I just read this article by Marisa from Food in Jars, which is a quick rundown on some canning techniques which are considered unsafe, and I thought that it was important enough that I should do my own version of it since I've seen these techniques and more used by friends and family. And of course because it is that time of year again and lots of people are hauling out their canners. I will not go into the full details of the techniques which are considered safe, so please do not treat this as any sort of "How To". In fact, one thing I always say to people over and over again, is that if you have never read a good modern book on home canning, you should absolutely not be doing it! And like Marisa says in the article I linked to, I also tell people that you should never, ever, ever learn home canning from a previous generation, because they almost certainly use techniques which are no longer considered safe today. Do not get me wrong - I love my mom, rest her soul, but I would never can anything the way she did. Same goes for my wife's grandmother - the dear sweet woman that she is.
What is Safe?
First let us take a look at what is considered safe. Basically, 2 techniques - one is called "boiling water canning", and the other "pressure canning".
Boiling Water Canning
With this technique, you have a large pot that is a fair bit taller than the tallest jars you want to use. You fill the jars with your preserves, put them into the pot, and then fill the pot with water so that the jars are submersed with at least an inch of water over the top of them. Then you boil - or "process" in the vernacular - the whole thing for a given period of time as specified in your recipe, and according to the size of jars you are using - larger jars requiring more processing.
This method is only suitable for high acid (low pH) foods. The reason is that one of the key nasties in home canning is botulism, and while the live bacterium is killed by boiling water, the spores are not. But high acid (low pH) does denature the spores such that they are not a risk.
Pressure Canning
With this method you fill your jars with the preserves, then put them into a pressure canner. A pressure canner is basically the same thing as a pressure cooker except that it is typically larger, and has a pressure guage on the lid so that you know exactly what the pressure is inside. The books I've read say that if you do not have a guage, you should not use it for canning. Unlike the boiling water method, you do not need to cover the jars with water because it is the pressure that is doing the work for you. So typically you put in enough water so that it will not boil dry while processing - typically an inch below the lids of the jars. With a tall enough pressure canner you can even put a trivet on top of the bottom layer, and stack more jars on top.
Then you turn on the heat, and once it starts streaming steam out the top you time 7 minutes before affixing the "pet cock". Once affixed, the canner will come up to pressure - either 5, 10 or 15 psi according to your recipe. Once at pressure, you start timing. Once done, you allow to come completely down to room temperature before opening.
This method is safe for all foods - even those which are high pH (low acid) because the high pressure denatures the botulism spores, as well as the bacterium itself. However, you may not want to use it for high acid foods because the high pressure can create a texture in the finished goods which is not as appealing. I personally use pressure canning for all my preserves just because I find it easier to always use the same technique.
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