How does a vacuum form inside a canned-vegetable container?

A vacuum forms inside a canned-vegetable container primarily through the process of exhausting and subsequent cooling.

To create this vacuum, processors use several methods:

• Exhausting: Before the container is sealed, air must be removed to prevent spoilage and pressure buildup. This is achieved by passing filled cans through a hot-water or steam bath, which heats the contents, causing them to expand and driving out the remaining air. Alternatively, some products are mechanically exhausted using specialized vacuum-can sealing machines.12
• Sealing: Once the air is removed, the container is hermetically sealed while still hot.31
• Cooling: After the sealed container undergoes heat sterilization, it is cooled to room temperature. As the contents cool, they contract, which creates a partial vacuum within the container.31

This internal vacuum is a standard feature of properly processed canned goods, often resulting in slightly concave can ends. If a can is bulged, it may indicate a loss of this vacuum due to mechanical, chemical, or physical defects, and such products should not be consumed.43