Part 2 in my little primer on home canning. For Part 1, click here.
I should mention that this post will be geared toward "How does it work?" rather than "How do I do it?" That will come next! Probably in a few days, because I’d like to take some stunning (heh) photos to illustrate the process and it’ll take me a bit to do that…but never fear! I’ll get there!
So! How does it work? Well, you’ve seen mason jars, right? The kind with the two-piece lid consisting of a flat bit and a screw band? That’s the key to home canning. What happens is this: you heat the glass jars, and the food to be canned, whether it’s scrumptious strawberry jam or perfect pickles (yeah, I just did the alliteration thing) and put the hot food in the hot jars, leaving the proper amount of space at the top. Then you place the lid on, screw the band on and put them in a hot water bath and process for the stipulated amount of time. (This is turning a little into the "How do I do it?" part, but, really, there will be many more details next time, I promise.) Now comes the fun part. The heating process causes gases and food to expand in the jar, leading to a buildup of pressure. The pressure is relieved by venting out from under the lid, which happens repeatedly throughout the heating process. This creates a vacuum inside the jar which means that when the jar is cool, the pressure outside is greater than the pressure inside, causing the lid to indent and the softened sealing compound to conform to the lip of the jar. This creates an air-tight seal preventing any microorganisms from entering the jar and recontaminating your precious food. And as the books say, heat-processing home-canned food is not optional! Without proper heat-processing, you run the risk of spoilage and health problems (I mean, I love my home-canned peppers as much as the next guy, but I’m not a fan of food poisoning…)
Next time, I’ll be getting down to the nitty gritty: how to actually do this canning stuff. I should note that I will only be discussing how to can high acid foods. Low acid foods are a whole ‘nother ball game, and one that I can’t get into, or at least not until somebody sends me a pressure canner. High acid foods include fruit, jams, jellies, fruit spreads, pickles, relishes, chutneys, and some tomato mixtures. These foods have a pH of 4.6 or lower, which means that they can be safely heat-processed in boiling water. Low acid foods are things like vegetables, meat, fish, poultry and some tomato mixtures (gotta watch out for those tricky tomatoes!) and they need to be brought to a much higher temperature in order to kill off toxin-producing bacteria, like Clostridium botulinum, which causes…you guessed it! Botulism! It thrives on low acid foods in moist environments with no air…just like inside a canning jar.