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