Sourdough is a naturally leavened bread representing one of the oldest forms of grain fermentation. The process relies on a self-sustaining microbial community, known as the starter, which is a simple mixture of flour and water. This stable ecosystem transforms the raw ingredients, creating the rise, texture, and complex flavor profile that distinguishes it from commercially leavened bread. Managing this microscopic culture requires a careful balance of biology and chemistry.
The Symbiotic Microbes of Sourdough
The foundation of a sourdough starter is a symbiosis between Lactic Acid Bacteria (LAB) and wild yeast. LAB typically outnumber the yeasts by a significant margin, often around 100 to one. These organisms are naturally present on cereal grains and colonize the flour and water mixture upon creation.
The bacterial component is primarily composed of species within the Lactobacillus genus, such as L. sanfranciscensis and L. brevis. These bacteria are responsible for acid production, which gives sourdough its characteristic tang and inhibits spoilage organisms. The wild yeast component often includes species like Kazachstania exigua and Candida milleri.
Yeast’s main contribution is leavening, achieved by consuming simple sugars and producing carbon dioxide gas and ethanol. The bacteria and yeast coexist through a mutually beneficial relationship. Bacteria create an acidic environment that wild yeast tolerates, and yeast produces compounds that bacteria can metabolize.
The Biochemistry of Fermentation and Rise
Fermentation involves microbes breaking down carbohydrates in the flour. Enzymes convert complex starch molecules into simpler sugars, such as maltose and glucose, which fuel the microbial community. This carbohydrate breakdown initiates leavening and acidification.
Lactic Acid Bacteria are classified into two metabolic groups: homofermentative and heterofermentative. Homofermentative bacteria, such as certain Lactobacillus species, metabolize glucose almost exclusively into lactic acid via glycolysis. Lactic acid provides a milder sourness and is the predominant acid in many sourdoughs.
Heterofermentative bacteria, including species like L. brevis and L. sanfranciscensis, utilize the phosphoketolase pathway. This route breaks down sugars into a mix of products: lactic acid, acetic acid, carbon dioxide, and/or ethanol. Acetic acid, the same acid found in vinegar, contributes a sharper, more pungent sourness.
Wild yeast primarily performs alcoholic fermentation, converting simple sugars into carbon dioxide and ethanol. The gas becomes trapped within the gluten network, creating the bubbles that cause the dough to expand and rise. While heterofermentative bacteria produce some carbon dioxide, yeast remains the primary engine for leavening. Ethanol mostly evaporates during baking but serves as a precursor for other flavor compounds.
Translating Acid Production into Flavor Profiles
The final flavor of sourdough results directly from the metabolic products created during fermentation. Core sourness is determined by the ratio of lactic acid to acetic acid produced by the LAB. Lactic acid contributes a milky, mellow tang, while acetic acid delivers a sharp, vinegar-like bite.
Bakers can manipulate this acid ratio to control the bread’s flavor profile. A ratio of approximately 80% lactic acid to 20% acetic acid is often considered ideal for a balanced aroma. Increasing the proportion of acetic acid results in a more pronounced sour flavor.
Beyond the acids, microbes generate hundreds of Volatile Organic Compounds (VOCs) that contribute to the complex aroma and flavor. These VOCs include chemical classes such as esters, aldehydes, and alcohols. Esters, which often carry fruity or floral notes, are produced by the yeast and contribute significantly to the overall bouquet.
Other flavor precursors are developed through the breakdown of proteins and lipids in the flour. During baking, heat triggers the Maillard reaction and caramelization, which interact with the fermentation products to form compounds like pyrazines and aldehydes. These reactions are responsible for the rich brown crust color and the toasted, nutty notes in the final bread.
Environmental Controls for Starter Health
The baker controls the starter by manipulating its environment, influencing the balance and activity of microbial populations. Temperature is the most powerful lever, as preferred growth ranges for bacteria and yeast differ significantly. Lactic acid bacteria generally thrive in warmer conditions, with optimal temperatures between 86°F and 95°F (30°C–35°C).
Wild yeast favors slightly cooler temperatures, typically around 77°F (25°C). Fermenting the starter at a higher temperature (80°F to 86°F) promotes lactic acid production, yielding a milder bread. Conversely, fermenting at cooler temperatures (68°F to 75°F) slows the rate but favors heterofermentative bacteria, increasing the ratio of sharp acetic acid.
The hydration level (ratio of water to flour) also plays a role in acid production. Lower hydration, resulting in a stiffer dough, encourages acetic acid production. A wetter, higher-hydration starter promotes acid diffusion and favors lactic acid bacteria, leading to a milder flavor.
Feeding ratios—the amount of mature starter mixed with fresh feed—dictate the rate and duration of the feeding cycle. A small inoculation slows fermentation, allowing microbes more time to produce complex flavor compounds. By adjusting temperature, hydration, and feeding ratios, the baker guides the microbial community to achieve the desired balance of rise and flavor.