How does home canning work?

Below I talked a bit about what the canning process does to rid a jar of micro-critters, but the physics of how canning works is every bit as interesting. Wait — where are you going? There’s no need to…what do you mean you’d rather be water skiing? Don’t hit that quit button, I was just starting to…

I’ll be serving Ovaltine later!

Dang. Just us nerds again. Oh well, at least I won’t feel self-conscious wearing my pocket protector. But I’m not lying when I say that canning is interesting stuff. As you probably already know from having received a jar of home made jam or fuit preserves before, a home canning rig consists of three parts: a jar, a flat lid with a rubber lining on the underside and a screw-on metal ring that holds the lid on while the jar is in the canner.

The way it works is as follows. Once the jar is placed in the boiling water canner, it begins to absorb heat. That heat soon creates steam in the inside of the jar in the “head space” or air gap between the food and the lid. Since steam takes up much more space than water (about 1600 times), it begins to push outward on the lid. Even though the lid is being held on tightly by the ring, it can’t prevent some of the steam — and some of the air in the head space — from escaping.

When the jar is ultimately removed from the canner, the contents of the jar being to cool. As that happens the steam that once occupied the head space condenses back into water, only now there’s less air in the jar than there was when the process started. A vacuum is thus created in the jar which pulls inward on the lid , holding it fast even when the metal ring is removed. (Actually, technically, it’s the weight of the atmosphere that pushing the lid in, but that’s WAY too nerdy, even for this discussion).

Pretty darn cool, and why you often see home-caned foods with only the lids on and not the rings (though between you and me it’s better to leave them on, especially if you’re going to transport them).