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Shelf life is the length of time that corresponds to a tolerable loss in quality of a processed food and other perishable items. Alternative terms are sell by date or best before date, both usually used in reference to foods with attached "do not sell past X" labels or some equivalent. In some jurisdictions, such dates are required on all packaged perishable foods. In the US, milk is an example; it universally has a sell by date.
Shelf life is different from expiration date: the first one relates to food quality, the latter to food safety. A food that has passed its shelf life is still safe, but optimal quality is no longer guaranteed.
Shelf life is most influenced by five primary events: light transmission, gas transmission, heat transmission, humidity transmission, or mechanical stresses. Product quality is often mathematically modeled around a single parameter: the concentration of a chemical compound, a microbiological index, or a physical parameter.
Under some circumstances, the shelf life is critical to health. Some medicines begin to deteriorate (eg, in potency) or begin to accumulate toxic breakdown products immediately after manufacture or packaging. Depending on the material involved, this can be dangerous to life. Bacterial contaminants are ubiquitous, and foods left unused too long will often acquire substantial amounts of bacterial colonies and become dangerous to eat. Food poisoning is the result, and can be fatal.
Nearly all chemical reactions will occur (at various rates depending on the individual nature of the reaction) at common temperatures. Examples are the breakdown of many chemical explosives into more unstable compounds. Nitroglycerine is notorious. Old explosives are thus more dangerous (i.e., liable to explode without warning) than recently manufactured explosives. Rubber products also degrade as sulfur bonds induced during vulcanization revert; this is why old rubber bands and other rubber products soften and get sticky as they age.
These breakdown processes characteristically happen more quickly at higher temperatures. The usually quoted rule of thumb is that chemical reactions double their rate for every 10 degree Celsius increase in temperature. The reason has to do with activation energy barriers.
The same is true, to a point, of the chemical reactions of life. 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 breakdown so quickly that they cannot be replaced. It is the reason why 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 increase speeds up reactions, temperature decreases reduce them. Therefore, to make explosives stable longer, or keep rubber bands springy, or force bacteria to slow down their growth, they can be cooled. This is the reason why shelf life is generally extended by refrigeration and the reason some medicines must be refrigerated; the breakdown reaction paths at room temperature are so rapid the medicine becomes unusable very quickly. Only refrigeration keeps them potent long enough to be practical.
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