How do food jar factories put negative pressure into the jar to begin with?

Question

How do food jar factories put negative pressure into the jar to begin with?

In: 87

Answer 1

Hot air is less dense than cold air, so if you heat the jar first then you’ll close the jar with hot air in it, and when it cools the hot air shrinks, thus creating a negative pressure differential.

This is part of the reason why some jars may become much more difficult to open after storing in the fridge, as you filled the jar with room temp air when you opened it last, then when the air shrank in the jar after cooling in the fridge, it formed a sort of seal in the jar.