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Sourdough: Starting Something Wild

The first loaf of yeast bread may have arisen as a happy accident. Wild yeast, which is ever-present in the environment, got mixed into a batch of flatbread dough and went to work. The adventurous—or very hungry—baker tried the strangely puffed product and was intrigued by the light texture and touch of sweetness. A taste for yeast bread was born.

At some point, ancient bakers also learned that they could reserve some of the batter or dough from one batch of bread to start the next one. The chewy texture and tangy taste may have taken some getting used to, but today, fans of sourdough bread—a name acquired only in the 1800s—are glad their ancestors made the effort.

Taming the Wild Yeast
It’s still possible to “capture” local yeasts with an inviting, kneaded paste of flour and water set on the counter. For more predictable results, you can buy dried packets, similar to other yeast, from natural-food stores or artisan bakeries, which make specialty or ethnic breads.

Any yeast will sour dough if left long enough, but those chosen to make sourdough bread are known for producing tasty results. They belong to a variety called Saccharomyces exiguus (sak-uh-roe-MY-seez EGG-zih-jus)—roughly translated “little sugar mold.” Unlike standard bakers’ yeast, which is grown specifically for rapid rising and consistent taste and texture, these strains are more individualized. While their domesticated cousin is given just a few hours to leaven bread, these yeasts are nurtured for days to stamp the loaf with their particular flavors.

A Little Help from Their Friends
Saccharomyces exiguus can’t take all the credit for the intense flavor and springy consistency that sourdough lovers crave. They work in tandem with a common, “friendly” bacteria called Lactobacillus (lak-toe-buh-SILL-us). Lactobacillus too comes in several strains, which also helps vary the flavors of different sourdough recipes. (The one responsible for making San Francisco’s famous version is aptly named Lactobacillus sanfrancisco.) The partnership works out well for humans and microorganisms alike, as described here:

  • Yeasts are added to the mixture of warm water and flour, where they start fermentation, digesting sugars in the flour and emitting carbon dioxide gas and alcohols. This process starts to leaven and flavor the mixture. Regularly stirring in additional flour and water, or “refreshing,” keeps them well fed and adds oxygen.
  • The water also activates the Lactobacillus. These bacteria don’t need oxygen to survive, leaving more available for the yeasts. Lactobacillus enjoys a varied diet. It eats sugars that the yeasts cannot digest, breaking them down into even simpler sugars that the yeasts can digest. In turn, it feeds on the amino acids and fatty acids provided by dead yeast cells. (One sugar that Lactobacillus cannot eat is sucrose. Bakers’ yeast thrives on sucrose, however, and soon overwhelms the Lactobacillus in other kinds of bread.)
  • Lactobacillus produces lactic acid as a by-product of digestion. This adds to the increasingly acidic environment. These conditions would kill most yeasts, but Saccharomyces exiguus tolerates them better. The two types of microorganisms live in symbiosis (sim-bee-OH-sis), a relationship in which each partner benefits the other without competing with the other.
  • By the time this extended fermentation is complete, the dough is pleasantly pungent with alcohols, amino acids, and acetic acid (the kind found in vinegar). These substances and many others all become ingredients in numerous, complex flavor compounds.

The risen dough, now called a sponge or starter, is ready for the rest of the bread-making process. It’s kneaded with more flour, allowed to rise again, shaped, and baked. A small amount is held out to begin the life cycle, and loaf making, anew.

Skilled sourdough bakers’ know how long to let fermentation continue, by the sight, smell, and feel of the sponge. This is important, since too much acid breaks down the gluten. Without this protein framework to trap the air, the bread will literally fall flat. Properly cared for, a starter can give rise to many generations of loaves. One San Francisco bakery is making bread from a starter “born” in 1849.


Compared to bakers’ yeast, wild yeasts are ____.
B)more rare
C)more acid-resistant
D)more uniform

The yeast and bacteria in sourdough bread “get along” because ____.
A)both are heat-resistant
B)they prefer different environments
C)they have different life cycles
D)each provides food for the other

One contribution of Lactobacillus to sourdough bread is ____.
A)creating a suitable environment for bakers’ yeast
B)producing acids that flavor the dough
C)producing alcohols that flavor the dough
D)controlling the growth of wild yeasts

An example of symbiosis is ___.
A)a cow eating grain and producing milk
B)earthworms eating fallen leaves and fertilizing the plant with their waste.
C)plants turning simple sugars into carbohydrates
D)body cells storing excess sugars as fats

Sourdough bread’s distinct flavor comes from ____.
A)a combination of acids and alcohols
B)a longer fermentation period
C)the particular strains of yeasts and bacteria
D)all of the above

Explain what might occur if you substituted conventional bakers’ yeast for wild yeasts in a sourdough recipe.
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