Jam

Good citizenship blog entry…

I was asked about sealing jam jars in my capacity as a scientist because the internet is full of misinformation on the subject.

The idea is to put the lid on while the jam is hot.

So long as you have a seal, as the jam shrinks whilst it cools (roughly ten percent in volume I guesstimate), you will create a low pressure air gap in the jar.

A vacuum is over-stating things. At best you will have half the atmospheric pressure. That is, the air that you sealed in the jar occupies around twice the original volume

This is enough to invert a thin metal lid which is a good indicator that the gas tight seal remains functional.

I should note that the pressure differential over the seal actually helps keep it tight; it pulls and pushes the flexible polymer into the glass if the seal is properly designed.

In these conditions bugs and excess moisture cannot get to the jam. And the air and water in the jar was hopefully sterilized prior to sealing by the high temperatures. Therefore the jam should last quite a while.