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.

What is Unsafe?

There is a long list of things that people used to do, or even still do today, but are not considered safe. In all cases the problem is that the jars have not been brought up to boiling temperatures after the lids have been affixed - so there is a chance of spoilage. Deadly spoilage. I can honestly say that in my 15 years of canning and using proper techniques, I have never had a single spoiled jar. Not one. (knock on wood :-))

Open Kettle Canning

This is a bit of a funny name which is non-intuitive, but a lot of people do it without knowing the name. They basically prepare a hot preserve - let's say jam - and then just fill the jars while the mixture is still really hot, then affix the lids and let it cool. This will often produce decent seals on the lids, but it is not considered safe, so please do not do it!

Wax Seals

This is what my dear mom used to do with her jams - it is basically like the Open Kettle method but before the lids are affixed, melted wax is poured into the top of the jar. It is not safe, so please do not do it.

Inverted Jars

This is once again another version of Open Kettle. The jars are filled, lids affixed, and then the jars inverted and left to cool. Not only is this not a safe way to preserve, it is also dangerous because if that lid is not affixed properly, when you invert the jar you will get boiling hot liquid squirting all over the place! I know this because I've had it happen! I actually use this method to prolong the life of an opened jar of pasta sauce in my fridge. It works really well for doing that but again I am storing it in my fridge. After opening a jar of my pasta sauce, I put it in the microwave and cook it on high long enough for it to come up to boiling, and then boil for 30 seconds to a minute. I then take it out, affix a lid very tightly, and invert the jar an let it cool. Once cooled, it will store in the fridge weeks longer than it would have if I'd just put the lid back on and put it in. But you must put it into the fridge!!! In any case, as mentioned, I did this once with a used snap lid that did not seal very well, and got a heck of a scalding burn as a result when I inverted the jar.

Steam Canners

I'd never even heard of these and do not know much about them, but Marisa mentions it so I will as well.

Oven Canning

This is one that Marisa did not mention, but it is definitely not safe. The books I've read go into detail as to just why it is not safe, so I will leave that as an exercise to the reader.

Dishwasher Canning

I was utterly shocked about a year ago when a friend of mine said he saw this on a TV. Wow. Just do not do it, OK?

Further Reading

Here are some good resources for you - in my opinion you absolutely need to read at least one of these from cover-to-cover before you dabble in home canning. Better yet, read several. Check your local library if you do not want to buy a copy.

• USDA Home Canning Guide is an excellent free book that you can download and read. It is free, so you have no excuse not to!
• Putting Food By was my first book on Canning, and I am very glad to see that it finally has a new revision out as of a couple of months ago! The 1994 edition that I used is actually getting kind of dated - I'm going to go out and get myself a copy of the new one, in fact.
• The Ball Blue Book is very highly regarded and recommended, although I have not read it myself.