How Sourdough Bread Works

If you have ever been to San Francisco, California (where it seems like every restaurant serves sourdough bread), or to many other parts of the world where it is a staple, then you have experienced the distinctive tangy taste of sourdough bread! The taste is what makes sourdough bread unique. But have you ever wondered where that taste comes from?

It turns out that sourdough bread represents a centuries-old technology for preserving and storing yeast for long periods of time, and it is this technology that creates the amazing flavor. Today we buy yeast as a powder in nice foil packets at the grocery store, but centuries ago there were no grocery stores (and no foil, for that matter). People cultured yeast themselves and kept it alive using a medium called sourdough starter. Bakers who use a starter today are using this old but still-useful technology. In fact, some batches of starter have themselves been around for decades, passed from friend to friend or generation to generation!


In this article, we'll look at how sourdough starter works and how it helps make a loaf of sourdough bread. We'll tell you how you can make your own traditional starter, and give you a basic recipe for sourdough bread as well. When you get done, you will look at sourdough bread in a whole new way!

Yeast, Gluten and Carbon Dioxide

You make sourdough bread from the same basic ingredients that you use for any other bread. The two most important ingredients are flour and yeast.

  • Yeast is a single-cell fungus that breaks down the starches in wheat flour, forming sugar. This is fermentation. When the yeast works on the starch and sugar molecules, it gives off carbon dioxide gas and alcohol. Yeast is a leavening agent for bread. It is what makes the bread rise.
  • Flour comes from any kind of ground grain, but most bread contains wheat flour. Two proteins found in wheat flour, gliadin and glutenin, form a stretchy substance called gluten. When you knead dough, you help gluten form long, threadlike chains. These gluten chains help hold the carbon dioxide gas in, creating those tiny holes that create the airy texture of bread.

The big difference between sourdough bread and the "normal" bread you buy or bake today is the source of the yeast. Most bakers today use cultivated yeast that comes in a package. The package contains live yeast fungi in suspended animation! The yeast has been dried, preserved and formed into a powder. You add flour, water, sugar and salt to the yeast to make a loaf of bread. The water re-activates the yeast fungi, which feeds on the sugar and starch to make the bread rise.


Sourdough bread deals with yeast in a completely different way. Sourdough yeast fungi are actually kept alive constantly in a liquid medium called a starter. The baker either captures wild yeast that floats in the air to create starter from scratch or gets a cup of active starter from a friend and expands it.

It turns out that the starter is what gives sourdough bread its distinctive taste...

Catching the Wild Yeast

Hundreds of years ago, before there was packaged yeast, bakers used sourdough starter to keep a supply of yeast alive and handy. They kept a pot of live culture in a flour/water medium, and "fed" it daily or weekly so that the yeast remained alive and active. To understand how sourdough starter works, let's look at how you can create a batch of starter using live yeast that is floating in the air!

To perform this experiment you will need:


  • A pottery crock, plastic container or glass jar, preferably with a loose-fitting lid
  • A wooden spoon
  • A piece of cloth
  • Some flour (preferably without any preservatives in it) and water

To start a culture, mix two cups of flour and two cups of water in a glass or pottery bowl (in the old days, a baker probably had a special clay crock for starter). Lay a cloth over the top and let it sit on the kitchen counter. It turns out that there is yeast floating in the air all around us all the time, and some of this yeast will make its way to your flour/water mixture. It will then start growing and dividing.

After 24 hours, you pour off about a cup of the mixture and feed it with another cup of flour and another cup of water. In a few days, the mixture will become frothy as the yeast population grows. The froth is caused by the carbon dioxide that the yeast is generating. The starter will also have a bacteria, lactobacilli, in it. This lends to the slightly acidic flavor of the bread by creating lactic acid! The alcohol that the yeast creates and the lactic acid together are the source of sourdough bread's unique flavor!

A common question at this point is, "why doesn't the flour get infested with all sorts of mold and bacteria and become a disgusting health hazard????" For example, if you put a bowl of sugar water or orange juice out on the counter, that is exactly what would happen. It turns out that the starch in bread flour is something that not a lot of bacteria can easily handle, while sugar is (see How Food Works for some details). Yeast, on the other hand, creates special enzymes to deal with starch. The yeast and lactobacilli also "poison" the culture with the alcohol and lactic acid they produce, and that keeps other bacteria out.

Leave the starter on the kitchen counter for five days. As the starter ferments, it will develop a strong aroma -- bready and alcoholy and not particular appetizing. Feed it every day or two by dividing it in half and adding a cup of flour and a cup of water to one half of it (you can throw the other half away). When you see a watery substance floating to the top, stir it. Sourdough bakers call this "hooch." Over the week the starter became a thick liquid, like pancake batter. It will be slightly yellowish.

At this point you can do one of two things:

  • You can store it in the refrigerator to slow down the yeast. Then you will only have to feed it every 5 or 6 days.
  • Or keep it on the counter and feed it every day.

If you don't like the "wild yeast floating in the air" idea, there are other ways to start a starter:

  • Get a cup of starter from a friend or another baker. You take a cup of the starter and add flour and water to make more of it. The starter can go on for years.
  • You can make a starter with normal packaged yeast you buy at the store. Start the same way as described above and simply add a package of yeast to it.
  • Or you can buy a packaged sourdough starter mix at the grocery store or by mail-order.

Some starters use milk instead of water, and some starter recipes call for sugar or honey, which boosts the fermentation. There are starters that use potatoes, too. Potatoes have starch in them, and that supplies more sugar for the yeast to feast on.

When it comes time to bake bread, you add a cup of this live culture to the dough to provide the yeast needed to leaven the bread. You replenish the pot by adding back an equal amount of flour and water, and regular feeding keeps the culture alive.

Baking Sourdough Bread

You can find scads of sourdough bread recipes in cookbooks and on the Internet. You can find recipes for sourdough white bread, rye bread, whole wheat bread, biscuits and even cake. Once you have the starter, you can bake sourdough in countless ways.

The recipes use various terms related to starter, including sponge and poolish. Sponge often refers to the mixture of starter with the flour and other dry ingredients of the recipe added. Poolish is another name for sponge. Some recipes call for proofing the starter. To proof a starter, you take a portion of it out of the refrigerator and feed it for a day or so to get a foamy "proof" that the yeast are active.


Here's a simple sourdough recipe from a very popular cookbook that you might have on your kitchen shelf -- the "Better Homes and Gardens Cookbook," 1989 edition. This recipe cheats a little by using both sourdough starter and packaged yeast at the same time. In this case, the cup of starter provides the flavor and the packaged yeast guarantees that the bread will rise quickly and reliably.

Here is the recipe:

  • 1 cup of sourdough starter
  • 5 1/2 to 6 cups of all-purpose flour
  • 1 package of active dry yeast (2 1/4 teaspoons)
  • 1 1/2 cups of water
  • 3 tablespoons of sugar
  • 3 tablespoons of margarine or butter
  • 1/2 teaspoon of baking soda
  • 1/2 teaspoon of salt

You need to have the sourdough starter at room temperature, so if you have your starter in the refrigerator, put a cup of it out on the counter for a while before you start mixing the bread. You combine 2 1/2 cups of flour and the yeast in a big bowl (a 4-quart glass bowl will do). Heat the water, sugar, butter and salt until warm (110 degrees Fahrenheit or so). Add the liquid to the flour and yeast mixture. Then you pour in the sourdough starter. Mix with an electric mixer on low speed for 30 seconds. The mixture looks very smooth and creamy at this point, and smells very yeasty. Then you mix with the mixer on high speed for 3 minutes. This is when you begin to see the elasticity develop in the dough. It practically climbs up the beaters to the mixer! You need to keep a scraper or a spoon on hand to push the dough back down. It's fascinating to watch!

Combine 2 1/2 cups of flour with the soda in a separate bowl. Then add it to the yeast mixture. Stir this until the dry ingredients and the starter mixture are combined. Then try to add as much of the remaining flour -- 1/2 to 1 cup -- as you can. The dough gets pretty stiff at this point.

The next step is kneading -- a part of making bread that many bakers find most satisfying, because you can feel the dough changing in your hands as you knead. Put the dough on a floured surface and start pushing and pulling. It will take about six to eight minutes to get the dough to the right stiffness. You will know it is done when you can push on it with your finger and it pops right back instead of denting.

Shape the dough into a ball and put it into a greased bowl. Cover it and put it in a warm place to rise until it doubles in size -- about 45 to 60 minutes.

When it has doubled, punch it down -- another satisfying part of bread baking. Put the dough on a floured surface and divide into two parts. Cover these two lumps for about 10 minutes and let them rest. Make them into two round loaves. Put the two loaves on a greased baking sheet, and cover again to let them rise until they are about double in size (about 30 minutes). Then it's time to bake the loaves, in a 375-degree oven for about 30 to 35 minutes.

You should get a crusty bread with a hearty, chewy texture and that amazing sourdough taste!

You can find many recipes for starter and for sourdough bread in the Links section. You can even develop a starter that you can pass along to friends to start a new tradition!