Ask the Goats: Why Ovens Don't Count

Ask the Goats is a (mostly) weekly feature in which we, the goats, attempt to answer your food preservation questions. Got a question? Drop us a line at dorisandjilly@gmail.com.

Q. I made a beef stew in a 350°F oven. That’s hotter than a water bath, so why can’t I just can the stew as is? Why do I need a pressure canner?—anon (to me, anyway) via Kaela at Local Kitchen

A. This is a great question, just at that point where common sense bumps up against the laws of physics. We say, over and over again, that the heat of a water-bath canner is not sufficient to kill off the kinds of nasties (botulism, I’m looking at you) that can live in low-acid, anerobic environments (i.e., beef stew). But, obviously, a 350°F oven is much hotter than the 212°F of boiling water. So, what’s the problem?

The problem has to do with what’s inside the stew pot. When an oven thermometer reads 350°F, that means that the air temperature is 350°F. Your stew, on the other hand, is going to be hovering at its boiling point. This is a consequence of what’s called the Phase Change Law, which says (more or less), that a substance will maintain the temperature at which it changes phase until the phase change is complete. Water melts at 32°F, and it boils at 212°F. If you put a pot of boiling water in the oven, it’s not going to get above 212°F until all of the water has turned to steam. Then, and only then, can the temperature start increasing.

Now, in beef stew, you’ve got a liquid that, depending on what it has in it (water, wine, stock, etc.) is boiling probably somewhere between 190°F and 212°F. But you’ve also got solids that are probably at a lower temperature. Think of what happens when you put a large roast in the oven. You might cook it for hours, and it might still be at 150°F. All of which is to say that your beef stew is nowhere near 240°F.

But how does the pressure canner do it? Well, when I said that a substance that’s changing phase will maintain an even temperature, that’s not quite true. It will maintain an even temperature under conditions of constant volume and pressure. Crank up the pressure, and the temperature goes up. And hence, safe canning conditions for low acid foods.

Want more canning advice? Check out the liveblogging stream from the panel on canning, preserving, and foraging that I participated in with Marisa from Food in Jars, Hank from Hunter Angler Gardener Cook, and Sean from Punk Domestics at last weekend’s BlogHerFood conference in San Francisco. I had a blast, and I hope those in attendance found it as informative as I did entertaining.

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