Shelf life is that length of time that food, drink, medicine and other perishable items are given before they are considered unsuitable for sale or consumption. In some regions, a best before, use by or freshness date is required on packaged perishable foods.
Shelf life is the recommendation of time that products can be stored, during which the defined quality of a specified proportion of the goods remains acceptable under expected (or specified) conditions of distribution, storage and display. Most of these labels do not guarantee the safety of food and should be used more as a guide.
Shelf life is most influenced by several factors: exposure to light and heat, transmission of gases (including humidity), mechanical stresses, and contamination by things such as micro-organisms. Product quality is often mathematically modelled around a single parameter (concentration of a chemical compound, a microbiological index, or a physical parameter), though this approach can miss relevancy .
For some foods, the shelf life is an important factor to health. Bacterial contaminants are ubiquitous, and foods left unused too long will often acquire substantial amounts of bacterial colonies and become dangerous to eat, leading to food poisoning. However, the shelf life itself is not an accurate indicator to the food safety. For example, pasteurized milk can remain fresh for five days after its sell-by date if it is refrigerated properly. In contrast, if milk already has harmful bacteria, the use-by dates become irrelevant.
The expiration date of pharmaceuticals specifies the date the manufacturer guarantees the full potency and safety of a drug. Most medications are potent and safe after the expiration date. A rare exception is a case of renal tubular damage purportedly caused by expired tetracycline. A study conducted by the U.S. Food and Drug Administration covered over 100 drugs, prescription and over-the-counter. The results showed that about 90% of them were safe and effective as far as 15 years past their expiration date. Joel Davis, a former FDA expiration-date compliance chief, said that with a handful of exceptions - notably nitroglycerin, insulin and some liquid antibiotics - most expired drugs are probably effective.
Preservatives and antioxidants may be incorporated into some food and drug products to extend their shelf life. Some companies use induction sealing and vacuum pouches to assist in the extension of the shelf life of their products.
Some degradation factors can be controlled by provisions in the ed packaging. For example, the amber bottle used for many beers blocks damaging wavelengths of light. Transparent beer bottles do not. Packaging with barrier materials (eg, (low moisture vapor transmission rate, etc) extends the shelf life of some foods and pharmaceuticals.
These breakdown processes characteristically happen more quickly at higher temperatures. The usually quoted rule of thumb is that chemical reactions double their rate for each temperature increase of 10 Celsius degrees (C°) because of activation energy barriers become more easily surmounted at higher temperatures. However, as with all rules of thumb, there are many caveats and assumptions. This particular one is most applicable to reactions with activation energy values around 50 kJ/mole; many of these are important at the usual temperatures we encounter. It is often applied in shelf life estimation, sometimes wrongly. There is a widespread impression, for instance in industry, that "triple time" can be simulated in practice by increasing the temperature by 15 C°, e.g. storing a product for one month at 35 °C simulates three months at 20 °C. There is enough variation that this practical rule cannot be routinely relied upon.
The same is true, to a point, of the chemical reactions of life. They are usually enzymatically catalyzed which changes reaction rates, but with constant catalytic action, the rule of thumb is still mostly applicable. In the particular case of bacteria and fungi, the reactions needed to feed and reproduce increase at higher temperatures, up to the point that the proteins and other compounds in their cells themselves begin to break down, or denature, so quickly that they cannot be replaced. This is the reason high temperatures kill bacteria and other micro organisms; 'tissue' breakdown reactions reach such rates that they cannot be compensated for and the cell dies. On the other hand, 'elevated' temperatures short of these result in increased growth and reproduction; if the organism is harmful, perhaps to dangerous levels.
Just as temperature increases speed up reactions, temperature decreases reduce them. Therefore, to make explosives stable for longer periods, or to keep rubber bands springy, or to force bacteria to slow down their growth, they can be cooled. This is the reason shelf life is generally extended by temperature control: (refrigeration, insulated shipping containers, controlled cold chain, etc) and the reason some medicines and foods must be refrigerated.
Best before is sometimes indicated on food and drink wrappers, followed by a date, and is intended to indicate the date before which the supplier intended the food should be consumed. The term best before is similarly used to indicate the date by which the item will have outlived its shelf life, and is intended to ensure that customers will not unwittingly purchase or eat stale food. Sometimes the packaging process involves using pre-printed labels, making it impractical to write the best before date in a clearly visible location. In this case, a term like best before see bottom or best before see lid might be printed on the label and the date marked in a different location as indicated.
Best Before is usually advisory and refers to the quality of the product, in contrast with Use By which indicates that the product is no longer safe to consume after the specified date.
Generally, foods that have a use by date written on the packaging must not be eaten after it has expired. This is because such foods usually go bad quickly and may be injurious to health if spoiled. It is also important to follow storage instructions carefully for these foods (for example, product must be refrigerated).
Foods that have a best before date are usually safe to eat after the date has passed, although they are likely to have deteriorated either in flavour, texture, appearance or nutrition.
Bathroom products/toiletries usually state a time in months by which, once the product is opened, they should be used. This is often indicated by a graphic of an open tub, with the number of months written inside (e.g., "12M" means use the product within 12 months of opening).
Most stores will rotate stock by moving the products with the earliest dates to the front of shelving units, which allows them to be sold first and saving them from having to be either marked down or thrown away, both of which contribute to a loss of profit.
The Agriculture Dept., which regulates fresh produce and meats, only requires labeling of the date when poultry is packed at the farm. However, many manufacturers are allowed to also add sell-by or use-by dates.
Beer is perishable. It can be affected by light, air, or the action of bacteria. Although beer is not legally mandated in the USA to have a shelf life, freshness dates serve much the same purpose and are a marketing tool.