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

Just put the contents into the jar ehile it’s hot. Put the lid on and flip it upside-down. Make sure the heat difference between the jar and the contents isn’t too great, else it will burst or crack. The hot content reduces in size while it cools down, creating the negative pressure. Flipping the jar upside-down also sterilizes any remaining air as the hot contents pass through it.

Source: making jams at home.