The Invention of Canning
Canning is a process that was invented at the behest of Napoleon and the French Army. In 1809 a man named Nicholas Appert developed a method of cooking food inside a sealed glass jar. The food did not spoil unless the seal was broken. Glass was not very durable, however, so the tin cylinder that has been associated with canning ever since was soon introduced. Canned tuna was first marketed in the United States in 1903.
Why Canning Works
Canning works by sealing the food (in this case, tuna) and whatever air there may be inside a container and then cooking it. The cooking sterilizes the interior contents, killing the bacteria that cause spoilage. So long as the can remains undamaged, new bacteria cannot reach the food, causing spoilage. Some foods require different, additional measures, such as salts or acids, and some companies use ionizing radiation in place of cooking. Tuna, for example, is one of the foods that must be canned under pressure.
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Not all bacteria can be killed by cooking, at least not at the temperatures used in canning. For example, a member of the botulism family can survive the canning process.