Short answer
Commercial baker’s yeast is a concentrated yeast ingredient added to dough for leavening. A sourdough starter is a maintained mixture of flour and water hosting yeasts and lactic-acid bacteria. Both can raise bread, but the starter is a culture that must be refreshed and can contribute acidity and flavor as well as gas. 1 2
A sourdough starter contains yeasts and lactic acid bacteria in a maintained culture, while commercial baker’s yeast is a cultivated yeast ingredient.
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At a glance
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| Question | yeast | sourdough starter |
|---|---|---|
| What is it? | Commercial leavening agent | Cultured mixture of microbes |
| How is it used? | Added directly to dough | Fed and maintained, then used |
What each thing is
Yeast. In US bread baking, yeast usually means commercially cultivated baker’s yeast sold for adding to dough. It ferments sugars and produces carbon dioxide that helps dough rise. 2
Sourdough starter. A sourdough starter is a maintained flour-and-water culture containing yeasts and lactic acid bacteria; bakers use portions of it to leaven and flavor dough. 1 2
Key differences
Commercial baker’s yeast is a cultivated yeast ingredient. Sourdough starter is a maintained flour-and-water culture containing yeasts and lactic acid bacteria. Their distinction is ingredient versus mixed culture, not a claim that every culture has a fixed species count. 1 2
How to tell them apart
You can identify commercial yeast by its packaging (packets, jars, or blocks) and its use as a direct ingredient. Sourdough starter is typically kept in a jar, requires regular feeding, and is visibly active with bubbles, but appearances can overlap if not labeled. 1 2
Where they overlap
Both can produce carbon dioxide to leaven bread. A flour-and-water preferment made with commercial yeast may also harbor other microbes over time, so its exact microbial composition is not a reliable naming test. 1 2
Edge cases
A poolish or biga is a flour-and-water preferment started with commercial yeast. It may look like a starter while fermenting, but it is generally made for a particular batch rather than maintained through repeated feedings. Some bakers also combine commercial yeast with sourdough in one dough. 2
Why the distinction exists
Commercial yeast gives bakers a concentrated, predictable leavening ingredient. A starter is maintained as a mixed microbial culture; its yeasts produce gas while bacteria contribute acids that shape flavor. The distinction is between an ingredient and an ongoing culture, not a claim that one method is superior. 1 2
Common misconceptions
A common misconception is that sourdough starter is simply ‘natural yeast.’ In fact, it is a symbiotic culture of both wild yeast and bacteria, not just yeast alone. 1 2
Examples
A US home baker buys a packet of instant yeast to make sandwich bread, adding it straight to the dough. For sourdough, they keep a jar of starter in the fridge, feeding it regularly before using a portion to leaven rustic loaves. 1 2
Sources
Sources checked October 3, 2026.
- King Arthur Baking — Sourdough baking guide. Starter maintenance and fermentation.
- King Arthur Baking — Baking with preferments. Commercial yeast and preferments.