How Does Yeast Work?
Yeast is a single-celled fungus that ferments sugars, producing carbon dioxide gas and alcohol as byproducts. In bread baking, the CO2 gets trapped in gluten networks, causing dough to rise. The alcohol evaporates during baking. Yeast is most active between 75-95°F (24-35°C) and needs food (sugars/starches), moisture, and warm temperature to work.
Key Takeaways
- Yeast cells consume sugars (from added sugar or broken-down starches in flour) through a process called fermentation.
- Salt inhibits yeast - add salt to flour, not directly to yeast/water mixture, to avoid killing yeast.
- Yeast also creates flavor compounds beyond just CO2 - longer, slower rises produce more complex bread flavor.
Explanation
Yeast cells consume sugars (from added sugar or broken-down starches in flour) through a process called fermentation. For each sugar molecule consumed, yeast produces two carbon dioxide molecules and two alcohol molecules. In bread, CO2 inflates air bubbles trapped by gluten strands, making the dough expand and become light.
Temperature dramatically affects yeast activity. Below 50°F, yeast goes dormant (useful for refrigerator rising). At 75-95°F, it's most active. Above 120°F, yeast cells begin to die. That's why bread recipes specify 'lukewarm' water for dissolving active dry yeast - too hot kills it, too cold slows it.
Different yeast types work slightly differently: active dry yeast must be dissolved in warm water to 'wake up'; instant/rapid-rise yeast can be mixed directly with dry ingredients; fresh cake yeast is highly perishable but very active. All produce the same fermentation, just at different speeds.
The alcohol produced during bread fermentation typically reaches 1-2% concentration in raw dough before baking. During baking at 350-400°F, virtually all alcohol evaporates by the time the internal temperature reaches 190°F, leaving behind no measurable alcohol in finished bread. In beer and wine making, the same fermentation process is used but without the baking step, allowing alcohol to accumulate to 4-15% depending on the sugar content and yeast strain's alcohol tolerance.
A single gram of active dry yeast contains approximately 10 billion individual yeast cells, predominantly Saccharomyces cerevisiae. During a typical 1-2 hour rise at room temperature, the yeast population can double 2-3 times, meaning a standard 2.25-teaspoon packet (7 grams) can grow from about 70 billion cells to over 200 billion by the time dough is fully risen. This exponential growth is why over-proofing (letting dough rise too long) causes collapse - the yeast exhausts the available sugars and produces too much CO2 for the gluten structure to contain.
Things to Know
- Salt inhibits yeast - add salt to flour, not directly to yeast/water mixture, to avoid killing yeast.
- Yeast also creates flavor compounds beyond just CO2 - longer, slower rises produce more complex bread flavor.
- Sourdough uses wild yeast plus bacteria, which produces more acid and distinctive sour taste.
- High sugar levels (over 10% of flour weight) slow yeast - sweet doughs need more yeast or special osmotolerant varieties.