How Yeast Works

• Yeast is a single-celled organism that reproduces when exposed to moisture and warmth. When yeast is mixed with flour and water, it begins to feed on the starches and sugars, releasing ethyl alcohol and carbon dioxide gas as byproducts. Gluten strands in the dough trap those carbon dioxide gas bubbles and force the bread to rise. The heat of baking kills the microorganisms, while the bubbles are fixed inside, creating bread’s light, airy texture.

Starter, Explained

• Making a starter requires mixing flour and water and leaving it exposed to the air. Wild yeast is a tiny fungus that is naturally in the air, on some work surfaces, and in flour. Friendly bacteria, called Lactobacilli, are also all around us. Together these microorganisms colonize in the starter and begin to ferment. As the Lactobacilli ferment, they produce acids, and it is those acids that produce the distinctive tang of sourdough bread. Different wild yeasts and bacteria live in different areas, so no loaf of bread from wild yeast starter will be exactly like another. The famous San Francisco sourdough bread has such a distinctive flavor that the bacteria in its starter have a scientific name: Lactobacillus sanfranciscensis.

Sourdough History

• Before yeast was commercially available, sourdough starter was the only way to leaven bread, and the term sourdough refers to both the starter and the bread made from it. Starters were so highly valued by early prospectors that they slept with them to keep them from freezing when temperatures plummeted. Some European bakeries have starters that have been preserved for hundreds of years.

Different Starter, Different Loaf

• The wettest type of starter is called a “sponge,” which is made by allowing some of the yeast and dough in a recipe to ferment before adding it to the rest. Other starters include a “poolish,” the starter used as the foundation of French bread baking, and a “biga,” a thick starter made with more flour than water that is used to create Italian breads such as ciabatta and foccacia. The spongy leftovers in the bottom of the bowl that are scraped down and reused as starter for weeks or decades, is termed, appropriately “the mother.”