The Chemistry of Flavor and Rise

The distinct roles of yeast and bacteria define the final characteristics of sourdough bread. Yeasts serve as the primary leavening agents. They metabolize simple sugars in the flour and produce carbon dioxide (CO2) gas as a byproduct. This gas becomes trapped within the dough's elastic gluten network, causing it to expand and rise, ultimately creating the light, airy crumb of a well-baked loaf.

Lactic Acid Bacteria, on the other hand, are the primary architects of flavor. They are broadly classified into two functional groups:

• Homofermentative LAB produce mainly lactic acid. This acid imparts milder, creamier, and yogurt-like flavor notes to the bread.
• Heterofermentative LAB produce both lactic acid and acetic acid (the primary component of vinegar), as well as some CO2. Acetic acid contributes a sharper, more pronounced tanginess.

The signature sour flavor of any given sourdough loaf is the direct result of the ratio between these two acids. This balance is not fixed; it is a dynamic variable that the baker can actively manipulate through factors such as fermentation temperature, dough hydration, and the types of flour used. Beyond these primary acids, the fermentation process generates over 300 identified volatile organic compounds, including alcohols and esters, which contribute to the nuanced and complex aroma profile that distinguishes sourdough from commercially yeasted bread.