However, consumer demand for 'fresh', preservative-free foods has been growing, and there has been growing interest in packaging concepts that reduce or eliminate consumption of preservatives.

Enter antibacterial packaging. An effective preservative-release system, for example, would release into the pack contents only the amount required to prevent microbial growth, thus reducing the preservative intake of the consumer. Of course, a sophisticated mechanism would be required to ensure that neither too little nor too much preservative is released into the pack, and this represents a major technological challenge for scientists.

Other antibacterial packaging targets surface bacteria, suitable for use in the packaging of bread, cheese and other items where mould growth tends to concentrate on the surface. In such instances, technology suppliers usually seek to immobilise the chosen active agent into the packaging film, and because the agent does not migrate into the pack contents, this represents a less controversial solution that is far less likely to raise concerns over additive levels in food.

Techniques for immobilising antibacterial agents on plastic substrates

Cornell University's Department of Food Science has been undertaking work into attaching bioactive materials and enzymes to polyethylene films, with potential applications in food packaging as well as possibly certain types of pharmaceuticals. The Department has investigated ways in which polyethylene film surfaces can be modified so as to develop non-migratory active packaging systems - as opposed to migratory systems, given negative consumer attitudes towards food additives.

One technique adopted by the Department involves the surface oxidation of polyethylene film with chromic/sulphuric acid, further functionalised by the attachment of diamino-polyethylene glycol (PEG) oligomers via carbodiimide coupling.

An additional carbodiimide coupling reaction was then used to link a peptide to the free amine functions of the PEG oligomers. Films were subjected to chemical, microscopic and spectrographic analyses after each step.

Contact-angle analysis showed the expected decrease in hydrophobicity on oxidation, and a further decrease on PEG attachment. The acid-base nature of the surface groups was further characterised by contact angles derived for phosphate buffer at a range of pH levels. Bromophenol blue dye was used to show up the appearance of free amines on the PEG functionalised surface.

The development of this system allows the attachment of biologically active molecules to the surface of polyethylene. Scientists at Cornell believe that active packaging incorporating tethered bioactive compounds will open the doors to in-package enzyme processing of food products, as well as novel antimicrobial packaging systems.

Aerosol-assisted atmospheric plasma (AAAP) barrier coating

Similarly, former UCB subsidiary Surface Specialties has also undertaken work into the incorporation of antimicrobial components onto plastic substrates, in this instance using coating technologies to immobilise the active agent.

Surface Specialties is now trialling the use of the aerosol-assisted atmospheric plasma (AAAP) barrier coating technique, developed as part of the EU-funded SOLPLAS research project, which began in 2002. VTT Finland, one of the partners in the project, undertook initial testing on the coating, which showed that it was effective in preventing microbial growth in cheese, meat and fresh produce, although VTT believes that further testing may well extend application into other areas. The barrier coating can be applied using atmospheric pressure plasma technology.

The AAAP process involves the injection of the film-forming precursor as an aerosol via atomisation into the plasma volume adjacent to the base substrate. The influence of the plasma on the aerosol droplets of the coating material enhances their reactivity, thus improving cross-linking and curing results, and limitations due to film thickness are also reduced.

The effectiveness of the process can be further improved by decreasing the size of the droplets to the nano-scale. A nano-sized aerosol will be generated by an electrospray process, which offers control on aerosol diameter and charge. The electrospray process has been developed and optimised by the Technical University, Delft.

The combination of the plasma process with designed hybrid polymers significantly increases the performance of the resulting coatings in terms of curing behaviour, degree of cross-linking and network density. Key properties to be improved are mechanical protection and barrier performance. In combination with the ability of chemical functionalisation of Ormocers, the field of applications of these materials can be extended significantly. Moreover, the plasma can also have effects on functionalisation of coatings and on adhesion.

Naturally occurring antimicrobials

A great deal of research into antibacterial packaging solutions is currently being undertaken worldwide. To date, much of this has been concentrated in Japan where the use of antibacterial films is relatively well-established, although of late interest in active packaging solutions has been growing in the US and Europe.

The most common antimicrobial agent in use in Japan is silver-substituted zeolite. Silver ions are initially adsorbed to the surface of, and then absorbed into, the microbial cell. These then react to stem the growth of metabolic enzymes necessary for the development of fungi and yeast. Although an extremely effective agent in the combating of yeast and fungi development, the silver ions must not migrate from the zeolite skeleton, and the technology cannot be applied to heat-resistant bacteria.

In addition, its effectiveness can be reduced when used in conjunction with contents that have a high sulphur content, as the silver ions will react with sulphates, hydrogen sulphide and sulphur-containing amino acids.

Another drawback is that the costs associated with silver-substituted zeolite antimicrobial technology are very high. Consequently, concentrations vary from just 1-3 per cent, with the material either laminated as a thin layer or applied on the surface of formed containers.

Wipak has been among those companies involved in the development of antimicrobial films using silver-based substances to combat bacteria, in this instance preventing the growth of E.coli in meat and sausage products as well as combating other bacteria. The use of silver in food packaging applications in the US and Europe has been slow to take off due to regulatory concerns, but this could change in the next few years.

By way of alternative, a range of natural ingredients has been put forward for use in antibacterial packaging solutions. In part, this is down to negative consumer perceptions regarding synthetic active chemicals, with active chemicals derived from natural sources less likely to degrade into harmful by-products. Substances under investigation in the past year have included chitosan, horseradish/wasabi, oregano and cranberry, strawberries and basil.

However, it has been pointed out that the fact that these chemicals are natural does not necessarily mean that they are safer, and research has been undertaken into ensuring that the active agents employed have no impact upon the composition of the packaging itself.

Chitosan

DuPont has been engaged in the development of a compound using chitosan, a naturally-occurring product found in fish waste such as shrimp shells to prevent antimicrobial cross-contamination. Work on the new compound is in the early stages and the company has stated that it is too early to say for which food packaging applications the product will be suitable, but believes that it will be more effective than products based upon the mould-inhibitor concept.

Research into chitosan has also been undertaken by Swedish research centre STFI-Packforsk, which has also found that that chitosan has high oxygen barrier properties. STFI-Packforsk has been engaged in a project for the past five years on developing renewable packaging materials to compete with synthetic polymers such as PET and EVOH, and in the second stage of its investigations - which began in October 2004 - began to look at adapting the polymers for commercial processing equipment.

Chitosan is non-allergenic and can be applied to a variety of food products. This differs from other high-barrier solutions being developed by STFI-Packforsk, which can only be employed in certain products such as dairy and cereals due to their tendency to provoke allergic reactions. These other agents include whey protein, a by-product from cheese manufacturing; and wheat gluten, a by-product from the manufacture of ethanol and sweeteners. STFI-Packforsk's goal is to bring these materials onto the market in around ten years' time.

Experiments conducted in Japan on the effectiveness of chitosan in active packaging applications have yielded positive results. One experiment evaluated the action of edible chitosan coatings on growths of L. monocytogenes and L. innocua. The chitosan film fully inhibited growth of the L. monocytogenes bacteria for a period of at least eight days, although the antibacterial effect fell off with time, ascribed to the decreasing availability of amino groups sourced from the chitosan. Subsequent experiments under more realistic conditions - this time using Emmental cheese samples and L. innocua rather than simply growths - confirmed the antibacterial action of the chitosan film.

Allyl-isothiocyanate (AIT)

One of a number of sulphydral groups that can be derived from wasabi and horseradish, AIT (also known as wasaouro or mustard oil) is able to inhibit mould growth in a range of products including bread, cheese, fresh meat and fresh produce.

In 2004, Clemson University completed a project that showed that AIT is able to suppress mould on lettuce, turnip greens, fresh ground mince, kiwi fruit, strawberries, rock cake and bread rolls. In addition, research also revealed that AIT keeps bakery products moist for longer periods. The main challenge when using AIT in food packaging applications is that it can leave the food with a slight horseradish or garlic flavour if used in too high concentrations.

AIT is already used in Japan to extend the shelf life of strong-tasting products such as Kim Chi, where its own strong flavour cannot be easily detected. AIT is not yet approved for use in food packaging applications in the US and Europe, although its use in other applications should make the active agent acceptable to authorities. Avery Dennison is known to have begun a project to develop antimicrobial labels incorporating AIT, although development work is at a very early stage.

A separate research project on the use of horseradish extracts in antibacterial packaging is being undertaken by Danish scientists from the Danish Veterinary & Food Administration, Danish Technical University and the film producer Neoplex - funded in part by the Danish government and EU. Researchers are using a delivery system developed during the EU-sponsored Biopack projects that test the effectiveness of the natural additive in conventional processing. A key aim of the project is to produce an odourless and tasteless antimicrobial agent, with researchers testing the agent on a variety of products with large surface areas such as bread and cakes.

The natural antimicrobes are to be inserted into the ring-formed sugar-type molecule delivery system developed by Biopack, used in conjunction with conventional polyolefins. When the system is stored at room temperature, the starch container dissolves and the antimicrobial agents are released in gas form into the pack. As with other natural antimicrobial compounds, they do not require physical contact between the food and package to operate effectively.

The Biopack project tested the effectiveness of mustard oil in antibacterial applications. The mustard was incorporated into a cyclodextrin and added to film resin, used to pack cheese slices supplied by the Danish company Arla.

Basil

Because the use of wasabi in antibacterial food packaging applications can result in flavours that are unappealing to consumers, researchers have looked into alternative natural ingredients that do not produce any noticeable food taint.

One such alternative is basil, or more precisely the basil extracts methyl clavicol (an ether) and linalool (an alcohol). Both these organic molecules contain chemical groups that are able to attack and destroy the cell walls of a range of pathogens including E.coli 0157, listeria and six other bacteria.

Scientists at the Technion Institute of Technology in Haifa, Israel, and the Victoria University in Melbourne, Australia (working alongside the Kasetart University in Bangkok, Thailand) announced in 2003 that preliminary tests on a new plastic food wrapper incorporating the basil extracts had yielded positive results, delaying bacterial growth in cheddar cheese by one week in comparison to standard packaging, without producing any basil flavour - partly because the basil extracts are less soluble in water than wasabi, for example, and as such are less likely to adhere to the food. Further research has shown the wrapper to be effective in slowing bacterial growth in meat and poultry.

In order to provide an effective antibacterial packaging solution, the methyl clavicol and linalool need to be released slowly into the pack contents. However, the incorporation of these volatile chemicals into film wrap is complicated by the fact that the standard process of using heat to make the material would lead to the chemicals evaporating during manufacture. As such, researchers have sought to develop a polymer mix, such that the methyl clavicol and linalool molecules bind fast to the plastic in the first instance, before diffusing into the food through permeable inner plastic layers, impelled by the water content of the food.

A major US packaging manufacturer was said to be interested in the new basil-based technology into 2004, undertaking investigations into the likely cost of the film material and also its commercial potential. A number of packaging companies have also expressed an interest in the technology, although the lead researcher on the project has stated that commercialisation is unlikely to occur for 2-3 years.

Oregano and cranberry

The University of Massachusetts has developed an additive made from a mix of oregano and cranberry able to inhibit growth of listeria in processed meats, with a view to using the mix as an active agent in food packaging. In tandem with Cornell University, South Carolina, it is now developing the technology in both an edible format for food producers and non-edible film format for shrink-wrap used with meat trays by supermarkets.

According to researchers, the antimicrobial activity of both oregano and cranberry is linked to the phenolic moiety (specific segment of a molecule) and as such are suitable as antimicrobial natural extracts when effectively combined with lactic acid. Tests have shown that antimicrobial activity increased when oregano and cranberry extracts were mixed in a weight ratio of 75 per cent oregano to 25 per cent cranberry with 0.1 milligram of phenolic per disk or millilitre, and their efficacy was further enhanced by lactic acid. Inhibition by "phytochemical and lactic acid synergies" was most effective when beef and fish slices were stored at 4°C.

At present, the oregano/cranberry mix is only effective in combating surface bacteria, and can leave a slight spicy, acidic flavour. US meat producers are currently testing the mix for use as a food additive, and the University of Massachusetts is hopeful of commercialisation in this application within the next year. The University of Massachusetts is also testing a number of other botanical antimicrobial agents, partnering legumes like soy and fenugreek with small fruits such as raspberries and blueberries.

Strawberries

The Spanish Institute of Agrichemistry and Food Technology has developed a new antimicrobial packaging solution that uses strawberry extracts to combat the growth of bacteria in berries. Funded by the Spanish government, research into the packaging began in 2001 with commercialisation of the pack expected before year-end 2005.

The pack was developed for members of the Huelva Association of Strawberry Producers, which accounts for 85 per cent of all strawberries eaten in Europe. Some 20 per cent of the producers' fruit crop, however, is lost to yeast infection each year, but adoption of the new pack is set to reduce this figure significantly, able to extend shelf life of strawberries by three weeks.

In the first instance, the antimicrobial packaging will be used with high-value items such as wild strawberries and raspberries - due to its relatively high cost. In the longer-term the packaging is likely to find application across a broader range of products, combating fungi growth in blueberries, blackberries, cherries and mushrooms.

Natural antioxidants

Antioxidants are already widely used as food additives. On the one hand, antioxidants are able to improve the oxidation stability of lipids, and can also increase the shelf life of dried goods and foods sensitive to oxidation. In the case of packaging films, antioxidants are already used simply as a means of protecting the film itself from degradation from oxidation. While oxidation does lead over time to lower concentrations of antioxidants in such film, concentrations are also affected by the process of antioxidant evaporation, as the antioxidants make their way to the film surface. 

The main issue for technology suppliers revolves around the choice of antioxidant material. In the past, the main focus of research was on the use of butylated hydroxytoluene (BHT), but of late a number of other natural alternatives have been considered, including vitamins C and E.

BHT is a small, mobile antioxidant that can be incorporated into (and released from) packaging films. It has been shown to be effective in extending the shelf life of oatmeal-based cereals when used in relatively high (around 0.3 per cent) concentrations in HDPE film. Over time, levels of BHT within the film fall to zero, in part due to the expected reactions within the pack, but often also as a result of leakage from the pack. The effect of the latter can be limited by use of multilayer films including a film layer with low permeability levels for BHT.

Albeit an effective technology, BHT is unlikely to be employed on a widespread basis (if at all) due to its tendency to accumulate in human adipose tissue on ingestion, this raising serious health questions. Given the health and consequently likely regulatory problems associated with BHT, technology suppliers have sought to examine the effectiveness of natural antioxidants when incorporated into packaging films. Key natural antioxidants under consideration include vitamin C and vitamin E.

Vitamin E is regarded as having the most potential, as it can be dissolved into polyolefins and does not break down when processed into film materials, able to tolerate the high temperatures involved in the thermoforming process. One Danish company is said already to be using vitamin E in a cheese wrapper to scavenge for oxygen within the pack.

Vitamin E has the advantage over other natural ingredients such as vitamin C in that it is cheaper and has superior polymer performance characteristics. In addition, it is also more effective than synthetic antioxidants at avoiding organoleptic substances. Although 2-3 times more expensive than synthetic antioxidants, vitamin E

is more environmentally-friendly, and a smaller amount of the ingredient is required to produce the same results. Furthermore, in this application, since oxidation of the food only occurs at the surface, a smaller amount of the antioxidant reacts to prevent oxidation than would be the case were the vitamin E used as a food additive.

Although vitamin E is a naturally-occurring substance already used as a food additive, approval for its usage in flexible plastic packaging could take some time, with the US Food & Drug Administration (FDA) needing to check that the additive does not change the structure of the plastics, which could lead to alteration of the pack contents.

The Matforsk Norwegian Food Research Institute recently published findings on research into the use of vitamin E in films supplied by Wipak Germany. The film was tested with frozen processed turkey breasts stored for a period of 12 months, and proved just as effective as film containing synthetic antioxidants.

Outlook

The use of antibacterial substances has clear potential in food packaging applications, but it is likely to be several years before the technology gains widespread acceptance in Europe. Proponents for the usage of antibacterial packaging have faced opposition in some quarters, in part relating to concerns over the efficacy of the new technologies, as well as issues relating to consumer safety.

The EU-funded Actipack project provided potential ammunition for opponents of the technology. Research into the effectiveness of antimicrobials indicated that they were ineffective in preventing bacterial growth - and thus failed to extend shelf life - in foods such as cheese, yogurt and milk, despite being able to inhibit single strands of bacteria in scientific experiments.

However, the methods employed by researchers were criticised by DuPont and others, claiming that the results were compromised by the use of films with poor barrier properties, effectively cancelling out the effectiveness of the antimicrobial components - acknowledged as a possibility by Actipack project co-ordinator TNO.