Imagine putting Tupperware in the fridge. When the air inside the container gets cold, it pulls the lid in tighter.
When the pickles are made, they are canned at a high enough temperature to kill germs, that’s important.
When the hot air inside the jar cools down to room temperature, it sucks the button down.
When you hear it pop, you know it was canned at a high enough temperature to kill the germs AND not opened since then.
I actually know a more precise answer from canning with my Grandma.
If you’re canning a food, like pickles or jams, using Kerr jars, one of the last steps is to heat up the food before putting it in the container. This not only cooks to food to help prevent botulism, but also creates a low pressure zone inside the jar due to the heat, so that when everything cools back down, the lid self-seals onto the jar until it’s opened again.
When the jar is sealed, there is a vacuum on the inside. This means the pressure outside the jar (air in the room) is greater than the pressure on the inside. The outside air exerts a force on the lid, just as if your finger would when you try to hold down the poppy thing. When you open that jar, air rushes in and the poppy thing is raised, because the pressure has equalized. Afterward, putting the lid back on just traps the air with the same pressure inside, so there is no force pushing down on the poppy thing.
the bottling process creates a lower pressure environment inside the jar that causes the air outside the jar to press the button down. once you open the jar for the first time, the air inside the jar mixes with the air outside the jar and they are equal. so unless you can seal the jar again and then lower the pressure inside the jar, it’s never going to pop again.
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