Home > Shelf-life testing – ‘Use-by’ dates for food safety

When is date marking required?

If food must be eaten within a certain period for health or safety reasons a use by date is required:

• The ‘health reason’ is applicable to food that is intended by the manufacturer to form the sole source of nutrition for a person’s diet for a specified period. This will apply where storage affects the critical nutrient profile of products such as infant formula or special dietary foods manufactured to provide the sole source of nutrition to persons who are ill or are unable to eat normal foods.
• The ‘safety reason’ is applicable to food that can become microbiologically unsafe before the food noticeably spoils. This will not apply to shelf stable, frozen or most raw foods but may apply to certain chilled ready-to-eat foods, for example chilled meals and salads. Development of use by dates for safety reasons is discussed below.

A best before date is applicable to food where deterioration affects consumer acceptance without impacting on health and safety. Many product changes will affect consumer acceptance, including:

• rancidity
• texture changes
• moisture loss
• moisture gain
• staling
• flavour loss
• light induced changes
• enzymatic browning
• chemical browning, and
• microbial spoilage

There is an exemption from best before date marking for products with a shelf life of two years or longer. A scan of supermarket shelves identified few products that were not date marked. These included sugar, golden syrup, treacle, honey, some soft drinks, canned fruit, canned vegetables, canned soup, vinegar, some canned fish and some cook-in sauces. Apart from these products, a shelf life of two years or longer is uncommon. While is it not mandatory in Australia some international food regulators recommend that all food be date marked to support ‘first in – first out’ food consumption.

How is shelf life determined?

There are many answers to this question, depending on the type of deterioration that the product undergoes. The NSW Food Authority has food safety as a primary objective and so considers reliable use by dates to be essential. If use by or best before dates are overstated there is a substantial risk of damage to a business’s reputation and brands. Food businesses must attend to all date marking requirements but this document will focus on the setting of reliable use by dates.

Product development

Product development must be well advanced before a business can have confidence in shelf-life estimates. The business must be able to reliably produce homogeneous product with consistency from batch to batch. Physical and chemical factors that impact on the capacity of bacteria to grow, such as pH, water activity and evenness of mix (distribution of moisture, salt, preservative or food acid) must be well controlled. The physical, chemical and microbiological profile of several batches of finished product must be established. The potential for product contamination must be evaluated. The packaging materials must be identified.

Product stability and food safety:

Initial studies are likely to consist of storage trials under the recommended storage conditions. Refrigerated storage trials should be run at 5°C and under conditions of mild temperature abuse equal to what might be encountered in the commercial cold chain. Products should be inspected at suitable times and samples tested for stability of the critical physical and chemical characteristics. These trials also provide an opportunity to commence microbiological tests for both spoilage organisms and the cold-tolerant pathogens named in the flow chart above. The trials should continue beyond the targeted shelf life unless the product fails earlier.

Subtle changes during storage can be significant:

• Does pH change eg due to microbial growth or failing buffering systems?
• Is water redistributed within the product eg due to syneresis, condensation or a broken emulsion?
• Is the preservative system stable?
• Are there any signs of surface growth of microorganisms?
• Does any modified atmosphere persist for required times?
• Do chemicals migrate from the food-contact materials into the food?
• Are there any unforeseen changes to the product?

The trials should lead to an understanding of target levels and ranges for the critical physical and chemical characteristics of the product over the intended shelf life. The worst case or least restrictive values (maximum or minimum as the case may be) of the key chemical characteristics can then be compared to published values to evaluate the opportunity for microbial growth, particularly for the key cold-tolerant pathogens.

Challenge tests:

A challenge test is similar to the microbiological testing used in storage trials mentioned above. The difference is that specific cold-tolerant pathogens are added to the product prior to packaging. Storage tests on products where pathogenic bacteria are only occasionally present are likely to be misleading. The design and evaluation of challenge tests should be done by an expert food microbiologist.

If ‘growth – no growth’ is not clear from other studies then challenge tests can be very useful. Their use is not required where it is obvious that either growth won’t occur or growth will occur. Challenge tests are used to clarify areas of uncertainty and verify growth models for products with chemical characteristics near to the growth – no growth border.

Cultures of pathogenic bacteria should never be introduced into a food processing factory. Challenge tests should only be done in an off-site laboratory or research facility.

Expiry testing program:

Retained samples from commercial batches of product should be tested for microbiological and chemical quality upon expiry of shelf life. Newly launched products should be tested more frequently while proven products should be tested occasionally. Samples for testing should be stored under realistic rather than ideal temperature conditions. Expiry testing provides further assurance that manufacturing systems are under control.

Use by estimation – an auditable program:

A series of linked documents can be used to provide objective evidence that use by dates are reliable.

Product specification: this should list the physical, chemical and microbiological characteristics of the product. In some cases the document might specify both release (start of shelf life) and expiry (end of shelf life) criteria. The specified criteria should be traceable by formulation / recipe / block code to product development records.

Even seemingly minor changes to ingredients, manufacturing procedures, packaging or distribution can have adverse effects on product safety and use by dates. The shelf life of the revised product should be verified, for example, by expiry testing of trial batches.

For the same reason, similar precautions should be taken when applying the same use by date to a family of products with very similar characteristics. The product where growth is least restricted should be used in the shelf-life study.

The specification should also correlate with quality assurance test results for manufactured batches. The specification should include recommended storage conditions and the shelf life. This should be traceable to the shelf-life study.

Shelf-life study: this should summarise the storage trial results and any required microbial growth modelling and challenge tests. The report should justify the shelf-life estimate.

Expiry testing records: quality assurance test results on retention samples stored for the product’s shelf life at realistic temperatures should verify the shelf life claims.

Determination of best before dates:

There are potentially many questions to be answered about product deterioration and many approaches to getting the answers. If products have a long shelf life then real time testing is not likely to fit in with product development cycles. In many cases accelerated stability testing can be used to estimate shelf life; but technical expertise is a necessity. Real time testing— after the product has been launched if necessary—should be used to verify estimated best before dates.

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