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The Bread Bakers Forum



The history of "sourdough" is as old as the history of leavened bread itself. Way back in ancient days (around 6,000 years ago, some say) humans first figured out how to promote the fermentation and leavening of grains to first be brewed into beverages and then, later, to be baked into bread.  This probably happened by accident time after time, until someone smart figured it out. Next our ancestors figured out how to save a portion of the fermented grains to use to "start" the fermentation of their next batch of bread. Since that time, humankind has been using and making "sourdough."

That fermented mix of grain and water that was saved and used to start the next batch of dough is what we now call a "sourdough starter" and bread made from such a starter, "sourdough bread."

From a scientific perspective, a sourdough starter is a natural leaven - a mixture of grains and liquid (usually flour and water) inhabited by so-called "wild" yeasts and bacteria which leaven and flavor bread dough. These yeasts are the yeasts that thrive naturally on the surface of grains, fruits and vegetables, in the air and in the soil. The bacteria are certain strains of the so-called benign or "friendly" bacteria Lactobacillus, rod-shaped bacteria that can convert simple sugars into lactic and other acids.

To understand more of what a sourdough is, we need to understand what yeast is. In simplest terms it is a plant. More specifically a fungus, a one-celled life form which digests sugars (such as those contained within the starch in flour) and produces a bit of ethanol (alcohol) and some carbon dioxide (which is what causes the bread to rise). The natural yeasts in a sourdough starter are strains of a yeast family whose scientific name is Sacchraromyces exiges. They are of the same family of yeast as commercial bakers' yeast, whose scientific name is Saccharomyces cerrivasae. The two have what might be called a distant family relationship but differ in one important way. Commercial bakers yeast cannot survive in a very acidic environment whereas natural yeast is very happy to live in such an environment. This is important because the lactobacilli in a sourdough culture produce a lot of lactic and acetic acids (which are what gives sourdough bread its flavor). The acids create an environment too acidic for commercial bakers' yeast, so only natural yeast can live with them.

In a healthy sourdough starter, yeast and lactobacilli thrive in a harmonious symbiotic relationship. This means that they do not compete for the same food and the yeast may actually help feed the lactobacilli. In turn, lactobacilli produce an acidic environment that the yeast like but which is inhospitable to other organisms. Thus the acids in the culture act as a sort of "antibiotic" so that lactobacilli contribute by providing a protective environment for the yeast.

Lactobacilli help bread rise, too. Just like yeast, they digest simple sugars found in flour and produce ethanol and carbon dioxide. In addition, the lactic and acetic acids that they produce flavor the bread with a rich complexity of flavors, sometimes giving it a sour tang.


While sourdough starters and bread made from starters has been around for thousands of years, the term "sourdough" has a pretty short history. It is an American term that came into use during the California Gold Rush days of the late 1800's.

Before the advent of commercial bakers' yeast, the folks who traveled and settled the Western U.S. in the 19th century carried starters with them for making bread. Folklore of the time abounds with stories of chuck wagon cooks making biscuits frombarrels of starters and Alaskan gold miners sleeping with their starters at night to keep them from freezing. More stories are told of the tragedies of pioneer families losing their starters and of passing down highly prized starters from generation to generation.

Many California and Yukon gold miners obtained provisions in the booming coastal town of San Francisco before heading up into the mountains to stake their claims. Over time, it was discovered that starters from that area produced bread with a unique and particularly sour tang. Thus the starters and bread from that area because known as "sourdough". Later the term was even applied to gold miners themselves. More recently the term has generalized across the U.S. to mean simply a bread starter.

Although the miners did not know it at the time, particular strains of yeast and lactobacilli took up residence in starters from the San Francisco Bay area and they are responsible for the unique flavor identified as "San Francisco Sourdough." In the 1970's the microbes were isolated and identified. The scientists studying San Francisco sourdough cultures identified the dominant yeast strain as a variety of Saccharomyces exigus called Torulopsis holmii (later they renamed it Candida milleri sp. nov). The dominant lactobacillus is a species that had not been seen before so they classified it Lactobacillus sanfranciscensis sp. nov (note: "sp nov" = "new species").


Things happen to language through imprecise use over time. Originally "sourdough" referred to cultures of specific microbes from the San Francisco area, later it evolved to refer to miners who carried sourdough starters with them. More recently, since the advent of large-scale commercial baking and the invention of various dough flavoring agents, the term "sourdough" has become generalized in common cultural usage to refer to any sour bread, be it one leavened with a natural leaven starter or with commercial bakers yeast.

This makes the term "sourdough" a difficult term to use, as it may mean one thing to one person and something very different to someone else. Since the distinctions between "sourdough" and "sour bread" are important in the method and process of bread making as well as in the resultant baked loaf of bread, I offer the following definitions of "sourdough" as they are used throughout these pages. For related definitions see SOURDOUGH DEFINITIONS.

Sourdough Starter: A starter or culture of wild/natural yeast and lactobacilli in a medium of flour and liquid which is propagated through ongoing refreshments (or "feedings") for the purpose of leavening bread dough, is on-going and is continued on from one bake or activation to the next.

Sourdough Bread: Bread which has been leavened with a sourdough starter. It may or may not be a sour bread, depending on the characteristics of the starter.

Sourdough : An American term for a natural leaven of "wild" or natural yeast and lactobacilli. Also the process of leavening bread with a natural leaven.

Yeasted Starter: A starter containing commercial bakers' yeast.

Sour Bread (or Faux Sourdough): Any bread given a sour flavor by means of a flavoring agent (such as souring salts), ingredients (such as yogurt or vinegar) or process not including a sourdough or other natural leaven starter.

Sourdough bread is not necessarily sour bread. Sourdough bread may be somewhat or very sour, or it may not be. It may actually be quite mildly flavored with rich, complex delicious wheaty flavors. With sourdough, the degree of sourness depends on many factors including the temperature, length of fermentation, type of grains, amount of water, and most importantly, the particular strains of yeast and lactobacilli that live in the starter. This will be discussed in more depth elsewhere in these pages.


Because the term "sourdough" is so imprecise, you'll find the term "natural leaven" used in its place throughout these pages. The definition of a natural leaven is the same as the definition of a sourdough starter: a culture of wild/natural yeast and lactobacilli maintained over time and continued from one activation to the next.

Other terms for natural leaven starters are :

Levain (French)
Desem (Flemish
Barm (British) (not in traditional usage)
Lievito naturale (Italian)
Friendship Starter (if not yeasted)

The major distinctions between the various terms for natural leavens, other than the terminology, are the grain used, the degree of hydration and, to some extent, the storage medium and temperature, all of which affect the degree of complexity and sourness of each type of starter. They are all discussed in more depth elsewhere in these pages.

Starters made with commercial bakers' yeast are not natural leavens. They are yeasted starters. They do not produce the same results in terms of flavor, texture and keeping qualities as natural leaven starters do. You will never obtain true sourdough bread from a starter that contains commercial bakers yeast. However, it is possible that a yeasted starter might be taken over by natural yeasts and converted into a natural leaven.

Credits: My thanks to the following for their contributions
The Sourdough Faqs of Rec.Food.Sourdough
The participating members of the Rec.Food.Sourdough newsgroup
Boudins San Francisco Sourdough Bakery
"Bread Alone" by Daniel Leader & Judith Blahnik
"The Book of Bread" by Jerome Assire

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