I store whole coffee beans in a mason jar. The jar has a screw ring that retains a lid with an integral rubber seal. This jar is meant for canning and, even without processing, the seal is very tight-- airtight, apparently.
On a daily basis, i haven't noticed anything. However, i went away for a week. When i opened the jar on the first morning back, the lid and ring flew out of my hands, accompanied by a loud "foomf" noise. Obviously, the beans are outgassing something. What? What's going on?