A sourdough culture is a symbiosis of two kinds of micro-organisms: a species of yeast, and a species of lactobacillus. The particular species varies from culture to culture, but the basic mechanism is the same. The lactobacillus makes the culture acidic, which repels most hostiles and pathogens and gives sour bread its characteristic sourness. The yeast is what leavens bread; it is not the same species as you would buy in the grocery.
The two basic things you must understand about a sourdough culture is that you need to feed it, and you need to remove its waste products – particularly you need to dilute the acid periodically so that the environment doesn't become hostile to your pet microbes (too much acid will do them in too, not just the bad guys).
There are many ways of making a sourdough culture by scratch. Here's what I did. You'll need a container with a cover, about a kilogram of (preferably organic rye) flour and lots of water.
Take a clean container and mix in it 200 grams of flour and 400 grams of water. Cover the container and leave it covered in room temperature. Feed it every 8–24 hours by discarding most (but at least half) of it and adding 200 grams of flour and 400 grams of water, and mixing well. In a couple of days it should start being bubbly; at that time, I recommend switching to feeding it equal parts of flour and water (I use 200 grams of both) to make it a bit thicker. When it expands itself in a couple of hours after feeding (if your container is transparent, you should be seeing holes all over it, but not necessarily on top), it's ready (I expect it to take a week to get to this point). It should at that point smell sour but not stink.
A live culture should be fed every 8–24 hours and kept in room temperature. You can refrigerate it just after feeding, in which case it should keep a week or two, and when you want to use it, take it out to room temperature and let it sit for hours, and feed it once before using it. Freezing and drying are reported to work, though I haven't tried them.
To use the culture, take some (but not all) of it several hours after the latest feeding and mix it in the water you intend to use in the dough. Remember to feed the culture.
Note: fruit flies like sourdough, so do remember to cover your culture when not working on it.
ETA The important thing is to dilute the acid when you are feeding. If you can't stand throwing stuff away, just feed the culture exponentially, at least doubling its size in each feeding. You might want to start small if you want to go this way...
ETA Something I forgot to mention... I added some apple cider vinegar to the mix during the first couple of feedings, before it started showing life, to discourage the acid-averse critters.
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3 comments:
I do in fact feed my sourdough culture exponentially, starting with a glob of the previous dough (or, when starting completely from scratch, with about 3 tsp rye flour and enough water to get a consistency like thickish pancake batter). I add flour morning and evening, and only water if it's too thick; the fermentation seems to make it thinner.
When I bake, I use the whole culture, which by then is about half a litre (it's in an earthenware one-litre jar covered with muslin against the fruit flies). I take a handful of dough to continue the culture when it's risen, never mind salt, oil, treacle and/or sunflower seeds. One sunflower seed once made it through five successive bakings, after which I used up the whole culture because I was going on holiday.
Could the thinning be caused by alcoholic fermentation?
I wouldn't know, but it seems plausible.
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