Pressure cooker and canner basics

Following are excerpts from a conversation that I recently had with HistoricFoodie:

Rod Franklin wrote: I think the pictures show a pressure canner, just not a very big one, and then they've misnamed it a cooker instead of a canner.

So, if the main difference between a cooker and a canner is the size, and because of that you need to change the times, wouldn't you need to do the same with a smaller presser canner like the one shown?

I'm confused.

I have to disagree with you somewhat, Rod.

It's true that the operating method is the same for both. Heating water in a pressure vessel allows it to reach higher temperatures than normal.

However, all reputable recipes for canning include both the heat-up and cool down times as part of the processing. And that's where the differences lie.

Here's a representative example. Caveat: All figures are made up out of thin air, and are not part of a real recipe.

Let's say you are canning venison. The recipe says to work at 10 psi for 90 minutes. That 90 minutes starts when the weight starts to jiggle or the gauge says 10 psi, not when you close the pot.

So let's assume it takes 20 minutes to reach the 10 psi point. 90 minutes later you turn off the heat. It then takes 30 minutes for the pressure to drop and you can open the lid.

Those 50 minutes are part of the processing time.

If you repeat the recipe in a canner, it might take only 5 minutes to heat up. And only 15 or 20 to cool down. So you have eliminated 30 minutes worth of measured processing.

Among food safety professionals that is considered an unsafe practice.

Is it actually unsafe? Perhaps not. But safe, in this context, is defined as having the food reach, and be sustained at, 240F. And you just don't know if that's the case in a smaller cooker.

The key here, as in anything to do with food preservation, is to understand the why of the rules. Then you can make an informed decision as to whether to slavishly follow the rules or not.

I agree with you that the unit shown in the pix is probably a small canner, a size that is now considered outmoded. Chances are the original instructions came with it's own set of recipes. Modern recipes, from Ball, the National Center, and even USDA are based on canners with capacity ranging from 19-22 quarts.

An interesting side-not on pressure measurements. Most recipes actually have two different pressure recommendations, depending on the type of unit being used. Those with jiggler weights specify 10 psi, because the weights are scaled for 5, 10, and 15 psi. For units with gauges, the recipes specify the more precise 11 or 12 psi.

What this means in practical terms is that 10 psi is the minimum safe processing pressure. Going higher could, as you note, have effects on the quality of the food. But it won't be an unsafe practice.

The one thing to avoid is having the pressure fluctuate greatly, up and down, over the course of the processing time. Doing so can lead to some surprises, such as finding soup in the bottom of the pot because the jars overflowed due to the pressure changes.