Go back in time ten years or so and you would find the food industry anticipating a revolution in food science and technology. At the centre of the excitement was the new discipline of nanotechnology, which promised to provide the answer to all manner of food processing problems and spawn a new generation of innovative and highly profitable products. With hindsight, that view now looks impossibly optimistic. The pace of progress towards practical commercial applications of nanotechnology in the European food industry could be described as glacial, amid fears of consumer rejection, restrictive legislation and economic downturn. Only in the food packaging sector does nanotechnology still seem to have a bright future. Even here there are concerns, but the range of potentially useful applications in the pipeline may be persuasive enough to overcome them.

Nanotechnology has been enthusiastically embraced by many industries, including medicinal products, electronics, aerospace, chemical engineering, construction, textile manufacture, cosmetics and even sports equipment. Several hundred consumer products incorporating some aspect of nanotechnology are reported to be on the market around the world, but very few are foods. Most of the commercial interest in so-called nano-foods comes from Japan and the USA, rather than Europe, where the food industry remains extremely wary. Although there is no shortage of ideas – products containing nanoparticles of functional ingredients and nanocapsules for delivering flavours, biofilm-resistant nanocoatings for food processing equipment and even futuristic notions of using nanotechnology to create foods at a molecular level – manufacturers remain reluctant to risk major investment, or even discuss the technology openly. The exception is food packaging, where commercial products exist and are being used, and where research and development continues to come up with promising new ideas. To understand why this should be, it is necessary to look at the factors holding back nanotechnology in foods and why packaging is different.

Fear of the unknown

What exactly is nanotechnology? The US National Nanotechnology Initiative defines it as "the understanding and control of matter at dimensions of roughly 1 to 100 nanometres, where unique phenomena enable novel applications." It is that last part of the definition that is really important. Materials do not necessarily behave in the same way at the nano-scale as they do at a macro-scale. Take titanium dioxide for example. At the macro-scale it is a brilliant white pigment used in a wide range of applications, but nanoparticles of titanium dioxide are transparent, though still resistant to ultraviolet radiation and thus suitable for a whole range of new applications. Many materials have been found to have entirely different physical, chemical and biological properties at the nano-scale. This is why nanotechnology has the potential to be such a powerful agent for change in many manufacturing industries, but also explains why caution has so far prevailed in the food industry.

Food is unique among consumer products in that it is inextricably linked to general health and wellbeing. This is why food production is so closely regulated. The prospect of new nano-foods entering the market has inevitably attracted attention from the regulatory authorities. EFSA published a draft scientific opinion on the topic back in 2008, which concluded that engineered nano-materials (ENMs) in food should be evaluated by established risk assessment methods on a case by case basis. The opinion went on the conclude that, "in practice, current data limitations and a lack of validated test methodologies could make risk assessment of specific nano products very difficult and subject to a high degree of uncertainty." It seems clear that the regulatory hurdle is likely to remain a substantial one for any business wanting to bring a new nano-food to market.

Consumers too have repeatedly expressed concerns over the safety of nanotechnology in foods. For example, as recently as April 2011 the UK Food Standards Agency (FSA) published a report, FSA Citizens Forums: Nanotechnology and food, compiled from surveys carried out by market research organisation TNS-BMRB. This report found that the public questioned the need for introducing nanotechnology into foods and whether the promised benefits were worth the possibility of long-term effects on public health and the environment.

The report concluded that the views expressed were often founded on cynicism about food technology and a belief that, "..technological advances in food are developed in the interests of business rather than consumers, and that consumers ultimately bear the costs, either through increased food prices, lower quality produce, or reduced health."

In such a generally negative environment it is hardly surprising that not even the biggest food manufacturers are willing to take the risk of substantial investment in nano-foods. But packaging is different. For one thing, the regulatory position for nano-packaging in the EU is clearer than for nano-foods. This is mainly a consequence of Regulation (EC) 1935/2004, which includes special requirements for active and intelligent packaging. The main limitation on active and intelligent packaging over traditional materials is that it should not cause changes, or give information, that might mislead consumers as to the freshness and condition of the food. This has allowed a more flexible approach to new packaging technologies, although there may still be issues over whether nanoparticles in packaging materials behave in the same way as larger particles in terms of migration into food. Secondly, consumers seem to be much more relaxed about nanotechnology in food packaging than in foods. The FSA Citizen’s Forum report found that, "Participants were relatively more open to the idea of applying nanotechnology to food packaging." Nevertheless, those surveyed wanted assurances that nanomaterials would not migrate into foods and that the resulting packaging would not cause environmental problems. The report also found that, "Specific nanopackaging applications were generally welcomed", provided that they carried benefits for consumers as well as for industry.

Packaging materials containing nano-materials have been commercially available for some time and there are some interesting ideas now being developed for the packaging industry. The market for nanotechnology-based food and beverage packaging is predicted to grow by at least 12% each year until 2014, by which time it is expected to be worth US$7.3 billion. Interestingly, active packaging applications are predicted to make up the biggest segment of that market.

Barrier materials

The application for nanotechnology in packaging that has seen most commercial development to date is the manufacture of polymers with improved barrier properties. Incorporating nanomaterial fillers into polymer matrices to produce ‘nanocomposites’ can alter the physical characteristics of the polymer, making it stronger, or less permeable to gases. The weakness of many polymers used in food and beverage packaging is their comparatively poor barrier properties, but nanocomposites can overcome this problem at a comparatively low cost. Nanocomposites are made by embedding the filler into the polymer matrix. Almost any kind of polymer can be used, but polyamides form the basis of many commercially available products. The filler could be nanoscale particles of a metal or oxide, nanotubes or fibres, but the most commonly used fillers are nanoclays.

Nanoclays are usually produced from naturally occurring clays, such as montmorillonite (also sometimes known as bentonite). The clay has to be purified and then chemically treated to ensure that the normally hydrophilic clay particles will disperse properly in the resin matrix. The clay filler is then mixed with the resin, either during polymerisation or by a ‘melt compounding’ process. This is the most difficult step in clay-based nanocomposite production, since the particles need to be evenly dispersed at the correct density. The process must also cause the clay filler to ‘exfoliate’ – separate into single plate-shaped particles about 1 nanometre thick and 100 or more nanometres in diameter – and the plates must disperse so that they sit parallel to the surface of the resulting nanocomposite. This is the secret of their functionality. The effect of evenly dispersed nanoclay particles in a plastic is to greatly increase its barrier properties, especially for gases. This is because the layers of clay platelets lying parallel to the surface greatly increase the distance that the gas has to travel before it can penetrate the film, producing a so-called ‘tortuous path’, thus slowing gas transmission. In effect, the clay filler does the same job as a much thicker resin layer. These clay-based nanocomposites also have UV-light barrier properties, yet remain transparent, and show greater strength than non-composite resins.

Several companies currently offer commercial products to packaging developers. Honeywell has developed three products for different applications making up the Aegis® range of nanoclay-based barrier nylon resins. Aegis OXCE is an oxygen-scavenging nylon resin designed for use as a barrier layer in PET containers where high oxygen barrier properties are needed, such as beer bottles. It is also resistant to carbon dioxide transmission and delamination. PET bottles incorporating Aegis OXCE are claimed to compare favourably with glass bottles in terms of performance and cost. Aegis HFX is also an oxygen-scavenging nylon film with high barrier properties, but is used in bottles for juices, teas and condiments. It is also claimed to stand up well to hot filling processes without delaminating. Finally, Aegis CSDE is a non-scavenging resin with high carbon dioxide retention properties, designed for carbonated soft drinks and water.

US-based nanoclay producer Nanocor has developed its own range of high-barrier nanocomposite nylon resins, called Imperm®, working in an alliance with the Mitsubishi Gas Chemical Company. Imperm is an ultra high barrier nylon, providing protection for oxygen-sensitive products and exceptional CO₂ retention for carbonated soft drinks, waters, beers and flavoured alcoholic beverages. Imperm is said to be most useful in multilayer bottles, films, and thermoformed containers, but it can also be used for coated paper cartons. Other commercially available nanoclay-based products include Durethan® from Lanxess AG and NanoPack Inc.’s NanoSeal™ coating made from vermiculite platelets in a polyvinyl alcohol (PVOH) matrix.

Nanoclays are not the only nano-fillers to have been used successfully to produce nanocomposites, although they are by far the most common in commercial products. Others include silica nanoparticles, starch crystals, cellulose nanofibres, chitosan nanoparticles and carbon nanotubes. All of these can be used to modify the characteristics of the polymer and applications for food packaging are being investigated.

Nanocomposite technology may also have a key future role in improving the performance of bioplastics, such as polylactide (PLA). The use of PLA and other bioplastics for food packaging has been restricted to some extent by their relatively poor mechanical strength and by their permeability. Adding nanofillers to biopolymers can help overcome these drawbacks. For instance, Rohm and Haas supplies Paraloid™ BPM-500, an acrylic nanoparticle product designed to be blended with PLA to improve its impact resistance. Additional benefits could be gained by using biobased nanomaterials, such as cellulose nanofibres or starch nanocrystals, as fillers. Research studies indicate that such materials actually have a faster biodegradation rate than PLA alone, so that biobased nanocomposites could provide environmental as well as performance advantages.

Antimicrobial packaging

Active packaging applications are predicted to be one of the biggest growth areas for nanotechnology in packaging materials. Oxygen scavenging nanocomposites already exist and a number of other ideas have been actively pursued. But antimicrobial packaging is probably the application that has generated most interest so far.

Most of the attention has been focused on materials coated with, or containing nano-silver particles. Silver is one of the oldest known antimicrobials and has been used in the treatment of wounds for centuries. Like other materials, silver nanoparticles behave differently from larger particles. Possibly because silver in this form has a much larger surface area and releases silver ions more efficiently, silver nanoparticles are far more effective at destroying bacteria and fungi. The technology has been used effectively in the medical field for some time to protect medical devices from biofilm formation and to make antimicrobial dressings. Antibacterial fabrics have also been developed using nano-silver. By coating materials with nanoscale silver particles, or bonding silver cations into nanocomposites, it should be possible to create effective and safe antimicrobial packaging. In fact, the technology has already been used in some parts of the world, such as South Korea, China and the USA, to produce food storage containers, though not food packaging as yet.

Although silver nanoparticles appear to be a very promising material for antimicrobial packaging, doubts have been expressed over their safety and the possible effects of silver ions accumulating in the aquatic environment. For instance, at the end of 2009 the German Federal Institute for Risk Assessment (BfR) published a recommendation that nano-silver should not be used in "foods and everyday products" until enough data is available to allow a conclusive risk assessment demonstrating that such products are safe. More recently, a Chinese research study produced evidence that silver nanoparticles migrated from commercially available food storage containers into "food-simulating solutions" (2). Migration was found to increase with storage time and at higher temperatures. Such concerns may turn out to be unfounded, but widespread use of nano-silver particles in food packaging seems unlikely while they remain.

Other nanomaterials have also been shown to have promise as antimicrobial additives in packaging. Examples include titanium dioxide particles, which are photocatalysed by UV light and thus only active when illuminated, magnesium oxide, copper and its oxides, zinc oxide, chitosan and carbon nanotubes. There has also been interest in coating nanoparticles with natural antimicrobial compounds, such as lysozyme and nisin. In this form the antimicrobial activity of such compounds has been found to persist for much longer than when they are present as free molecules.

Smart packaging and future developments

An area of special interest for developers of intelligent packaging applications, or so-called smart packaging, is the possibility of using nanosensors to detect gases and indicator molecules within food packaging. Sensitive and rapid oxygen nanosensors have already been investigated for use in modified atmosphere packaging (MAP) packs where the presence of oxygen could indicate a leak or faulty seal. One of these, developed at the University of Strathclyde in Scotland, is based on a cellulose polymer containing titanium dioxide nanoparticles and a blue indicator dye. The dye is bleached by exposure to UV light after the pack is sealed, but regains its deep blue colour within a few minutes on exposure to oxygen.

Other nanosensors have been developed that change colour when exposed to mechanical stress or changes in temperature, indicating that the pack and its contents could be damaged. Time temperature indicators (TTIs) are also in development. An existing example is the iStrip® device developed by Timestrip® plc, which uses a system based on colloidal gold nanoparticles to indicate when a product has been subjected to a freezing event in the cold chain. Above 0^oC the indicator is red, but on freezing the nanoparticles agglomerate irreversibly and the colour disappears. Nanosensors may also be able to detect small concentrations of the amines produced during the spoilage of meat and poultry, giving early warning of end of shelf life. Detection of chemical contaminants using nanosensors is a further possibility. It may even be possible to detect microbial pathogens like Listeria monocytogenes using fluorescent nanoparticles with attached antibodies.

Finally, nanotechnology may have a part to play in product security and traceability. Radio Frequency Identification (RFID) tags are already in widespread use for tracking products through the supply chain. But they are currently based on semiconductor technology and are relatively expensive, and so are used mainly for high value products or to track large containers rather than individual packs. But researchers are looking at developing printable electronics based on conducting polymers like pentacene and using metallic inks containing nanoparticles of gold and other metals. By combining this technology with carbon nanotubes acting as antennae, it may eventually be possible to replace the conventional RFID tag with a nanotechnology-based alternative that is cheap and simple enough to print directly onto the packaging of a single food item, allowing complete traceability for each unit. In the more distant future, printable electronics could even be combined with nanosensors, so that each pack has the capability to transmit information about the condition of its contents to a reader and then to a central control point. This sounds like the stuff of science fiction, but it may not be so far away.

References

1. Duncan TV. Applications of nanotechnology in food packaging and food safety: Barrier materials, antimicrobials and sensors. Journal of Colloid and Interface Science 2011; 363(1): 1-24.

2. Huang Y et al. Nanosilver migrated into food-simulating solutions from commercially available food fresh containers. Packaging Technology and Science 2011; 24(5): 291-7.