Canning is a way to store food for long periods of time. It is a method of
preserving where food is placed in airtight, vacuum-sealed containers and heat
processed at 250 °F (121 °C). This destroys microorganisms and inactivates
enzymes. As the food cools, a vacuum seal is formed that prevents any new
bacteria from getting in. Since the food in the container is commercially
sterile, it does not spoil. Once the container is opened, however, bacteria
can enter and begin growing in the food. Any unused portions must then be
refrigerated in clean containers.