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The main purpose of food packaging is to protect the food from microbial and chemical contamination, oxygen, water vapour and light. The type of packaging used therefore has an important role in determining the shelf life of a food.

'Active' packaging does more than simply provide a barrier to outside influences. It can control, and even react to, events taking place inside the package.

WHAT CAN HAPPEN INSIDE A PACKAGE?

Fresh foods just after harvest or slaughter are still active biological systems. The atmosphere inside a package constantly changes as gases and moisture are produced during metabolic processes. The type of packaging used will also influence the atmosphere around the food because some plastics have poor barrier properties to gases and moisture.

The metabolism of fresh food continues to use up oxygen in the headspace of a package and increases the carbon dioxide concentration. At the same time water is produced and the humidity in the headspace of the package builds up. This encourages the growth of spoilage microorganisms and damages the fruit and vegetable tissue.

Many food plants produce ethylene as part of their normal metabolic cycle. This simple organic compound triggers ripening and aging. This explains why fruit such as bananas and avocados ripen quickly when kept in the presence of ripe or damaged fruits in a container and broccoli turns yellow even when kept in the refrigerator.

Extensive trials have shown that each fresh food has its own optimal gas composition and humidity level for maximising its shelf life. Active packaging offers promise in this area; it is difficult with conventional packaging to optimise the composition of the headspace in a package.

The shelf life of processed foods is also influenced by the atmosphere surrounding the food. For some processed foods, a lowering of oxygen is beneficial, slowing down discolouration of cured meats and powdered milk and preventing rancidity in nuts and other high fat foods. High carbon dioxide and low oxygen levels can pose a problem in fresh produce leading to anaerobic metabolism and rapid rotting of the food. However, in fresh and processed meats, cheeses and baked goods, carbon dioxide may have a beneficial antimicrobial effect.

CONTROLLING THE ATMOSPHERE AROUND PACKAGED FOODS

The concept of extending shelf life of foods by controlling the gases in their immediate environment is not new.

Controlled and modified atmosphere storage (CA and MA respectively) are terms used to describe the addition or removal of gases from storage rooms, transport containers or retail food packages. The levels of oxygen, carbon dioxide, water vapour and ethylene are manipulated to achieve an atmosphere different from normal around the food. The nitrogen concentration may also be changed, particularly when an inert replacement for oxygen is required.

Controlled atmosphere usually indicates monitoring and control of gaseous composition. This is the case with bulk stores for fruit and sometimes with transport containers. It is not practical with small packages in the distribution system.

The term modified atmosphere is used when the composition of the storage atmosphere is not closely controlled. This is usually the case with wholesale or retail packs. The initial atmosphere is intentionally adjusted to give a gas mix as close as possible to that which will optimise the shelf life. Subsequent movement of gases and moisture into and out of the package is controlled only by the ability of the packaging film to act as a barrier.

ACTIVE PACKAGING SYSTEMS

Active packaging, employs a packaging material that interacts with the internal gas environment to extend the shelf-life of a food. Such new technologies continuously modify the gas environment (and may interact with the surface of the food) by removing gases from or adding gases to the headspace inside a package.

The table below sets out some areas of atmosphere control in which active packaging is being successfully used.

USES OF ACTIVE PACKAGING
Active Packaging System	Application
Oxygen scavenging	Most food classes
Carbon dioxide production	Most food affected by moulds
Water vapour removal	Dried and mould-sensitive foods
Ethylene removal	Horticultural produce
Ethanol release	Baked foods (where permitted)

Recent technological innovations for control of specific gases within a package involve the use of chemical scavengers to absorb a gas or alternatively other chemicals which may release a specific gas as required.

CSIRO is conducting research to develop the right package for each commodity. The technology being developed by CSIRO incorporates chemical scavengers in packaging films to control such gases as ethylene or oxygen.

Ethylene scavenging

A chemical reagent, incorporated into the packaging film, traps the ethylene produced by ripening fruit or vegetables. The reaction is irreversible and only small quantities of the scavenger are required to remove ethylene at the concentrations at which it is produced. A feature of the CSIRO system is its pink colour which can be used as an indicator of the extent of reaction and shows when the scavenger is used up.

It is expected that the film will be produced in Australia and used as a valuable means of extending the export life of fruit, vegetables and flowers.

Systems developed in other countries are already commercially available. These usually involve the inclusion in the package of a small sachet which contains an appropriate scavenger. The sachet material itself is highly permeable to ethylene and diffusion through the sachet is not a serious limitation. The reacting chemical for ethylene is usually potassium permanganate which oxidises and inactivates it.

Oxygen scavenging

The presence of oxygen in food packages accelerates the spoilage of many foods. Oxygen can cause off-flavour development, colour change, nutrient loss and microbial attack. Several different systems are being investigated by CSIRO to scavenge oxygen at appropriate rates for the requirements of different foods.

One of the most promising applications of oxygen scavenging systems in food packages is to control mould growth. Most moulds require oxygen to grow and in standard packages it is frequently mould growth which limits the shelf life of packaged baked goods such as cakes and crumpets and of packaged cheese. Laboratory trials have shown that mould growth on some baked products can be stopped for at least 30 days with active packaging and significant improvements in the mould-free life of packaged cheese have also been obtained.

Another promising application is the use of active packaging to delay oxidation of and therefore rancidity development in vegetable oils.

Again the use of discrete sachets containing oxygen absorbents has already found commercial application. In this instance the scavenging material is usually finely divided iron oxide. These sachets have been used in some countries to protect the colour of packaged cured meats from oxygen in the headspace and to slow down staling and mould growth on baked products, e.g. pizza crusts.

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.

Humidity control

Condensation or 'sweating' is a problem in many kinds of packaged fruit and vegetables. It is of particular concern in cartons of fresh flowers for which there is an important export trade.

Unless the relative humidity around flowers is kept at about 98 per cent, water will be lost from the bunches. Such high humidities mean there is a very real risk of condensation occurring during transport as the temperature of the flowers may fluctuate by several degrees. When one part of the package becomes cooler than another, water is likely to condense in the cooler areas.

If the water can be kept away from the produce there may be little harm. However when the condensation wets the produce, nutrients leak into the water encouraging rapid mould growth.

CSIRO has developed technology to control condensation inside packages. This allows the food to remain dry without drying out the product itself. Therefore sensitive products such as flowers and table grapes are protected from contact with water. This helps to reduce growth of mould.

Carbon dioxide release

High carbon dioxide levels are desirable in some food packages because they inhibit surface growth of microorganisms. Fresh meat, poultry, fish, cheeses and strawberries are foods which can benefit from packaging in a high carbon dioxide atmosphere.

However with the introduction of modified atmosphere packaging there is a need to generate varying concentrations of carbon dioxide to suit specific food requirements. Since carbon dioxide is more permeable through plastic films than is oxygen, carbon dioxide will need to be actively produced in some applications to maintain the desired atmosphere in the package.

So far the problems associated with diffusion of gases, especially carbon dioxide, through the package, have not been resolved and this remains an important research topic.

Release of microbial inhibitors

Ethanol

Ethanol (or common alcohol) is not a permitted food preservative in Australia. However its antimicrobial activity is well known and it is used in medical and pharmaceutical applications. Ethanol has been shown to increase the shelf life of bread and other baked products when sprayed onto product surfaces prior to packaging.

A novel method of generating ethanol vapour, recently developed in Japan, is through the use of an ethanol releasing system enclosed in a small sachet which is included in a food package. Food grade ethanol is absorbed onto a fine inert powder which is enclosed in a sachet that is permeable to water vapour. Moisture is absorbed from the food by the inert powder and ethanol vapour is released and permeates the sachet into the food package headspace. This system is approved in Japan to extend the mould-free shelf life of various cakes.

Sulphur dioxide

Sulphur dioxide is a permitted preservative in Australia for some processed foods. It can also be used to control mould growth in some fruits.

Serious loss of table grapes can occur unless precautions are taken against mould growth. It is necessary to refrigerate grapes in combination with fumigation using low levels of sulphur dioxide.

Fumigation can be conducted in the fruit cool stores as well as in the cartons. Carton fumigation consists of a combination of quick release and slow release systems which emit small amounts of sulphur dioxide.

When the temperature of the cartoned grapes rises due to inadequate temperature control, the slow release system fails releasing all its sulphur dioxide quickly. This can lead to illegal residues in the grapes and unsightly bleaching of the fruit.

CSIRO is working on developing systems which gradually release sulphur dioxide and are less sensitive to high temperature and moisture than those presently used. These systems have potential use for fresh grapes and processed foods permitted to contain sulphur dioxide such as dried tree fruits and wine.

Other developments

The examples given above are only some of the commercial and non-commercial applications of active packaging. This technology is the subject of research in many countries and rapid developments may be expected.

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© CSIRO Food and Nutritional Sciences, 11 Julius Avenue Riverside Corporate Park Delhi Road (PO Box 52) North Ryde NSW 2113 Australia