I’m a bit confused about sourdough bread. I know it’s supposed to have a tangy flavor and a characteristic texture. Some people talk about a "sourdough starter" but I’m not entirely sure what that is. So, is commercially produced yeast, like the kind you buy in packets at the grocery store, needed for sourdough bread, or does the leavening come from something else entirely? If it doesn’t use commercial yeast, what does make the bread rise and give it that distinctive sourdough flavor and texture? Is it just the starter, and if so, what’s in the starter that causes it to do that?

Answer

Yes, yeast is needed for sourdough bread, but it’s wild yeast, not commercially produced baker’s yeast.

Sourdough bread relies on a "starter," a culture containing wild yeasts and lactic acid bacteria (LAB). This starter is created by combining flour and water and allowing it to ferment over time, capturing the naturally occurring yeasts and bacteria present in the flour and the environment.

Here’s a breakdown:

• Wild Yeast: These are different strains of yeast than those found in commercial baker’s yeast. They’re naturally present in flour (especially whole grain varieties) and the surrounding environment. The yeast consumes sugars in the flour and produces carbon dioxide, which leavens the bread, and alcohol, which contributes to flavor.

• Lactic Acid Bacteria (LAB): These bacteria are also crucial. They ferment sugars and produce lactic acid and acetic acid. Lactic acid contributes to a mild, tangy flavor, while acetic acid gives a sharper, more vinegary taste. The balance between these acids influences the overall flavor profile of the sourdough. LAB also help to inhibit the growth of undesirable microorganisms and contribute to the bread’s keeping qualities.

• The Starter: The starter is a living ecosystem where the yeast and LAB coexist and thrive. It needs regular feeding (adding fresh flour and water) to maintain the microbial population and ensure it’s active enough to leaven bread.

• No Commercial Yeast: True sourdough bread relies solely on the wild yeasts and LAB in the starter for leavening and flavor development. Adding commercial yeast would alter the fermentation process, flavor, and texture, resulting in a different type of bread, not authentic sourdough. Some recipes use a tiny amount of commercial yeast to supplement the wild yeast and LAB but this can affect the characteristics of traditional sourdough.