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

Canned and in-glass foods are heated in a retort to over boiling temperature. They are then sealed. While being heated, any air is displaced by steam/water vapor. After sealing the steam condenses. Water vapor/steam has about 1700 times the volume of liquid water. As the volume decreases, a partial vacuum is formed. That’s what you perceive as ‘negative’ pressure.