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Packaging food to protect its shelf life

06 April, 2006

Shelf life is an important property of any food and is of interest to everyone in the food chain from producer to consumer.

Food Science Australia

The legal requirement for manufacturers of packaged foods to open date mark foods was introduced in Australia in 1978. This followed similar moves internationally and the publication of a standard by the Codex Committee on Food Labeling. It was argued by consumer groups that with the rapid changes occurring in food manufacturing, packaging and retailing that consumers could no longer rely on traditional wisdom and habits to dictate how long a food may be stored.

Changes to the requirements for date marking of food have occurred since 1978. The current Standard 1.2.5, Date Marking of Packaged Food of the Australian Food Standards Code (AFSC) reads in part: ' the Standard requires packaged food, with some exceptions, to be date marked, and prohibits the sale of packaged food after the expiration of the use-by date, where such a date mark is required'.

A use-by date means 'the date which signifies the end of the estimated period, if stored in accordance with any stated storage conditions, after which the intact package of food should not be consumed because of health and safety reasons'. Thus, it is an offence to sell packaged food past its use-by date and this form of date marking is tied to food safety.

The best-before date is the open date which most packaged foods carry and is defined in the AFSC as 'the date which signifies the end of the period during which the intact package of food if stored in accordance with any stated storage conditions, will remain fully marketable and will retain any specific qualities for which express or implied claims have been made'.

An exemption from the open date marking still applies if the best-before date of the food is two years or more. However many manufacturers whose products fall into this category elect to date mark these foods.

Whose Responsibility?

The responsibility of determining the shelf life of a food, and thus its best-before or use-by date, lies with the manufacturer/packer.

The most direct way of doing this is to conduct properly constructed storage trials under realistic, defined conditions. This may not be possible for smaller manufacturers, in particular those just entering the market. They frequently are looking for some rapid method to measure and estimate shelf life to allow them to put their product on the market as quickly as possible.

Established manufacturers also are reluctant to rely solely on conventional storage trials to determine shelf life. Commercial pressures usually mean that a product must be marketed as quickly as possible after its development. Retailers may also have their own demands about shelf life of particular product categories especially those with a relatively short shelf life. Indirect methods of shelf life determination are therefore frequently used to yield results which otherwise would be time consuming to obtain.

These may involve so-called accelerated shelf life tests usually based on storage of the product at higher than normal temperatures or computer-based models.

While these methods may prove useful, they must be applied with care. Storage of foods at higher than normal temperatures can induce changes in the food which would not occur at normal ambient temperatures. Also the rate at which normal changes are accelerated by higher temperatures must be known with acceptable accuracy. Models are useful only if they have been shown to mimic spoilage rates in like foods and not merely been developed under laboratory conditions. These models are not designed to be used beyond the range of data used to construct them.

Factors Influencing Shelf Life

No single factor may determine the shelf life of a food but the most important to be considered in shelf life studies are:

Microbiological changes
Moisture and water vapour transfer
Chemical or biochemical changes

1. Microbiological Changes

Unless a food has undergone a commercial sterilisation process (e.g. canned foods) or has a water activity which will not permit microbial growth (e.g. sugar, breakfast cereals), the rate of growth of spoilage microorganisms is likely to be the major factor determining shelf life. This rate is determined by a number of factors including:

- food properties (e.g. pH, total acidity, water activity, presence of preservatives either natural or added);
- environmental factors (temperature, relative humidity, gaseous atmosphere);
- any process designed to kill or retard growth of microorganisms (thermal processing, freezing, packaging); and
- the type of microflora present on the food, and the initial population.

2. Moisture and Water Vapour Transfer

Not only is water (measured as water activity) a critical factor which determines which, if any, microorganisms will grow in a food, many foods are sensitive to loss or gain of water. This in turn can be affected by the choice of packaging and in many instances will determine which packaging is used. Many biscuits and savoury snacks including nuts suffer in quality as a result of moisture gain. Some baked foods such as cakes may suffer from moisture loss.

3. Chemical or Biochemical Changes

Numerous possible reactions which could limit shelf life fall into this category. The most important are oxidation, non-enzymic browning, enzymic browning and, in some cases, food and packaging interaction.

Oxidation of fats and oils leads to the development of rancidity marked by off odour and flavour. This may limit the shelf life of fats and oils but can also limit the shelf life of many other foods containing fats and oils. Examples of foods stored at ambient temperatures which can develop rancid off flavours are nuts, potato crisps and biscuits. Storage of these foods in high oxygen atmospheres can sometimes be used to accelerate shelf life studies but atmospheric oxygen is not the only initiator of oxidative spoilage. Many frozen foods can also have their shelf life limited by fat oxidation. While freezing arrests microbial activity, chemical reactions proceed at a much reduced rate even at recommended storage temperatures. Examples of frozen foods whose storage life is limited by oxidation include fish and meats.

A number of different vitamins are sensitive to oxygen including vitamin C (ascorbic acid) and vitamin B (thiamine). When vitamins are added to fortified foods such as breakfast cereals or sports drinks and a label declaration made, then shelf life determinations will have to take account of any vitamin degradation which will occur with time in addition to any other changes in quality.

Food Packaging Interaction

Most food products rely on their particular packaging to achieve their expected shelf life. In some circumstances the desired shelf life can be a major factor in the selection of a packaging material. For example, if for a particular market segment, the manufacturer of a snack food packed under a modified atmosphere determines that a shelf life of only six weeks is necessary, it may be possible for the product to be successfully packed in a plastic with lesser barrier properties than if the manufacturer was seeking a six month shelf life. In either case, the expected shelf life will be dependant on the integrity of the package seal to maintain the atmosphere within the package (beyond any expected gas transmission across the packaging film) for the nominated shelf life.

A special case of food and packaging interaction is the use of tin plate cans with welded side seams in the canning industry. While food standards limit the concentration of certain metals in all foods for public health reasons, there is a maximum tin content requirement for canned foods of 250mg/kg. Most canned foods are now processed in lacquered cans which substantially reduces the possibility of tin dissolution in the food. However tin dissolution from the can is essential in some canned foods which would otherwise be subject to discolouration. Asparagus is one example of such a food. Sufficient tin is therefore left exposed in such canned foods to ensure the expected quality of the food through its nominated storage life without it exceeding the regulatory limit.

Food Safety and Shelf Life

All shelf life studies will include an assessment of the safety of the product and this assessment will normally precede any assessment of shelf life.

Today it is widely recognised that the most effective way to ensure food safety is to meet the internationally recognised Hazard Analysis and Critical Control Point (HACCP) system as adopted by the Codex Alimentarius Commission and written into legislation in many countries including Australia.

Standard 3.2.1 of the Food Standards Code, Food Safety Programs, which is based on the HACCP system, is being progressively introduced into food businesses in Australia based on risk ranking by Commonwealth and State authorities.

Standard 3.2.2, which has been introduced throughout Australia, is titled Food Safety Practices and General Requirements, and says in part, 'this standard sets out specific requirements for food businesses and food handlers that, if complied with, will ensure food does not become unsafe and unsuitable'.

The most important of these is the requirement that potentially hazardous foods be stored below 5°C throughout their storage life.

Correct temperature storage of chilled foods will not prevent their becoming 'unsuitable'. Changes will occur at storage temperatures between 0°C and 5°C both as a result of microbial activity and chemical reaction. These will ultimately limit the shelf life of the food and determine its best-before date. Because of the wide range of foods in this category, which includes cook-chill foods, shelf life studies and safety evaluations are usually assessed via a challenge study rather than from accumulated knowledge as is the case with most frozen foods.

Challenge studies

Challenge testing is a laboratory investigation which is used to determine what can happen to a food product during processing and storage following inoculation with one or more appropriate microorganisms. A challenge study is most frequently used to determine if pathogens will be controlled or to estimate the time it takes for them to grow to potentially hazardous levels, but can also be used for shelf life studies using potential spoilage organisms. This is particularly so for acid products which receive no pasteurisation. Challenge testing is a specialised procedure that is time consuming and expensive but it remains an important procedure in both safety studies and shelf life determination.

Using chilled foods as an example, the aim of a shelf life test should be to measure as far as possible the acceptable life of the product under conditions of time and temperature likely to be encountered during manufacture, distribution, retail storage and in the consumer's hands. Standard 3.2.2 requires that foods in this category, if they can support the growth of pathogens, must be stored at 5°C or lower. Therefore 5°C is the obvious baseline temperature to use during storage trials. However the effect of temperatures fluctuating above 5°C needs examination as the possibility of this occurring in actual practice is very real. It is obviously impossible to duplicate the whole range of temperature abuse which might occur in a product's life time.

A shelf life estimate made at, for example, 8°C is likely to vary significantly from an estimate made at 5°C and some judgment must be exercised in reconciling the two figures. It is essential that a cautious approach be taken until significant real life data can be assessed. Any challenge test will only yield a specific result for the samples under test and the conditions under which the test was conducted.

Domestic Storage

Most shoppers look for the open date marking on foods in retail stores. This is especially the case with short shelf life products usually those purchased from the chill cabinet. They should also check carefully the required storage conditions and endeavour to maintain these as closely as possible on the journey home (see factsheet Handling Food in the Home).

The shelf life as determined by the manufacturer applies to the unopened container. When a container is opened in the home and not all of its contents used at a single time, the excess should be returned as quickly as possible to the manufacturer's recommended storage conditions. Some manufacturers will include in their label recommendations for handling the product after opening with regard to time and temperature.

If they do not, the onus is on the consumer to handle the product hygienically, store it at recommended temperatures and use it within a reasonable time, certainly within the time frame specified by the manufacturer for the unopened container.

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