The short answer, for people who don’t want to read: keep it on the counter and feed it its own weight of flour and water every twelve hours, until you have enough for your first batch of bread plus an extra cup or two — the extra will be your new mother starter. If you don’t bake often, you can keep the mother in the fridge, and just feed it once a week or so.
Sourdough starter = the blob that ate everything
I have a new sourdough starter! And so can you. In fact, if you have friends who bake, it’s pretty much guaranteed that sooner or later you will receive a “gift” of sourdough starter.
You see, a sourdough starter is a living colony of wild yeast and lactobacilli, happily doing their thing in a paste of flour and water. The yeast eat the flour and fart carbon dioxide, which makes your bread rise. The lactobacilli eat the flour and make lactic acid, which makes your sourdough sour. (The acid also keeps the pH low enough to keep undesirable organisms from taking over your starter.)
In order to keep your colony alive and happy, you need to keep feeding it more flour and water. Proportions vary, but a typical ratio is 1:1:1 (by weight). To feed 100g of sourdough starter, you stir in 100g of flour and 100g of water. With me so far?
Here’s the thing: at room temperature, a sourdough starter needs to be fed every twelve hours or so. Think about this for a second. Every twelve hours, your sourdough starter doubles (at least) in volume.
So say that on Monday morning your friend gives you a cup of sourdough starter and a 20th generation photocopy of instructions with hand-scribbled notes in the margins. If you keep feeding it and don’t actually do any baking, by Sunday night you’ll have 512 gallons of the stuff.
You’d be happy to give it away, too.
Now, if you’re baking every day, you can just about keep up with your starter’s output. Alternately, you can give away — or throw away — half the starter each time you feed it. Of course, you can only give away so much, and people who bake their own bread are the type of people who feel guilty about wasting food, so instead of throwing it away you’ll end up looking for ways to use it up, and making sourdough pancakes and sourdough waffles and sourdough biscuits until you can’t fit into any of your jeans.
There is a better way.
What to do with your new sourdough starter
When you get your new blob of starter, feed it its own weight in flour and room-temperature water.
If you don’t have a kitchen scale, start by stirring down the starter. Notice the consistency: is it more like pancake batter or bread dough? Now, feed it its own volume of water and flour — for 1/2 cup of starter, mix in 1/2 cup of water and 1/2 cup of flour. Keeping track, add more flour or water until it’s about the same consistency as your original starter. Write this down for the next feeding. Mine usually ends up around 1/2 cup starter : 1/2 cup water : 3/4 cup flour.
Stir or knead everything together and put in a container. Use one big enough for the starter to double or triple in size, or it will overflow and you’ll have a big mess. Cover the container it loosely (so air can escape) and leave it at room temperature. Within 4-6 hours, it should bubble up to about twice its volume, then collapse (or you can stir it down).
After 12 hours, throw away* (or bake with) all but 1/2 cup, and feed it again. If you want to make sure it’s healthy and stable, keep doing this every 12 hours until it seems to be bubbling and collapsing on a regular schedule, or until you get bored, whichever comes first.
Now, stir it down and put it in the fridge. This is now your mother starter, and you can just feed it once a week or whenever you’re down to 1/2 cup. (If you find yourself running out, just feed it without reducing it until you have enough to last a week.)
*You don’t have to throw away the extra: google “what to do with extra sourdough starter” and you’ll get a billion recipes. You can even save up the extra — in its own jar in the fridge — for a few days until you get around to using it.
Whenever you want to bake, scoop out a glob, put it in a bowl on the counter, and feed it — use the same ratio of flour and water, unless your recipe says otherwise. After six hours on the counter it’ll be happy, thriving, and ready to turn into delicious bread.
Or to pop into a jar and send to a new would-be baker.
You can make your own sourdough starter from scratch
If you’re really hard-core, you can make your own sourdough starter by making a gruel of flour and water (and pineapple juice, if you’re using Peter Reinhart’s method) and leaving in an open container. Wild yeast and bacteria will colonize it, and as long as you feed and stir it regularly, they’ll settle down into a stable, healthy colony. Undesirable microbes will be killed off by the acid in the pineapple juice or out-competed and/or eaten by the desirable bacteria and yeasts. If you’re into this sort of thing, Peter Reinhart has complete directions in The Bread Baker’s Apprentice and Whole Grain Breads.
If you’re too lazy (like me!) or fear that your home has more ambient mold and mystery life forms than nice bready yeast and sourdough lactobacilli (like me!), just ask around — someone will be more than happy to give you a cup.