formation of tablets from granules. 

However it may also be undesirable, as would be true of a large amount of packaging material which is usually chosen for its ability to protect a product and therefore resist compaction to a reasonable extent.

Measure Abrasion Force

ABRASION is the erosion of material from a solid surface by the action of another solid. 

When material is removed by contact with hard particles, abrasive wear occurs. The particles either may be present at the surface of a second material or may exist as loose particles between two surfaces.

Abrasive materials are widely used across a variety of industries for polishing, grinding, honing and other similar processes. The efficacy of these materials relies on their frictional properties, which can be calculated as the force required to draw the material across a given surface. Assessment of these properties will allow manufacturers to evaluate the suitability of their products for a specific use. The test results will also highlight differences between grades or brands of abrasive materials.

Measure Consistency

CONSISTENCY is commonly the textural property possessed by dairy products, sauces, syrups and pharmaceuticals lotions (i.e. fluid products). 

It most often makes use of the back extrusion rig for its measurement. Consistency relates to the 'firmness', 'thickness' or 'viscosity' of a liquid or fluid semi-solid. 

Stirring a fluid or semifluid food with a spoon or a finger is frequently used by consumers to give an indication of viscosity or consistency.  

Measure Stringiness

STRINGINESS/SHORTNESS is the degree (distance) to which a product hangs on the probe when the probe is withdrawn. 

It is a major characteristic of mozzarella cheese or adhesives but is also commonly the textural property possessed by sauces (e.g. ketchup, dessert sauces), confectionery glazes, baked product toppings and undoubtedly, syrups. 

In the adhesive industry this characteristic is sometimes referred to as 'legging' or 'tailing', as stringiness may result in the formation of filaments or threads when the adhesive surfaces are separated. 

Measure Friction Force

FRICTION is a phenomenon that is part of our daily lives - in one sense we would find most difficult to survive without it yet in another it can be most expensive to minimise. 

It can be simply identified as the resisting force that arises when one surface slides, or tries to slide, over another. It is commonly the textural property measured from packaging materials, shaving gels, cosmetic sponges, etc.

Friction can be a limitation to us; for example, the friction of packaging can be a major limiting factor in the speed of packing machines. Additives are often used to improve the lubricity of surfaces, this is known as 'slip'. Polythene is added to packaging to improve slip characteristics.

Measure Swelling

SWELLING is associated with an increase in size or volume of a product.

Swelling applications include tablets, gels, films, pellets, mucosal tissues, super-absorbent polymers, sponges, aquaculture feed, chemical and yeast leavened products, etc. 

Depending on application, swelling can be quantified by measuring changes in force or distance by either (i) compressing a product a slight distance and then measuring how force increases as the product swells, or (ii) applying and maintaining a small force onto the product and measuring the distance change as swelling occurs. 

Measure Cutting Force

BITING/CUTTING FORCE gives an indication of the firmness/hardness of a product.  

Bearing in mind that if a row of top front teeth were pulled from a curve into a straight line they would represent a 'knife edge', using a knife blade gives a close representation of the biting or cutting action.

TOUGHNESS – whilst Hardness/Firmness is usually the textural property possessed by most products, the word 'toughness' or ‘chewiness’ may be substituted for a textural property more associated with the product.  

Measure Tensile Strength

TENSILE STRENGTH is the measure of the force or stress required (resistance to lengthwise stress) to pull a sample to the point where it breaks or before permanent deformation results.  

Usually it is the maximum amount of tensile stress that it can be subjected to before failure, although the definition of failure can vary according to material type and design methodology.

In the fields of material science, mechanical engineering and structural engineering there are three typical definitions of tensile strength:

• Yield strength: the stress at which material strain changes from elastic deformation to plastic deformation, causing it to deform permanently.
• Ultimate strength: the maximum stress a material can withstand.
• Breaking strength: the stress coordinate on the stress-strain curve at the point of rupture.

Measure Springiness

SPRINGINESS is the rate at which a deformed material goes back to its undeformed condition after deforming force is removed. 

It is a measurement of elastic recovery – the textural property possessed not only by baked goods such as cakes or breads but also by novel confectionery products and pharmaceutical materials (e.g. cosmetic sponges).

Measure Rupture Force

RUPTURE FORCE – the force that is required to produce a major break/rupture in a sample – can be related to the firmness and brittleness of a product. It is related to burst force – the force causing a sample to break suddenly and violently into pieces. Rupture force measurement can be crucial for such products as medical implants, which may be required to resemble soft tissue but must remain intact for the duration of their intended residence.

Several other products, such as gel capsules containing vitamins, possess bursting as a desirable property, but must withstand transport and handling before ultimately bursting in the mouth.

Measure Seal Strength

SEAL STRENGTH is the ability to resist forces that can separate materials that have been sealed together. 

It may be a flexible surface from a rigid surface or another flexible surface.

Seal strength tests are useful for a variety of products and can be performed on containers such as trays and packaging pouches as well as assessment of adhesive, cosmetic and medical products such as surgical pouches and sterile packaging. 

Measure Syringeability

SYRINGEABILITY can be described as the ability of a product to be successfully administered by a syringe and appropriate needle.

Syringes are used in hospitals, surgeries and homes around the world on a daily basis, and for a multitude of purposes. Successful administration, as well as comfortable receipt of the material under the skin, rely on the effective tracking of the syringe piston within its outer tube. 

Measure Firmness

FIRMNESS/HARDNESS/SOFTNESS are textural properties that are on the same general property spectrum. 

A soft product is one that displays a slight resistance to deformation, a firm product is moderately resistant to deformation, while hardness describes a product which shows substantial resistance to deformation. 

However, it is also likely that, depending upon the particular industry, one of these words may be more favourable or pertinent to a specific product.

Measure Gel Strength

GEL STRENGTH is a measure of the ability of a colloidal dispersion to develop and retain a gel form.

In the gelatine world, gel strength is traditionally referred to as Bloom. It is the force, expressed in grams, necessary to depress by 4 mm the surface of a gelatine gel with a standard 0.5" diameter cylinder probe.

Whilst gels are commonly accepted in the food industry the measurement of gel strength is also of widespread interest in the manufacture of pharmaceutical, medical and cosmetic products. 

Gel properties such as elasticity and rupture force of, for example, pectin, gelatine, agar etc. are important in the development of such products as coronary stents where hydrogel polymers are selected due to their soft, rubbery nature which gives them a strong, superficial resemblance to living, soft tissue.