food Marketing & Technology · June 201142

Packaging

Edible films – are they just an alternative to plastic wrap, a pretty coating for almonds or do they offer far wider scope for food manufacturers to offer exciting new products that stand up to the rigours of global transportation and deliver an acceptable shelf life? Edible films are not a new phenomenon, many of us are familiar with leaf gelatine and rice paper, but perhaps less well appreciated are the edible films that can be used as surface coatings to improve durability, enhance appearance, prevent moisture transfer or control the diffusion of preservatives from the exterior into the foodstuff. They can also be used to carry active food grade additives or functional ingredients to contribute towards the stability and nutritional value of a food.

What is the potential of edible films?

Edible films offer the promise of microbial stability, convenience, exciting new product developments and even environmental benefits. With the versatility and established safety of many gum polymers, novel applications for such films seem almost limitless. The US Department of Agriculture says the market for edible films has already experienced substantial growth, from less than $1 million in 1999 to more than $100 million last year. Following the launch of a breath freshener film in 2001, the North American market for edible films is becoming well established, but the European market is still relatively small. However, it is an exciting area with potential for strong growth in Europe, forecast to reach $2 bn (€1.36 bn) by 2012 . Clearly edible and biodegradable films and coatings are an important area of food research with enormous commercial potential and great significance to the environment.

global food regulations. As a packaging material it needs to be robust, meeting with predefined mechanical properties to protect food quality and maintain its integrity during handling and transportation.

A film fit for purpose?

Taking into account the desired function, method of processing, transport and storage of the finished food product, the film will be required to perform certain functions under a particular range of conditions. Once the critical parameters are identified, the mechanical properties and suitability of a film for a particular application can be determined very accurately.

Using a texture analyser, a range of measurements including tensile, shear and compression figures can be recorded. In addition to measuring stress and strain in a material, coating characteristics such as adhesiveness may be critical. The product may have to stick to a food, fix a coating or give a glossy finish, or indeed the function of the film may be to reduce adhesion.

Challenges in testing

As is the trend across much of industry, testing is increasingly controlled under ASTM or ISO methodology. However, many procedures are based on non-edible plastic packaging materials which frequently results in limitations when trying to comply with the defined sample preparation methods for testing edible films. Instruments and test procedures often require a large piece of film, and for inflexible materials this can be difficult to produce. There is also the challenge

The increasingly adventurous use of edible films can greatly enhance the organoleptic properties of many foods by incorporating flavorings, colorings and sweeteners into the edible packaging and may offer novel ways to pack individual portions of food. This opens up whole new areas of packaging for products such as soft fruits that for practical reasons are not currently individually wrapped.

Environmentally friendly

With so much focus on reducing packaging in the current climate, it may seem out of step to be highlighting opportunities for introducing packaging of individual portions. However, edible films and coatings have received considerable attention in recent years because of their great potential to reduce and simplify the packaging materials required for the protection of food products. They afford numerous advantages over conventional, non-edible synthetic polymeric packaging due to their biodegradability, inherent ability to be consumed as part of the goods and derivation from natural sources. They can be used as wraps or formed into pouches, bags, casings or sachets. But even if the films are not consumed, they can still contribute to the reduction of environmental pollution as a result of enhanced biodegradability. The films are produced exclusively from renewable, edible ingredients and are therefore anticipated to degrade more readily than their synthetic polymeric counterparts.

Stringent requirements for films

An edible film has a lot to contend with – as a food component it should be tasteless, non toxic and compliant with

Edible Films as Scrumptious Packaging?Jo Smewing, texture expert, explores the hidden applications of edible films in our foods, uncovering the array of fascinating functions they perform.

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food Marketing & Technology

of casting a film with well defined dimensions, particularly where thickness is critical. Depending on the composition of the film, it is not always practical to prepare samples in accordance with ASTM procedures and it may indeed be preferable to mimic the application conditions under which the film is likely to be used. The tensile strength results are still viable providing that the detailed test parameters are reported. An investigative approach to tensile testing can indicate where adaptations to formulations can deliver verifiable results for materials of varying type, dimension and response to environmental factors such as temperature and humidity.

The established definition of an edible film is “a thin layer of material that can be consumed”. Whilst there is no specific definition of “thin”, what is invariably more important is that the layer is consistent. It is this consistency that will ensure that the product will always be delivered in line with the customer’s expectation. As a result, it is important that any test procedures are reproducible, and to ensure this, a wide variety of accessories guarantee that the measurements are representative of the sample and not a function of the test procedure.

Tensile strength testing

Tensile strength is the most common measure of the strength and ductility of a film and typically uses standard method ASTM D882. Using this methodology, a rectangular strip of pre-cast film of less than 1.0 mm thickness is pulled apart at a carefully defined speed while resistance is recorded over time. The TA.XTplus texture analyser can capture force, distance and time data at a rate of up to 500 points per second and has a wide range of accessories to allow practical inspection of edible films according to their intended end use.

Tensile grips such as those shown in Figure 1 are attached to the base and cross head of the texture analyser. Strips of film are cut and tightened between the tensile grips which are set at a distance of 50  mm apart, in accordance with ASTM D882. The tensile test is then performed at a speed of 0.5 mm/s to failure. The resulting load extension curve is automatically plotted from

Pneumatic grips

When measuring tensile strength of edible films, rubber coated, pneumatic grips may be a preferred means of securing the sample as the gripping pressure can be precisely controlled so that specimen deformation does not produce any change in the gripping pressure. This grip clamps the specimen by lever arms actuated by compressed air cylinders built into the grip bodies. A constant force can be maintained on the specimen, compensating for any decrease in force that results from creep of the specimen in the grip. By adjusting the air pressure, the sample gripping

which a number of useful parameters can be calculated including: the peak force needed to break the film – the higher the force the stronger the film; tensile strength; elongation or extensibility which gives a measure of the flexibility of the film.

Whilst ASTM test methods recommend that the length of specimens between the grips should be at least 50 mm, this is not always possible and sample preparation methods and testing parameters may need to be adapted. A range of tensile grips is available to suit different sample sizes, material strengths and grip face requirements.

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food Marketing & Technology · June 201144

Packaging

force can be optimized which makes it possible to minimise specimen breaks at the grip faces.

Film strength and resilience

Where there is limited sample availability, a film support rig can be employed, see figure 2. This enables the film to be secured to measure the bi-extensional properties of the film via a puncture test. Typically a 5 mm diameter stainless steel ball probe is driven down onto the sample film. The force required to burst the film is recorded and is referred to as the burst strength or break point of the film. The rig can also be used to measure resilience and relaxation. By comparing the work of compression and withdrawal when depressing the film surface a certain distance a measure of resilience can be calculated. Similarly the coefficient of relaxation can be analysed by holding the probe on the surface for a predefined length of time. The coefficient of relaxation can then be calculated from a force time curve. The ability to calculate the relaxation and resilience of films can provide useful data for manufacturers to assess how well a film may withstand being handled in practical situations.

bound too tightly to the other material that they are designed to peel away from, that they possess sufficient adhesion not to secure whatever they are intended to hold in place, whether that be tiny seasoning particles or poppy seeds, and that the resulting surface is not too sticky to handle. To respond to the ever increasing needs of the food industry there is now a vast range of accessories that can be incorporated with a texture analyser including rigs to measure the peel strength of films, flexible clamps to measure the adhesiveness of films against various substrates and extrusion rigs to calculate the consistency or

“stringiness” of coatings.

The future of edible films

The concept of drug, vitamin and mineral delivery films is already accepted and likely to expand, but there is still great potential for food scientists and formulators to develop novelty products using films. It is predicted that a whole range of new applications for edible films will become apparent in the coming months and years as a result of their versatility and potential to offer technical solutions to difficult processing and packaging food challenges. Our company works closely with the food industry to develop a wide range of innovative equipment to test every physical aspect of these edible films, opening up new and exciting applications in an industry where consumers have such high expectations of consistent taste, texture and appearance. As the convenience, environmental appeal and microbial stability of food films is one of the biggest selling points for developing new products, it is likely that the future for edible films is limited only by the imagination.

Key No. 76042

The AuthorJo Smewing is Applications Manager, Stable Micro Systems, Godalming, UK

Adhesion

Some films are developed to deliver adhesive properties so the strength of this adhesion is a key measurement. An oil spray may be applied to a snack food followed by a coating with a powdered seasoning, for example. The tacky oil film coating needs to be sufficiently sticky to keep the seasoning on the food rather than staining the customer’s fingers or falling off during distribution.

During a simple adhesive test a probe, typically a 1” (2.5 cm) diameter spherical probe is lowered onto the film beginning the bonding process. This force is held for a predetermined time allowing the bond to form. The probe is then withdrawn, stressing these newly created bonds as the adhesive is stretched. The maximum force needed to withdraw the probe from the sample is a measure of the strength of the bond interpreted as the adhesiveness of the material and is highly dependent on the chemistry of the materials, forces applied and ambient temperature.

Measuring adhesion brings a wealth of challenges: ensuring that films do not stick to each other, that they are not

Figure 1: Tensile grips Figure 2: Film support rig

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