Why are cans sealed while hot during vegetable canning?

In the process of canning, vegetables are heated—often through a steam or hot-water bath—before the container is sealed to expand the food and drive out remaining air.1

This step, known as exhausting, is critical because it ensures that once the can is sealed, sterilized, and subsequently cooled, the contraction of the contents creates a partial vacuum within the container. This vacuum is a key indicator of a properly sealed product; it causes the ends of the can to remain slightly concave, whereas bulging ends may indicate a loss of quality or potential spoilage. By removing as much air as possible, processors also create anaerobic conditions that inhibit the growth of oxygen-dependent microorganisms.123