What Is Sourdough Starter?

G. Wiesen

Sourdough starter is, much as it sounds, the starting product used in the production of sourdough bread. Although it can be made in a number of different ways, a starter can be created simply through a combination of flour and water that is allowed to ferment and develop bacteria. The microorganisms within the starter ultimately produce the sour aroma and flavor that is the very namesake of sourdough bread. Sourdough starter is essentially self-replicating; as long as it is “fed,” the starter will continue to develop and remain usable, which means that some starters can be decades or even centuries old.

Sourdough starters contain microorganisms that produce the bread's characteristic taste.
Sourdough starters contain microorganisms that produce the bread's characteristic taste.

The basic purpose of sourdough starter is the development of two microorganisms: a bacterium called lactobacillus and yeast, which is technically a fungus. Since these are microorganisms, they are both alive and need to be “fed” to allow them to develop and thrive. This is done through the combination of water and flour. Any type of flour can work, including wheat, rye, and rice flours. There are already a number of microorganisms present on the flour itself, including yeast and many different types of bacteria.

Many bakeries have sourdough starter that is decades old.
Many bakeries have sourdough starter that is decades old.

What basically happens in the initial creation of sourdough starter is that flour and water are combined and left at room temperature to create an environment in which yeast and lactobacillus can thrive. Lactobacillus is usually introduced to the mixture either as a bacterium already on the flour, or from the environment. This initial mixture is then “fed” additional amounts of flour and water over several days to allow the yeast and lactobacillus to multiply, which creates an environment inhospitable to other types of bacteria that might be harmful.

As the sourdough starter continues to develop, this creates a potent mixture of these two microorganisms. The yeast and lactobacillus create the aroma and flavor of sourdough that is often described as tangy or sour. After several days or a week of the starter being fed and developing, it is typically ready for use in the creation of sourdough bread, through the combination of starter with other ingredients.

One of the advantages of the use of sourdough starter, beyond the signature flavor of sourdough, is that it can continue to thrive and be used for years. Only part of the starter is used to make bread, and the remaining starter can be “fed” flour and water and refrigerated to prolong its life. There are many bakeries and organizations that have sourdough starter that is decades old, some even going back more than a century. The development of a starter is also influenced by local conditions, including air quality and humidity, which means that starters from different parts of the world, such as San Francisco, have different flavors.

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Discussion Comments


@Soulfox -- You can jump start the sourdough starter process by adding some bread yeast when you first mix up the stuff. It will be ready to go faster.

Of course, you do pay a price for using that method. The "wild" yeast in your area won't be able to take hold because it will be strangled out of existence by the more aggressive cultivated yeast. If you want that local flavor, you will have to stick with the old fashioned way of mixing up a starter that will encourage local yeast to take hold and thrive.


Yes, there are some sourdough starters that are very, very old. San Francisco is famous for its sourdough bread and has been since the Gold Rush in the 1800s. The starter that was prominent back then is, reportedly, still in existence and bits of it are doled out to bakers to this day. How's that for history?

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