Canning is a method of preserving food in which the food is processed and sealed in an airtight container. The process was first developed as a French military discovery. The packaging prevents microorganisms from entering and proliferating inside.

To prevent the food from being spoiled before and during containment, quite a number of methods are used: pasteurization, boiling, other means of high temperature applied over a period of time, refrigeration, outright freezing, drying, vacuum treatment, antimicrobial agents that are natural to the indigenous recipe of the foodstuff being preserved, or otherwise are applied to the contents of the can, a sufficient dose of ionizing radiation, submersion in a strongly saline, acid, base, osmotically extreme (e.g. very sugary) or otherwise microbially challenging environments.

No such countermeasure is perfectly dependable as a preservative. E.g. spore-forming thermo-resistant microorganisms, such as Clostridium botulinum (the causative agent of botulism) can still survive.

From a public safety point of view, foods with low acidity, i.e. pH more than 4.6 need sterilization under high temperature (116-130°C). Foods that must be pressure canned include most vegetables, meats, seafood, poultry, and dairy products. The only foods that may be safely canned in an ordinary boiling water bath are highly acidic ones with a pH below 4.61, such as fruits, pickled vegetables, or other foods to which acidic additives have been added.

History

"An army marches on its stomach." —Napoleon

During the first years of the French Revolutionary Wars, the notable French newspaper Le Monde, prompted by the government, offered a hefty cash award of 12,000 francs to any inventor who could come up with a cheap and effective method of preserving large amounts of food. The new, larger armies of the period required increased, regular supplies of quality food; limitations on food availability were among the factors restricting effective military campaigns to the summer and fall months. In 1809, the French confectioner Nicolas François Appert observed that food cooked inside a jar did not spoil unless the seals leaked, thus developed a method of sealing food inside glass jars. The reason why food did not spoil was unknown at the time, since it would be another 50 years before Louis Pasteur demonstrated the role of microbes in food spoilage. However, glass containers presented challenges for transportation.

Glass jars were largely replaced in commercial canneries with cylindrical tin or wrought-iron canisters (later shortened to "cans") following the work of Peter Durand (1810). Cans are both cheaper and quicker to make and much less fragile than glass jars. Glass jars have, however, remained popular for some high-value products and in home canning. Tin-openers were not to be invented for another thirty years — at first, soldiers had to cut the cans open with bayonets or smash them open with rocks. The French Army began experimenting with issuing tinned foods to its soldiers, but the slow process of tinning foods and the even slower development and transport stages prevented the army from shipping large amounts around the French Empire, and the war ended before the process could be perfected. Unfortunately for Appert, the factory which he had built with his prize money was burned down in 1814 by Allied soldiers invading France.