September 2008 • Volume 33 • ISSue 36Food EnginEEring & ingrEdiEnts

The US National Nanotechnology Initiative defines nanotechnology as “the understand-ing 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 diox-ide 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.

The food industry is a latecomer to the nanotechnology field, and hasn’t yet been able to exploit its potential. There have been plenty of ideas – from products containing nanoparticles of functional ingredients and nanocapsules for delivering flavours, to biofilm-resistant nanocoatings for food processing equipment and even futuristic notions of using nanotechnology to create foods at a molecular level – but very few marketable products. Today, nan-otechnology is barely mentioned by most big food manufacturers. There are several

Back in 2000 the food industry was gripped by excitement about nanotechnology and what it might mean for future development and growth. Many of the larger food manufacturers enthusiastically embraced the new field and researchers in the academic sector took advantage of generous funding opportunities and collaborations to begin ambitious new programs that would herald a much-hyped “revolution” in food technology. Eight years later, much of the steam has gone out of nanotechnology in the food sector, and most proposed ‘nanofoods’ are still a long way from the market. The one sector that has not developed cold feet to quite the same extent is packaging. Despite lingering concerns over safety, consumer resistance, costs and regulation, the potential of the technology may be about to be realised.

Nanotech packaging

Nanotechnology in packaging:a revolution in waiting

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possible reasons for this. One is undoubt-edly cost – many applications are signifi-cantly more expensive to make than existing products. Another is uncertainty about regulation of nanofoods. For example, the European Food Safety Authority (EFSA) Scientific Committee has been conducting a risk assessment of the technology for the European Commission and was due to pub-lish an opinion on its deliberations in July, although at the time of writing, this had still not appeared. But probably the biggest reason is the fear of a consumer backlash like the one that greeted the introduction of GM products.

Packaging sector leads the way with nanocompositesFood packaging is the one sector of the industry where nanotechnology applications are beginning to live up to their promise. This is partly because packaging developers have been able to come up with some cost-effective nanotechnology applications that have real benefits for food manufacturers, consumers and the environment. Further-more, recent consumer surveys have tended to show much less concern over the use of nanotechnology in packaging than its direct application in foods. Crucially also, the regulatory position for nanopackaging in the EU is clearer than for nanofoods. This is mainly a consequence of Regulation (EC) 1935/2004, which includes special require-ments for active and intelligent packaging. The main limitation on active and intel-ligent packaging over traditional materials is that it should not cause changes, or give information, that might mislead consum-ers as to the freshness and condition of the food. This has allowed a more flexible approach to new packaging technologies than was previously the case. However, there may still be issues over whether nano-particles in packaging materials behave in the same way as larger particles in terms of migration into food, and testing to deter-mine this may be crucial to the success of some applications.

At the forefront of nanotechnology in food packaging is the rapidly growing field of nanocomposites. These materials consist of a resin matrix, such as nylon, in which

is embedded a ‘filler’ material made up of nanoparticles. These fillers may be nanoscale particles of a metal or oxide, nanotubes or fibres, or nanoclays, and their function is to modify the physical properties of the resin matrix. Most food packaging applications developed to date have incorporated metal or oxide parti-cles, or more commonly nanoclays. It is these nanoclay-based composites that have found commercial applications and have come on to the market.

Nanoclays are usually produced from naturally occurring clays, such as montmo-rillonite (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 den-sity. 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, 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 have already mastered the art of nanocomposite production and currently offer products to packaging developers. Honeywell has developed three products for different applications making up the Aegis range of nanoclay-based bar-rier nylon resins. Aegis OX 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 delami-nation. PET bottles incorporating Aegis OX 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 with-out delaminating. Finally, Aegis CSDE is a non-scavenging resin with high carbon dioxide retention properties, designed for carbonated soft drinks and water.

Food packaging is the one sector of the industry where nanotechnology applications are beginning to live up to their promise.

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US-based nanoclay producer Nanocor has also developed its own range of high-barrier nanocomposite nylon resins, called Imperm, working in an alliance with the Mitsubishi Gas Chemical Company. Imperm grade 103 is designed for use in multiplayer PET bot-tles for the beverage industry, whereas grade 105 is designed as a non-contact barrier layer for multilayer films and sheets in combina-tion with polyethylene, polypropylene and other resins. Imperm 105 has good gas bar-rier properties and is said to help extend shelf life in oxygen-sensitive food products, such as cheese, snack products and cooked meats.

Researchers at Bayer Polymers also devel-oped a nanoclay-based nanocomposite called Durethan KU 2-2601, which is now supplied by Lanxess AG. This material uses polyamide resin and nanoclay filler from Nanocor to produce a nanocomposite film with much improved gas barrier properties over plain polyamide. The film is also said to have an enhanced gloss – a result of the clay particles’ beneficial influence on the crystallisation of the plastic. Dr Ralph Ulrich of Bayer Polymers research department saw Durethan film as “suitable for applications in which conventional polyamides are too permeable and EVOH is too expensive.” A particularly promising application is said to be as a low-cost coating film for paperboard cartons containing oxygen-sensitive products like fruit juices.

Australian bioplastics producer Plantic Tech-nologies has also been looking at nanocom-posites in collaboration with the Coopera-tive Research Centre for Polymers and has successfully tested the incorporation of “substantially exfoliated hydrophobic clays” in their Plantic R1 sheet. Plantic is a biodegrad-able plastic made from corn starch and the addition of nanoclays improves its gas barrier performance, clarity and strength. The result is a material that can be used for rigid and flexible food packaging and also has strong green credentials. Nanocomposite technology may have a key future role in improving the performance of bioplastics, such as PLA.

Nanocomposites are the nanotechnology products that have made by far the most progress towards commercialisation. A 2004

report predicted that consump-tion of these materials in the food and drink industry could reach around 50,000 tonnes by 2011, with carbonated soft drinks and beer producers being the biggest customers. But there is no short-age of other ideas and potential applications for nanotechnol-ogy in food packaging. Many of these can be classified as active or intelligent packaging applica-tions. Proposals already being investigated include nanosen-sors for temperature, time and moisture monitoring, fluorescent nanoparticles with attached antibodies that can detect the presence of food poisoning bacteria, nanosensors that can detect chemicals associated with spoilage, nano-scale systems that release preservatives into the food in response to spoilage, and even self repairing plastic films. For example, researchers at the University of Pisa in Italy have developed a biodegradable plastic film that can indicate when it has been exposed to stretching and raised temperatures. The polyester film contains nanoparticles of a stilbene dye that normally forms aggregates in the film, which look green under UV light. Stretching and raised temperatures pull these aggregates apart, causing the film to appear blue. While some of these ideas are very much at the research stage, and may never be suitable for commercial exploitation, one area that shows promise for the emergence of practical products is antimicrobial films.

Nanotechnology could provide a boost for antimicrobial packagingPackaging materials incorporating antimi-crobials were one of the first active packag-ing ideas to be investigated some years ago. The idea was to use antimicrobials already approved as food additives and apply them to the inner surface of packaging to improve product safety and reduce micro-bial spoilage. This could be particularly useful for products vulnerable to surface

spoilage, like cheese, bakery goods and sliced meat products, and which could be vacuum packed or film wrapped so that most of the product surface is in contact with the packaging.

A number of antimicrobial compounds have been investigated by researchers for active packaging applications. For example, traditional antifungal preservatives, such as potassium sorbate, have been shown to inhibit mould growth on pre-packed cheeses when applied to the inner surface of packaging. Elsewhere, a so-called ‘natural preservative’, the bacteriocin nisin, has been applied to the inner surface of LDPE films, using methyl cellulose as a carrier, to help control the growth of Listeria on the surface of vacuum-packed hot dogs. A number of these developments have proved to be quite successful at a research level, giving longer shelf life and better control of pathogens. Although some products based on these ideas have been launched in Japan, it appears that only one type of antimicro-bial packaging has made it to commercial production in Europe.

Bakery goods could benefit from packaging materials which incorporate antimicrobials on the inner surface of the package.

Nanotech packaging

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That exception is a range of materials developed in Spain by researchers at the University of Zaragoza, led by Professor Cristina Nerin. Working with manufacturer Artibal SA, the Zaragoza researchers have been looking at essential oils from herbs and spices, including cinnamon, oregano and cloves, as the active components of packaging. These essential oils have been known to have a natural preservative effect for many years and have been investigated as preservatives for direct addition to foods. So far, the collaboration has produced a semi-rigid PET material with an added antimicrobial and a flexible film coated with a cinnamon extract that is claimed to significantly increase the shelf life of bakery products. They have also developed packag-ing incorporating natural antioxidants, such as an extract of rosemary. Artibal says that the packaging does not need to be in full contact with the surface of the food, since the antimicrobial and antioxidant effects can occur in the vapour phase within the pack. The technology can also be adapted to all kinds of packaging materials.

Other researchers have developed similar approaches based on essential oils. For example, a group of scientists working for the USDA Agricultural Research Service have successfully tested edible films made from apple puree and oregano oil as anti-microbial coatings for fresh produce that can kill food poisoning bacteria, such as E. coli O157. But all this research effort has not led to the development of many com-mercial products. This may be because of fears of consumer resistance and the danger of expensive litigation in the event of any failure of the technology. Despite this, some experts believe that the potential offered by applying a nanotechnology approach to antimicrobial packaging could change that concern in the future.

Leading the field of contenders for the nanoscale approach to antimicrobial packaging is nano-silver. Silver is one of the oldest known antimicrobials and has been used in the treatment of wounds for centuries. Unfortunately it isn’t very soluble and its applications have been quite limited until recently. But, like other materials,

silver nanoparticles behave differently from larger pieces of the metal. Possibly because silver in this form has a much larger surface area, silver nanoparticles are far more effec-tive 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, and anti-bacterial 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 antimi-crobial packaging. Some silver compounds have already been evaluated successfully by EFSA as antimicrobial additives for packaging materials, although the assessments did not

consider the implications of nanoparticles of silver. In fact, the technology is already in use in some parts of the world, such as South Korea, China and the USA, to pro-duce food storage containers, though not food packaging as yet.

One of the problems with nano-silver is its cost, which may prove prohibitive for disposable single-use packaging. Some researchers have therefore looked at alterna-tives. A project at the University of Leeds in the UK has been looking at the possibility of using nanoparticles of zinc, calcium and magnesium oxides and titanium dioxide in antimicrobial packaging. Initial results have been promising, and since these materials are much cheaper to produce than silver, they could be strong candidates for future antimi-crobial packaging applications. It may also be fruitful to look at other antimicrobials that have already been investigated as packaging additives on a macro scale, but not yet at the nanoscale. For example, the performance of natural preservatives, such as nisin and plant essential oils might be enhanced if they were applied to packaging as nanoparticles.

Another exciting idea is the development of packaging films that actively generate a biocide and release it into the pack. Based on nanoscale controlled release technology, Microsphère and Microlite are being devel-oped by the Microactive Corporation in the USA. The technology is designed to create something that the company calls a ‘Micro-atmosphère’ environment within the pack by the sustained release of chlorine dioxide. This is said to inhibit the growth of micro-organisms and neutralise undesirable odours. There is no need for direct contact between food and packaging with this system, since it produces a vapour in the pack. The Micro-atmosphère can be maintained for extended periods and can also be ‘switched on’ by UV light or raised moisture levels.

There is clearly a great deal going on at a research level to develop effective nanotech-nology-based antimicrobial packaging, but there are also some big obstacles to overcome before it can be successfully marketed. Safety will have to be assured and uncertainties over how nanoscale food contact-materials will be regulated will have to be resolved to make significant progress. Most importantly, the consumer will have to be won over and convinced of the benefits. The current indus-try approach to nanotechnology in general appears to be not to talk about it, but the wisdom of this as a long-term strategy must be in doubt. An open debate is surely a better option.

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Leading the field of contenders for the nanoscale approach to anti-microbial

packaging is nano-silver.