When canning was invented, no one knew why food packaged and heated inside cans was kept fresh for a long time.
Then, in 1860, French scientist Louis Pasteur discovered that food spoilage was due to bacteria present in all fresh food and air. These organisms contaminate the substances in the food and make it useless and harmful to man.
It was then understood how canning worked to conserve food: Heat kills bacteria in food, and hermetic sealing of the can prevents more bacteria from entering the contents. The food inside a can will be fresh until the container is opened.
In the 1920s, U.S. companies found that liquid packaging of food kept it much fresher and more nutritious. That's why most canned goods now contain water. All cans are also vacuum-packed, meaning that all the air has been removed from the can before it is hermetically sealed. This prevents bacteria in the air from contaminating the food.
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