Sourdough is a wild-caught yeast that can be used to make bread. You can use it for adding a fermented flavor to the bread while using some regular storebought yeast to do the leavening, or, if you're lucky, your strain of yeast will be so vigorous that it'll rise the bread enough to not need any additional storebought yeast. It's worth experimenting with different sources of sourdough yeasts so that you can find the best one.

There are many ways to capture the yeast. Here are just a few:

the bloom on the skins of grapes, cherries, plums, etc, is a yeast that works well for leavening and flavoring breads.
Soak some fruit in warm water, with a few spoonfuls of flour in the water, in a thermos or in a jar that's sitting in a picnic cooler full of warm water, overnight. Remove the fruit and keep fermenting the flour/water/yeast mixture for another week. I usually add fresh flour to it every few days, and some more water, to keep feeding the yeast new sugars. Anyway, eventually it'll start to smell like nicely fermented sourdough, though don't expect that to smell like bread made from storebought yeast- it's definitely 'sour'.

You can also capture sourdough yeasts just from the air by setting a bowl of flour-water batter in a warm place in your kitchen. if the kitchen has a history of bread baking, the cooking mythology says that some of those yeasts from the prior breads are supposedly floating around for you to catch. I've considered sneaking a bowl of flour into the back of a commercial bakery once get some of these yeasts.

I usually make my sourdough starter the Russian way- get some organic, non-heat-dried caraway seeds (like Frontier spices), soak the seeds in warm water in a thermos for 24 hours. then I strain and throw out the seeds, and mix the water with rye flour and let it ferment for a week in a warm place. The seeds have wild yeasts on them. The exact variety of yeast varies wildly in this method and sometimes I've gotten a really vigorous strain this way.


Caring for starter:
Once you've captured a starter yeast, you can keep it going indefinitely.
I usually store mine in the fridge, and have done so for months at a time with no care. Most cookbooks tell you to pull it out of the fridge once a week and re-feed it to keep the microbes from dying. The way that this works is that you add some flour and water, and let the starter sit in a warm place for a day before re-refrigerating it. I sometimes just add flour to the cold jar of starter and put it back in the fridge without re-fermenting it. All of this seems to work. Theoretically it should also work to dry some out and then use it dried in the future to start more starter if youre taking a break from baking bread.

Using the starter:
Recipes for bread are always approximate since flour varies wildly in moisture from one grind to another. I also found that the 'pumpernickel rye' flour sold at Berkeley Bowl and elsewhere doesn't seem to work as well as their regular rye- not sure what the difference is, might be a coarser grind.

I always improvise my bread, and tend to make rye-only bread that I can't buy easily in stores, but the formula looks something like this. It'll make a dense loaf that doesn't rise as high as a rye-wheat blend will, but resembles a large version of one of those cocktail bread pumpernickel things, especially if you use a lot more oil:

8-10 cups rye flour (or half rye half wheat, which makes a higher-rising bread)
salt
1 spoonful of honey or malt syrup
1 cup of starter. You can use more if your starter isn't a good riser, or you can use some starter and some commercial baking yeast.
2 TB flavorful oil such as olive oil or butter or coconut oil
Russians usually use butter and eggs in this dough also (I might be confusing the eggs part with another dough for filled pirogi, though ,so don't shoot me if I'm wrong) .
add lukewarm water and knead in till the dough is right

knead, knead, knead, knead. A mixer is immeasurably helpful here.

Put in an oiled bowl, cover with a towel, put in a warm place, and let rise till it substantially rises (it probably won't double like a wheat dough, but should expand by half again).

make loaves (I think 8 cups flour makes two loaves) and put into oiled loaf pans. Let rise again till it gets another 1/3 higher (or so)


Knowing how long to knead the stuff is tricky- all-rye dough is notoriously sticky and doesn't act like wheat dough at all. It'll never get to the point where it pulls away from your fingers if you're hand-kneading, unless you use part wheat/part rye. I joke that for this bread, you make rye glue and then bake it into rye bricks. Lately I've been experimenting with adding in wheat gluten to make it more stretchy and less sticky and help it rise better.

I can't remember how long I bake these bricks at . I use 375 and if it looks like it's cooking too much on top, I turn it down. The exact time seems to vary wildly depending on how much water I used and how well it rose and presumably dried out. I think it's something like 1 hour, but really can't remember since I improvise this recipe at this point.