If you're used to thinking about yeast as an ingredient that you add to flour to turn it into bread, you might be surprised to learn that all flour already has yeast in it—albeit in such minute quantities that it won't, on its own, cause your bread to rise. But with a little patience and natural chemistry, you can create something that will.
What Is a Sourdough Starter?
A sourdough starter is a medium for activating those naturally occurring yeasts so that they multiply and form a giant colony, then using a portion of that colony as the leavening agent in your bread.
You'll then maintain that colony by feeding it flour and water, so that in theory it will live forever, allowing you to make unlimited of loaves of sourdough bread.
Your Flour Is Alive!
Flour has two types of microorganisms in it: yeast and bacteria spores. Yeast produces CO2 gas, which causes the bread to rise. And the bacteria, among them lactobacillus acidophilus, convert starches into lactic acid and acetic acid, which combine to produce the rich, tangy, sourdough flavor.
The ratio of these two acids, along with other variables like time, temperature, pH level, how much (and what) you feed the starter, not to mention what species of wild yeast you happen to be cultivating, give each starter, and each loaf of bread baked from it, its unique combination of flavor, crumb and airiness.
A sourdough starter is a culture of live yeast and other microorganisms. When you want to bake, you grab some starter, combine it with flour and other ingredients and form a dough, which you then proof, shape and bake just like ordinary yeast bread.
How to Make a Sourdough Starter
Making a sourdough starter (sometimes also called a mother, a sponge, or a levain in French) requires just two ingredients: flour and water.
The preferred flour to use is whole wheat flour, which has more microorganisms in it and because its higher protein content provides better food for the yeast. Rye (or pumpernickel flour) is also great.
Bleached flour has the fewest microorganisms and thus is the least desirable. In general, whole grain, higher protein flours will be the best, and overall, the less the flour has been processed, the better.
Tap Water Is (Usually) Fine
To this flour, you will then add water. Although most tap water is fine, you should avoid tap water if it smells of chlorine.
Chlorine is added to tap water to kill microorganisms that can make you sick. But chlorine will also kill the microorganisms your starter needs. So if your water smells of chlorine, use bottled or filtered water. Otherwise, tap is fine.
The water should be about 70 F—cool, but not cold or warm. You can achieve this by letting the water sit out for an hour or so.
You're going to combine equal parts (by weight) of flour and water, say 110 to 115 grams of each, and stir to form a batter, then cover loosely and place it somewhere warm, like the top of your fridge. You've now established your starter colony.
Growing the Starter
Once you've established your starter, next you need to grow it. To do that, starting on day two, you'll perform a series of daily refreshes, which consist of scooping out 110 to 115 grams of the starter, transferring it to a clean container and throwing away the remainder from the first container.
You'll then add fresh flour and water, again around 110 to 115 grams each, to the portion you just saved, stir, cover the container and replace it atop your refrigerator. That fresh flour is the new food for the yeast colony.
You'll repeat that process every day, scooping out and saving 110 to 115 grams, feeding it with new flour and water, and discarding what was left in the previous container. Since the initial process can take two weeks or more, you could end up throwing away several pounds of flour during this time.
It's possible to create a micro-starter using much smaller amounts of flour. You still throw away half to three-fourths each day, but it's a much smaller amount.
The reason for not doing this is that starting with a small amount of flour makes it less likely that you'll achieve the proper balance of yeast and friendly bacteria. It can work, but it can also not. In other words, the more wasteful technique is also the more reliable technique.
What's the Deal With Discard?
The idea of throwing away starter might not jibe with stories you've heard about people nurturing their sourdough starter like it was a live baby. But there's a reason for it.
In part, it's because if you didn't, you'd soon have enough starter to fill your bathtub.
But also, when you discard starter you're also discarding the waste materials from the microorganisms in the starter. Some of those materials, like the lactic acid and acetic acid, you want. But others, you don't, because not all the bacteria in the flour are beneficial to the starter.
Starting a fresh culture every day (which is essentially what you're doing) helps keep those unwanted byproducts to a minimum, instead of overpowering the microorganisms you do want.
One useful trick is to save the previous day's starter as a backup. That way, if something goes wrong with your starter, it only sets you back one day, not all the way back to the beginning. Then simply toss the backup after two days and make today's discard your backup. And so on and so on.
You could (and indeed should!) also give your discard away to someone who wants to grow a sourdough starter of their own. They would have to continue the daily refreshes until the starter is ready.
Maintaining Your Starter
And then, one day, it'll be ready. How many days this takes can depend on many things, but don't rely on the calendar. Instead, just watch the starter. Because if you use it too soon, it'll have weak flavor and insufficient rise, producing a dense, heavy loaf.
You'll know the starter is ready to use when, after refreshing it, it doubles in size within 8 to 12 hours. It will show vigorous bubbles and have a strong, ripe, boozy aroma. You can confirm using the float test: spoon a tiny blob of starter into a bowl of water. If it floats, it's ready.
At this point you can go ahead and bake with it, or just keep it in the fridge, where it will, hopefully, live a long and productive life.
And that means you'll have to take care of it. Once you're past the initial stage of growing your starter, maintaining it is a matter of feeding it fresh flour and water (equal parts by weight) on a regular basis, say once a week or so. You'll store it in the fridge during this time, but it helps to let it sit at room temperature for 4 to 5 hours after each feeding before returning it to the fridge.
If you neglect your starter, like by not feeding it for a month or more, it will die. You can "revive" a dead starter by taking it out of the fridge and adding more flour and water, but you're really just creating a new colony. Once the yeast are dead, they're dead.
Using Your Starter
When you're ready to use your starter, it helps to know the day before, so that you can give your starter a double feeding (i.e. 220 to 230 grams each of flour and water), and again, let it sit out for 4 to 5 hours, followed by another 12 to 18 in the fridge.
When you're ready to make your dough, you'll measure out however much starter your sourdough recipe calls for, say 250 grams for an average loaf, and add the flour, water and salt as per your recipe. And you'll then proof and shape, mostly likely baking the dough the day after proofing, or two days after you gave it its double feeding.
Note that you don't want to use all your starter. Leave at least one third of it in the container, which you will afterward replenish with a fresh feeding of flour and water and continue to maintain so that you can make loaves, and other sourdough treats, endlessly.