Note: This is for peace of mind. Every time I get a little worried about the amount of produce just hanging out on my guestroom bookshelf, I remind myself that this is a process backed by science and very safe if done correctly.
Most fresh foods have a very high water content, which makes them deteriorate quickly due to micro-organisms, food enzymes, contact with oxygen, and moisture loss. Canning slows this decay by vacuum-sealing jars to limit oxygen exposure and moisture loss. The heating process prevents the growth of most micro-organisms and halts food enzyme activity.
The major concern with home canning is botulism. Clostridium botulinum spores can live in soil and water for a long time without causing problems. When provided with a moist, low-acid, low-oxygen environment at the right temperature (40° to 120° F), the spores produce vegetative cells that rapidly generate the botulism toxin. Because canned goods are moist, low-oxygen environments and water-bath canning can’t reliably heat jar contents above 120° F, the acidity level of your food is critical.
Many foods already have enough naturally-occurring acid to be safely water-bath canned, e.g. most fruits. Foods that aren’t already acidic enough can be canned with the addition of lemon juice or vinegar, e.g. pickles. Knowing that botulism hates acidity like the Wicked Witch of the West hates water makes canning much less scary.
See our Canning page for more info.