Question

Explain why it is difficult to remove lid from a preserving jar which was closed when the food was full of steam?

Answer 1

As it cools, the air in the space above the conserving contracts, forcing the safety button on the lid to drop. This forms a tight seal that makes the lid harder to remove. This is the seal that prevents the fear going off and is essential to give it a long shelf life.

Answer 2

As the contents cool, the pressure of the air left inside the jar falls and the metal on the lid pops downwards. The greater atmospheric pressure outside the jar keeps the metal button depressed and makes it more difficult to open the jar.