A Comparison of Sourdough Starters
Most commercially sold bread today is formed from refined flours and prepared yeast, plus a variety of other ingredients. These standardized, processed ingredients allow consistent and predictable production of thousands or millions of bread loaves on a high-paced schedule.
For the home baker who wants something a little more personal, and a lot more individual, there is sourdough. This is made with a sourdough starter containing live, active yeasts and bacteria, which is added to flour (and sometimes other ingredients) to create a unique and flavorful bread.
The Science of Sourdough Starters
A sourdough starter is a natural leaven. It includes not only the yeast and bacteria, but the grain as well, as a source of food for the microorganisms. These yeasts and bacteria are those that thrive naturally on the surface of grains, fruits and vegetables, in the air and in the soil.
Yeast itself is a kind of plant, or fungus: a one-celled life form that digests sugars and starts, and produces a little ethanol and some carbon dioxide. The ethanol is further converted by the bacteria in the sourdough. The carbon dioxide becomes trapped in the stretchy dough, making tiny bubbles and causing the dough to rise.
The bacteria in sourdough starter 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.
The natural yeasts in the starter are strains of a yeast family whose scientific name is Saccharomyces exiges. They are of the same family of yeast as commercial bakers' yeast, whose scientific name is Saccharomyces cerrivasae. The two types of yeast differ in one important way. Commercial bakers yeast cannot survive in a very acidic environment while natural yeast does very well in an acidic 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. The lactobacilli require maltose, and break it down into single glucose molecules. The yeast cannot metabolize the maltose (grain sugar) but uses other available sugars, including the glucose produced by the lactobacilli, and breaks down starches into other sugars that the Lactobacilli can eat. This means that the yeast and bacteria do not compete for the same food and actually support each other. In addition, lactobacilli produce an acidic environment that is good for the yeast but inhospitable to other organisms.
While sourdough starters and bread made from starters have 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 1800s.
Before the advent of commercial bakers’ yeast, it was common to make bread using starters, or sponges, of flour, water, and yeast. These could be passed from one family to the next, or kept by bakers for their breadmaking. The pioneers who settled the western United States in the 19th century carried starters with them for making bread, very likely starters that had been handed down from the early settlers of the eastern part of the continent. Folklore of the time abounds with stories of chuck wagon cooks making biscuits from barrels 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, and a good bread starter would have been a vital necessity. 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." Over time the term has generalized to mean any natural bread starter.
The unique “San Francisco Sourdough” flavor comes from particular strains of yeast and lactobacilli that were present in the Bay. In the 1970s 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, and it was named Lactobacillus sanfranciscensis sp. nov. ("sp nov" means "new species")
Varieties of Sourdough Starters
Cultures for Health has collected sourdough starters from all over the world. While only the San Francisco starter has a bacterial strain that has been specifically identified and named as being unique to its region, each of our starters has a particular characteristic that sets it aside from the rest.
A great deal of the flavor and sourness of any starter comes from the way it is proofed. Sourdough breads can stand up to a much longer proofing time than breads made with commercial yeast. The long, slow rise allows the proliferation of the yeasts and bacteria that produce the characteristic flavor.
With a reasonably short proofing time, you may not notice the difference between one starter and another, but the longer you let the dough proof, the more pronounced the flavor differences will be. Click here for more information on developing the flavor of sourdough bread.
Starter that has been grown in one type of flour can be converted to another type. Click here for more information on converting a starter to a new type of flour.
Starter made and fed with one type of flour can be added to another type of flour to make bread. (For instance, you can use a whole wheat starter with white flour, or a white flour starter with rye bread.)
A Comparison of Sourdough Starters
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