Canning was invented as a way to preserve food for Napoleon’s army. It was found that food that was cooked in sealed jars would not spoil. In those days they did not understand that they were killing off germs but they could see the salutary results nonetheless. Sometime in the 19th century, glass jars were replaced with lightweight, unbreakable metal cans.
Today, food is still cooked inside cans. The equipment that the cans are cooked in is called a retort. Sometimes cans are packed with hot product. As the product cools, it shrinks and creates a vacuum in the can. If the product is not packed hot, a vacuum sealer, which evacuates the can as it seals it, can be used. Sometimes, rather than a vacuum sealer, a steamflow sealer is used. This injects steam inside the can as it seals it. As with hot pack, the steam soon shrinks in volume and creates a vacuum.
In a few cases, glass jars are still used. Cappers, as built by Continental White Cap, inject steam in the jar as they cap it, thereby creating a vacuum. The caps have special lugs to make it possible to overcome the vacuum when the jar has to be opened.
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