Most foods degrade and decay over time due to reactions with oxygen, decomposition of the food’s structure or flavour compounds, or microorganisms (or “microbes”) causing the food to spoil. The result can be food that is rancid, smelly, slimy, has lost its colour or flavour, or is growing things like mould.

We apply our understanding of spoilage mechanisms to extend the shelf life of foods by slowing the rate of spoilage. We exclude oxygen by packaging, slow reaction rates by refrigerating, gently heat foods (blanching, pasteurisation) to stop enzymes and to kill microbes. Nonetheless, as consumers we want “fresher”, more natural foods but many fresh and lightly preserved foods will degrade quickly.

Food quality can deteriorate before we perceive obvious signs of spoilage. If allowed to grow to high levels, some microbes that may contaminate foods (collectively called “pathogens”) can cause foodborne illness, or “food poisoning”. Often these microbes don’t visibly spoil the food so we can’t tell if a food has become unsafe.

To protect consumers against poor quality or potentially hazardous foods, governments have introduced “date codes” to help consumers evaluate the “freshness” of foods. Food producers also embrace date codes to help them provide safe, high quality products to consumers.

In Australia and New Zealand, we specify “use-by” and “best-before” dates for perishable and semi-perishable foods that are ready-to-eat without cooking. Food expected to remain wholesome for more than two years, in unopened packages, doesn’t require these labels and there are other types of date codes for bread.

Similar regulations operate in European Union countries, but the United States and other nations have a plethora of food date codes that don’t readily correspond with our “use-by” and “best-before” dates. In general, there is little harmonisation internationally for food date codes which has been identified as a barrier to trade. Perhaps unsurprisingly, surveys in the United Kingdom and US showed that many consumers don’t understand the meaning of various date codes on foods.

Use-by

A “use-by” date is required for foods that might, over time, support the growth of certain pathogens under the specified storage conditions. A “use-by” date, then, relates to food safety and indicates the time when the food may become less safe to eat.

Foods cannot legally be sold, and should not be consumed, beyond their “use-by” dates and should be discarded. “Use-by” dates are also required on specially formulated foods for medical purposes given, for example, to people with inherited metabolic disorders whose dietary needs cannot be met by a normal diet.

For these foods, the “use-by” date indicates when essential nutrients in the food will have degraded to levels so low that the food no longer provides the intended therapeutic benefit.

Best before

A “best-before” date provides advice about the expected quality of the product and is the time beyond which, in the food manufacturer’s opinion, the product will no longer be in optimal condition. It will, however, probably still be edible and nutritious for some time after that date.

Food producers usually apply conservative “best-before” dates to ensure that consumers have enough time after purchase to use the food in good condition. Consumers can exercise judgement about foods beyond their “best-before” date: if it looks OK, and smells OK, its probably OK to eat, though not at its best.

This approach must not be used for foods beyond their “use-by” date, since pathogens can be present without affecting the sensory qualities of the food.

Waste not, want not

Accurately specifying “use-by” and “best-before” dates is challenging, requiring extensive scientific trials and/or knowledge because of the range of foods, spoilage processes and food preservation technologies.

Expert guidance is available to food producers from a number of sources. Sometimes, however, there’s uncertainty about whether a “use-by” or “shelf life” date should be applied if its not known whether pathogens are realistically likely to be present, and able to grow, in the particular type of food or not.

Our capacity to produce enough food for Earth’s population is becoming a real concern, yet 20-30% of the world’s food is wasted. A consequence of confusion about the correct specification of “best-before” and “use-by” dates is that some foods are unnecessarily discarded even though they are safe to eat and nutritious.

Developing appropriate “use-by” and “best-before” dates could help reduce this wastage. Helping consumers to minimise their wastage of food through understanding of “best-before” dates offers another small, but potentially useful, contribution to global food security.

As an additional tip to reduce wastage, if foods are unlikely to be consumed before the “use-by” or “best-before” date, they can be frozen (which essentially stops spoilage processes) and thawed for consumption later, provided they’re eaten soon after thawing.

Dr. Wayne Anderson, Director Food Science and Standards Division, Food Safety Authority of Ireland, co-authored this article. Wayne is also a member of the International Commission on Microbiological Specifications for Foods.