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

Pressure, volume and temperature are all related. If volume remains the same but temperature goes down, then pressure must also go down too.

So by heating up the contents before sealing, the later cooling of those contents to room temperature will cause the pressure in the sealed container to become lower than ambient pressure.

If you ever do any home canning, you can hear when it happens because the kids make neat little pinging sounds when they pop in.