Introduction

Packaging is described as "active" when it performs some role in the preservation of the food other than providing an inert barrier to outside influences. Although the ideal active package would sense the needs of the food and adjust the characteristics of the in-pack environment to meet these needs, we are faced at present with the need to work with more primitive systems. Indeed the active packaging systems currently available are based very much on first generation ideas.

Today we need to be content with seeking to maintain predetermined conditions and this state is provided by smart films or by active packaging in the general sense. Both of these approaches are the subject of research projects in the CSIRO Division of Food Processing.

There is a broad range of effects which can be achieved in-pack with varying degrees of success if one can believe the patent literature. Very few of the ideas have yet established themselves in the marketplace. An overview of these technologies is shown in Table 1 where the numbers indicate the estimated importance of the potential contribution to the food and beverage industries.

The removal of headspace and dissolved oxygen from a wide variety of food products is of paramount importance if they are to reach the desired shelf life. Although much of the oxygen in air can be removed by conventional gas flushing there is an increasingly appreciated need for in-pack oxygen scavenging.

The reaction of iron powder with the oxygen has been central to most of the commercial methods of oxygen removal used so far, with other reagents being added to improve the reaction rate. The iron powder and its associated compounds have normally been enclosed in a porous sachet made of non-woven high density polyethylene film or a tough laminate of a plastic to paper.

This approach of inserting a sachet into the package is effective but meets with resistance among food packers. The active ingredients in most systems consist of a non-toxic brown/black powder or aggregate which is visually unappealing if the sachet is broken. A much more attractive approach would be the use of a transparent packaging plastic as the scavenging medium.

Table 1. Active packaging technologies

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Process Packaging Location Home use

Insert

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1 Oxygen removal * + +

2. Oxygen barrier *

3. Water removal *+ + +

4. Gas indicator *+ +

5. Ethylene removal *+ + +

6. Carbon dioxide release * +

7. Antimicrobial action *+ +

8. Preservative release *

9. Aroma release *+

10. Taint removal *+

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* CSIRO research area

+ Technology available or patented by others

Oxygen scavenging plastics

Several processes are being developed in the CSIRO Division of Food Processing. One process utilises reactions inside the packaging film containing a photosensitising dye and an excited oxygen trap. On illumination of the film with visible light dye, molecules become excited and pass on their excitation to oxygen as it diffuses into the film from either the package headspace or the liquid food. The excited oxygen molecules react with the trap and become bound. While the film is illuminated the process continues until all oxygen or trap is reacted.

The reaction scheme is:

Dye + light ------> Dye*

Dye* + O2 ------> *O2

*O2 + Trap ------> Bound oxygen

where * represents an excited state of the species.

This photochemical process offers advantages over most of the other processes in that it requires no addition of sachets to the food, does not involve particulate matter in the transparent packaging material and is switchable with light. However, scavenging does not occur in the dark so a good barrier plastic would be necessary for highly sensitive foods.

The polymer system is being improved to bind the reaction products as part of the package to prevent migration into the food.

A small pouch lined with a scavenger film area of 150 cm2 and containing 25 ml of air can be deoxygenated in about 15 minutes. This result is in no way limiting as that system is far from optimised and utilises a low scavenger concentration.

Whereas the photochemical system has the advantage of being switchable, it is limited because of the need for continuous lighting until the desired amount of oxygen is removed. A second process, suitable for both transparent and opaque packages and which therefore continues to operate during the warehousing and distribution of the food, is the subject of active research in CSIRO at present. It is proposed that this process, which has additional control features, will be suitable not only for plastics containers but also for coatings on metal cans and on closures for glass containers.

Oxygen barriers through chemistry

The chemistry described in the preceding section has been applied successfully to blocking completely the diffusion of oxygen through a medium-barrier laminate of nylon 6 to polyethylene. The process involves lamination of the scavenging film on the inside of the barrier resulting in chemical trapping of the oxygen passing through the barrier.

Although the use of laminations of this type may appear to have only novelty value there may be application in the fine control of atmospheric composition in the controlled atmosphere packaging and storage of horticultural produce. Until truly intelligent film systems are available, our system will be based on the use of an oxygen sensor which will control illumination.

Ethylene removal

Although polymer films containing dispersions of minerals such as crystobalite are marketed for ethylene removal from the atmosphere, there is still debate as to whether they function by removal of sufficient ethylene to achieve their observed effects. The levels at which ethylene functions as a ripening hormone in fruit, flowers, etc, is known to be very low indeed in some cases.

Work by Dr Bob Holland in the Division of Food Processing has resulted in the development of a radically different polymer which removes ethylene at the levels found to be physiologically active. This polymer has been shown to prevent packaged carnations from wilting during storage. The ethylene level was kept at well below 0.1 ppm even with deliberate injection of ethylene into the package.

The conditions under which this technology will be introduced into the marketplace are still being determined. However, it is highly probable that a related chemistry will be released allowing for measuring the ethylene permeability of plastic films utilising the inexpensive spectrophotometric method devised by Dr Holland and his collaborators.

Carbon dioxide release

Carbon dioxide has long been used to suppress microbial growth in packaged cheeses, meat and other products. However, with the introduction of modified atmosphere packaging there is a need to generate varying concentrations of the gas. Further, in packages containing little or no oxygen there can be a need to remove oxygen diffusing into the pack and to replace that gas with carbon dioxide.

A polymer film system capable of achieving these effects has been developed for limited applications. One such application is in the storage, shipment and cultivation of specimens containing anaerobic or microaerophilic microorganisms. Combination of oxygen scavenging polymers with carbon dioxide producing systems allows interesting gas compositions to be reliably achieved.

So far the problems associated with diffusion of ingredients into the package have not been addressed so use with foods needs further work.

Other active technologies

Research in CSIRO into other areas of active packaging is continuing at a lower level than those described above. It is becoming clear that principles and problems learned using one system can often be applied in another.

Results to date lead to the suggestion that some of the processes now performed in the processor's plant will in future be performed by the active package.