Why are glass jars generally vacuum-sealed? How is the vacuum created during the production process?

Jars are vacuum-sealed in order to extend the preservation period for the contents (shelf life). There are three different methods of vacuum-sealing:

• mechanical or dry vacuum-sealing: using bell jars connected to pumps which remove the air (mainly for dry products).
• steam vacuum-sealing: this is the most commonly used system. A steam jet is injected into the container before sealing it;
• venting vacuum-sealing: once the jar has been filled with the product, it is closed in a tunnel or autoclave; a controlled increase in temperature allows the release of the air contained between the product and the lid.

There is another method which is currently used in households to pack products (e.g. fruit jam) which consists in filling the pot with the very hot product almost up to the brim with no empty space left at the top. When the pot is closed, the product cools down and automatically creates a vacuum seal.