Yeast

Natural Yeast (Sourdough Starter)

What It Is
Natural yeasts are present in nature in various forms, mainly on the surfaces of fruits and grains. These yeasts consist of a mixture of bacteria (lactic acid bacteria) and yeasts.

The French word levain, used to describe natural yeast, comes from the Latin levare, meaning “to lift.” In other regions, it is known by different names, such as massa madre in Italy or chef dough in the United States.

Studies have shown that mature sourdough starters can contain more than 25 strains of lactic acid bacteria and 50 types of yeast, primarily from the genera Saccharomyces and Candida. This microbial diversity is influenced not by the origin of the starter but by how it is maintained—specifically, the type of flour and water used for feeding and the fermentation time.

Behavior
Fermentation is a far more complex process than simply breaking down sugars into carbon dioxide and alcohol. During fermentation, the diverse microbiota in the sourdough starter produce a variety of compounds that contribute to unique flavors and aromas.

Among these products are alcohols, carbon dioxide, acids (from sugar breakdown), enzymes released by bacteria and yeasts, and amino acids from protein breakdown—such as gluten—from the flour.

Amino acids play a key role in the formation of the compounds responsible for bread’s aroma and color.

Two particularly important acids formed during fermentation are:

• Acetic acid: Typically produced in lower hydration and cooler temperatures, giving the bread a tangy, vinegar-like acidity.

• Lactic acid: Commonly found in dairy products like milk and butter, contributing to a creamy, buttery flavor in bread.

Baker's Yeast (Fresh or Dry Yeast)

What It Is
Unlike natural yeast, baker’s yeast—commonly found in supermarkets in fresh or dry form—is a monoculture, meaning it contains only a single yeast strain: Saccharomyces cerevisiae.

This yeast is widely used in the food industry as the primary fermentation agent for bread, wine, and beer, and even in the production of biodiesel and ethanol.

Its industrial success is due to its ability to reproduce quickly, produce high amounts of alcohol, and withstand high alcohol concentrations.

Behavior
Compared to sourdough starter, baker’s yeast has some crucial differences in bread-making:

• Produces more carbon dioxide, resulting in a lighter, airier bread.

• Reproduces quickly, meaning fermentation times are shorter.

• Does not release protease enzymes, which means it does not break down external proteins—so gluten and the gluten network remain largely intact.

• More predictable behavior, since it is a single-strain culture, leading to more consistent fermentation times, flavor, and volume.

Substituting Yeasts

The conversion ratio is more precise when substituting between fresh and dry baker’s yeast because they contain the same yeast strain. Typically, the standard ratio is 1:3—meaning 10 g of dry yeast is equivalent to 30 g of fresh yeast.

However, when comparing baker’s yeast to sourdough starter, there is no exact substitution, as they produce fundamentally different results.

Many bakeries combine both sourdough and baker’s yeast in bread production. Baker’s yeast provides better control over dough rise and volume, while sourdough adds unique flavors and complexity.