What makes canning lids seal?
At the time food is packed into canning jars, the head space is filled with air. There is also some air in the food, especially when fruits or vegetables are packed raw. During processing, air is driven from the jars and the head space fills with steam. After processing as the jars cool, the steam condenses, leaving behind a vacuum. The vacuum pulls the lid down to form a vacuum seal. When you open a vacuum sealed jar, you may hear a hissing sound. That is the sound of air rushing into the jar to fill the vacuum.
Barbara Willenberg, Nutritional Sciences, University of Missouri-Columbia
Last update: Tuesday, May 05, 2009