How do food jar factories put negative pressure into the jar to begin with?
In: 87
You mean for jarring / canning of food? You heat the jar / can to a temperature that sterilizes the contents and leave a small amount of air at the top. Place the seal / lid on, and as the jar / can cools the volume of the trapped air decreases (because it cools off), creating a vacuum and securing the seal / top to the container.
When I do this at home, I fill the jar with the hot contents (jelly or preserves) up to a line on the jar, I place a lid on, and lightly screw on a retainer ring. Then, I submerge the jar in boiling water and allow it to sit for a certain amount of time (depending on what I’m canning). The gasses at the top expand and leak out a bit. When the time’s up, I pull the jar out with tongs and set it on a towel on the counter to cool. If, after 45 minus, the lid has not deformed (pulled in by the vacuum), then I know the seal is bad and it can’t go on the shelf — I put it in the fridge and use it in the next few days. Those with a good seal will keep a few years on the shelf.
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