Active research

14 September 2004 (Last Updated September 14th, 2004 18:30)

Throughout history, packaging has protected food. George Sadler, professor of packaging at Illinois Institute of Technology, discusses the development of containers that can actively enhance their contents.

Packaging provides us with a means of enclosing items, which makes portability and long-term storage possible. Occasionally, unforeseen interactions between the container and the food produce unexpected modifications in the food it carried.

Milk stored in calf bladders separated into curds and whey. It was observed that milk stored in vessels which had already successfully fermented milk into yoghurt were more likely to successfully produce subsequent batches of yoghurt.

These containers are said to be 'active'. They not only passively protect food against deteriorative forces, but are also dynamically linked to favourable alterations - such as unique enhancements to food products or the storage quality of foods.

Most food preservation methods have existed for centuries. However, the thermal stabilisation of food is comparatively new. Nicholas Appert introduced the rudimentary principals of thermal processing in the first decade of the nineteenth century.

Appert discovered that a British invention, the tin canister, was particularly suited to this process. While Appert's adoption of the tin canister was initially based on the merits of metal for thermal transfer, it did mark the beginning of a systematic study to optimise container benefits for foods.

Canning developed slowly in the nineteenth century, and had to wait for Pasteur's declaration of germ theory in the 1860s before it was fully appreciated. Once the role of micro-organisms in disease and spoilage was understood, food processing could be developed around sound scientific principles.

Productive polymers

Significant twentieth-century packaging developments include cellophane at the start of the century; nylon in the 1930s; and polyethylene (PE), polypropylene (PP), polyvinylidene chloride (PVDC) and poly-vinyl chloride (PVC) during the Second World War.

While cellophane was commercialised for food by 1950, the broad application of PE, PP and PVDC for food use had to wait until the 1960s. Many additional packaging polymers were added in the second half of the twentieth century.

Polystyrene (PS), polyethylene terephthalate (PET) and EVOH are among the most important of these. Each of these polymers owes its success to distinctive niche properties directed at specific food vulnerabilities. These include sensitivity to moisture loss, oxidation, flavour absorption and light-induced reactions.

Concurrent with packaging developments, advances in biochemistry, microbiology and nuclear science have contributed to a broader understanding of the atomic and molecular behaviour of enzymes, catalysts and macromolecules. Anticipated active packaging concepts require a fundamental understanding of molecular level processes for optimal conception.

Modified atmosphere

Modified atmosphere packaging (MAP) relies on the differential permeability of selected polymers to establish a food-stabilising environment. It works best with actively respiring foods, such as fresh-cut vegetables and raw meats.

Carbon dioxide is produced as a by-product of respiration, and accumulates inside the package. Oxygen permeates from the outside of the package to the inside, and ensures that anaerobic conditions are not achieved.

Carbon dioxide migrates from the inside of the container into the external atmosphere to prevent excessive accumulation. If carbon dioxide accumulation is significant, nitrogen can also migrate into the package, until equilibrium is established with the external atmosphere.

The advantage of MAP is the fact that the reduction in oxygen slows the respiration of foods, and hinders the propagation of many spoilage micro-organisms. Consequently, foods age at a slower pace and spoilage organisms take longer to achieve critical spoilage numbers. MAP has been extremely successful in recent years in the packaging of retail bags for cut salad vegetables.

However, achieving and maintaining an optimal blend of gases by passive diffusion is difficult. The establishment of equilibrium can take several days. Also, gas permeation is determined by temperature. Broad fluctuations in temperature can tip permeation in favour of anaerobic conditions or allow excessive amounts of oxygen to penetrate the package.

Oxygen scavenging

Once processing has removed the threat of micro-organisms, internal chemical processes govern the shelf life of packaged foods. For many foods, the shelf life-limiting reaction involves oxygen. Air-saturated fluid foods contain approximately 7ppm oxygen at room temperature. Dry foods often contain air in interstitial areas between pieces or particles.

Despite scrupulous efforts, it is difficult to remove much more than 1ppm oxygen. The container headspace is a particularly rich reservoir of oxygen. Even when container headspace is commercially purged with steam or inert gas, residual oxygen may linger at single digit levels. When foods are packaged in polymer containers, oxygen migration through the contain sidewalls and seal areas is inevitable.

Virtually all oxygen can be removed with oxygen scavengers. These can be located in the packaging material itself, in screw-top closures, or in special sachets that can be inserted into food, or between package layers.

Oxygen scavengers are frequently powdered metals (often iron) in combination with a reducing agent (often ascorbic acid). However, aromatic nylons that chemically react with nylon are currently used as intermediate laminate layers in plastic beer bottles.

Tin is also an efficient oxygen scavenger in tin-lined cans. Tin's use is limited by the off-flavour imparted by tin oxides. While oxygen reacts with many compounds, price sets something of an upper limit to scientific imagination.

Electrochemical packaging

When an organism dies, electrons are gradually depleted. As a result of this process, tissue redox potential shifts from a reduced to a more oxidised state.

In thermally processed foods, cytochroms are destroyed and electrons stumble haphazardly down the energy ladder without regard for the policing effect of the cytochroms. Reactions result which are uncharacteristic of tissue - living or dead. The cascade of reactions responsible for Maillard browning falls into this category.

Reducing agents such as tin and ascorbic acid place food in an electrochemically reducing environment, and sacrificially combine with oxygen to maintain a reduced state.

In essence, these compounds lock electrons at the top of the energy ladder. In doing so, their participation in browning and other biologically unsupervised reactions is prevented.

It is possible to scavenge the oxygen in foodstuffs with the direct application of electricity. In this process, a metallic container sidewall serves as the cathode, the site of reduction. The anode is a second metallic surface, which is also in contact with the food, but insulated from the cathode. While this approach is not currently feasible for single serve containers, it may play an important role for bulk storage vessels.

Bioactive packaging

Many globular proteins, including enzymes, antibodies, haemoglobin, cytochroms and certain muscle proteins, exhibit biological activity. Binding (non-covalent) and grafting (covalent) methods exist, which allow the attachment of globular proteins to polymer surfaces. If properly executed, the proteins attached retain a portion of their biological activity, but do not become a component of the food.

Attached proteins can remove oxygen, improve or preserve packaged food, and reduce micro-organisms. The two principal disadvantages of protein attachment are the greatly reduced activity of some proteins during attachment and the drop in biological activity of the bound protein over time.

Once these drawbacks have been properly addressed, a single- or multiple-attached protein system could potentially confer the properties of living tissue to a package.

Antimicrobial packaging

Packaging materials can be impregnated with antimicrobial components, which slowly migrate to the food to inhibit microbial growth. This approach has been successfully used to inhibit the growth of mould on cheese rinds.

An antimicrobial component is incorporated directly into the wax used to coat the cheese. However, the local action of antimicrobial packaging is a serious drawback for chopped and ground foods, since they have many surfaces that fall outside the reach of the added antimicrobial agent.

Organic compounds diffuse at rapid rates through some polymers. This is especially true of polyolefin polymers (polyethylene and polypropylene) and many of their co-polymers (such as ethyl vinyl acetate, anhydride grafted polyethylene and ionomers).

In most cases, the absorption of organic compounds by polymers is considered undesirable. Absorption can lead to a concentration of intrinsic food flavours - a phenomenon known as flavour scalping. However, in some applications, the absorptive properties of olefin-derived polymers can be exploited.

If absorptive polymers are impregnated with high concentrations of flavour essence, flavour migration will be directed from the package into the food. Not only can scalping be eliminated, but fresh flavour notes, sometimes lost during processing, can also be returned.

Indicator packaging

Other recent developments in active packaging include packages that can announce the end of their shelf life or identify a tampering event. An indicator compound can be inserted beneath all, or a carefully selected portion of the bar code. If tampering has occurred, or an end of shelf life indicator develops, the bar code either becomes unreadable or changes to announce the problem.

Simple indicators include iron powder and dichloroindophenol. Both of these indicators react with oxygen if the product is tampered with, or detect its slow migration during storage.

Indicators which announce the presence of spoilage have also been proposed. If packaging materials are in intimate contact with a food, the reaction between attached antibodies and microbial antigens could be designed to deliver a message that spoilage has occurred in the package.

The main concern surrounding indicator packaging is the result of false negatives and false positives. False negatives can expose a processor to legal vulnerabilities, and false positives can result in the rejection of a good product. Unless false responses can be held to less than 0.1 per cent of all packages, indicators will probably be too risky to consider.

Active packaging has been a by-product of all the driving developmental forces in packaging: food processing, microbiology, consumer preferences and recent scientific developments.

Consumers have consistently demanded fresher, more natural foods, along with the minimisation of food additives and preservation of nutrients, so packagers must continually strive to meet these needs.