"Pickles" include the green type made of cucumbers, but also kimchee, sauerkraut, escabeche, and tsukemono, to name a few. Old-fashioned pickling is a type of fermentation. You create a salty, anaerobic environment and allow the naturally-occurring bacteria to multiply. As the bacteria consume the sugar in the food, they produce lactic acid. Eventually the pickled food becomes so acidic that no other bacteria can live in it. This is how bacterial preservation works: if you encourage enough good bacteria, they'll keep the bad bacteria away. [Fun science fact: some microorganisms produce lactic acid, and they're used for pickles and yogurt. Others produce alcohol, as in wine and beer. Vinegar is a two-stage process: first you grow the alcohol-producing critters, and later you introduce others that turn the alcohol into vinegar.] Kimchee is one traditional type of pickle (sort of a Korean version of sauerkraut). To make it, you begin by adding salt to shredded cabbage. The salt draws water out of the cabbage, creating a brine.

The good bacteria work anaerobically - that is, without oxygen. If the cabbage were allowed to float, bits of it would poke from the surface of the liquid and be exposed to the air - so we keep it underwater by weighting it down with rocks, jars, plates, or whatever works.

If a scum forms on top of the liquid, it's made of aerobic microorganisms, like yeast. Scoop out the scum when it appears, and throw it away. (Another helpful strategy: use a layer of oil to keep the air away from the pickle brine.)

Bacteria in the pickle jar come in three successive waves: first coliform, then Leuconostoc, and finally Lactobacillus.

Many American pickle recipes recommend adding vinegar to your starting brine, in which case you may not be fermenting the pickles at all. Vinegar is pre-fermented, and gives the right taste.

Safe pickling maintains the right temperature, acidity, etc. Read up before beginning:

• Pickled cucumber recipes of all types
• Dill pickles & sauerkraut suitable for hot-water canning
• Beginner recipe for cucumber pickle slices
• Pickle tips: reducing salt, making them crispier
• Sauerkraut recipe suitable for a microbiology class (with discussion)
• Japanese pickles (tsukemono and more)
• Escabeche recipe (Mexican pickled veggies with jalapenos)
• Toorshi (Armenian pickled veggies)
• Kimchi recipe (korean spicy pickled sauerkraut)
• Chow chow and other Pennsylvania Dutch pickles
• Buy kimchi-making equipment here