Product quality is often mathematically modelled around a parameter (concentration of a chemical compound, a microbiological index, or moisture content).
For some foods, health issues are important in determining shelf life.
The concept of expiration date is related but legally distinct in some jurisdictions.
Shelf life is the recommended maximum time for which products or fresh (harvested) produce 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. High-acid canned foods (tomatoes, fruits) will keep their best quality for 12 to 18 months; low-acid canned foods (meats, vegetables) for 2 to 5 years. A product that has passed its shelf life might still be safe, but quality is no longer guaranteed.
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One major exception is the Shelf Life Extension Program (SLEP) of the U. Department of Defense (Do D), which commissioned a major study of drug efficacy from the FDA starting in the mid-1980s. The SLEP and FDA signed a memorandum that scientific data could not be shared with the public, public health departments, other government agencies, and drug manufacturers.
The SLEP discovered that drugs such as Cipro remained effective nine years after their shelf life, and, as a cost-saving measure, the US military routinely uses a wide range of SLEP tested products past their official shelf life if drugs have been stored properly.
Shelf life depends on the degradation mechanism of the specific product.
Most can be influenced by several factors: exposure to light, heat, moisture, transmission of gases, mechanical stresses, and contamination by things such as micro-organisms.