What is Sourdough?

Excerpted from Wild Bread

Sourdough starters are referred to as “cultures.” Sourdough cultures are complex ecosystems composed of multiple species of yeast and lactic acid-producing bacteria. No baker’s yeast or chemical leaveners like baking soda are ever added to real sourdough breads.

Wild sourdough yeasts don’t live alone in a monoculture, like baker’s yeast. Dormant cells of bacteria and fungi float through the air all around us and live in flour. When they land on a suitable material, they begin to reproduce. Their digestive process is known as “fermentation.” These microorganisms live together in a mutually-beneficial symbiotic partnership like plants and animals do in larger ecosystems. The yeast and bacteria share the available nutrients rather than compete for them and cooperatively protect their ecosystem from harmful invaders.

Yeast are tiny, one-celled fungi. Sourdough yeast is more tolerant of varying environmental conditions such as temperature and acidity than baker’s yeast. When yeast have access to oxygen, aerobic fermentation produces carbon dioxide gas (CO2), which bubbles through bread dough, making it rise. Yeast fermentation in the absence of oxygen (anaerobic fermentation) produces alcohol. A sourdough culture that has been left to ferment for a long time without being aerated will develop a thin layer of alcohol (hooch) on the surface. The alcohol provides an additional flavor dimension.

Another flavorful byproduct of yeast metabolism is the amino acid glutamate. Glutamate produces a rich, savory flavor called umami (oo-MA-mee in Japanese). Umami is now known to be the fifth taste perceived by the human tongue, along with sweet, salty, sour and bitter tastes. Umami flavors are also found in other fermented foods such as miso and aged cheeses, as well as some vegetables, such as tomatoes and mushrooms.

Sourdough bacteria are lactic acid-producing bacteria (lactobacilli) that are also found in numerous other fermented foods. Sourdough bacterial genera include Lactobacillus, Leuconostoc and Streptococcus, including probiotic species such as Lactobacillus acidophilus, L. plantarum and L. brevis.

These bacteria produce unique flavors and textures and increase the nutritional value of sourdough bread.

Sourdough bacteria eat carbohydrates, fats and proteins and produce acids, notably lactic acid and to a lesser extent acetic acid (vinegar). The pH of sourdough is roughly 4 to 4.5. Baker’s yeast dough, which is lacking in lacto bacteria, has a less acidic pH of 5.5 to 6. Pathogenic microorganisms such as botulism bacteria, E. coli bacteria and spoilage fungi cannot reproduce in an environment with a pH below 4.6. Lactic acid bacteria produce other antibiotics to protect their ecosystems, too. Thus, the bacteria prevent pathogenic microbes from invading the sourdough ecosystem and upsetting the ecological balance. Furthermore, these natural preservatives keep sourdough bread mold-free much longer than bread made with baker’s yeast.

Real sourdough flavor comes primarily from these acids. The lactobacilli need a minimum of 12 hours to ferment in order to produce these wonderful flavors.

Sourdough breads are more nutritionally complete than those made with baker’s yeast. The acids produced by sourdough lactic acid bacteria and the long proofing times of sourdoughs confer a number of nutritional advantages over fast-rising breads made with baker’s yeast.

Healthy sourdough ecosystems are very resilient. Once you obtain a sourdough starter culture, whether you capture it yourself from the air or buy one from a reputable source, it will remain stable even if taken to a new location. Like heirloom seed varieties, heirloom sourdoughs can be passed down for many human generations.

It’s easy to obtain an authentic sourdough culture. You can:

  •     Capture wild yeasts and bacteria from the air
  •     Buy a commercial fresh or dried sourdough culture
  •     Get some healthy starter from a friend

Is sourdough always sour?

No! Wild bread can taste mild, sour, or anywhere in between. It’s all a matter of taste. In Europe, the French use natural leavens to produce very mild artisan breads and sweet doughs, whereas the Germans and Austrians prefer strongly acidic rye breads.

Local strains of sourdough yeasts and bacteria differ from one another just as plant varieties differ from one another in different localities. Because of this natural diversity, regional sourdough breads are as distinct from one another as regional artisan cheese, miso, wine and other fermented foods and drinks. Each sourdough culture has a unique leavening time, degree of sourness and balance of additional flavors. Some strains of yeast ferment very quickly, giving less time for the bacteria to produce their acids and producing milder flavors. Other cultures, including the famous San Francisco sourdough culture, have long leavening cycles of up to 24 hours and produce very sour tasting bread. When choosing a sourdough culture, consider your family’s tastes, time constraints, and other needs (this is the subject of chapter 3).

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