Yeast
Yeast is a single-celled fungus that feeds on sugars and releases carbon dioxide and alcohol as by-products. In bread, the carbon dioxide gets trapped in gluten and makes the dough rise, while the alcohol and other compounds add much of bread's flavour and aroma.
Nearly all baking yeast is one species, Saccharomyces cerevisiae, the same one used for most beer and wine.
How yeast makes bread rise
Flour contains a small amount of sugar, and its enzymes break starch down into more. Yeast absorbs those sugars and, through fermentation, converts them to carbon dioxide and ethanol.
The gas inflates tiny air pockets created during mixing and kneading. Stretchy gluten holds it in, so the dough expands. In the oven, gas expands further and alcohol evaporates, contributing to oven spring, until the heat kills the yeast and the crumb sets.
Types of baking yeast
| Type | What it is | How to use |
|---|---|---|
| Active dry | Dried granules with a protective outer layer | Traditionally bloomed in warm water 5–10 min |
| Instant (rapid-rise, bread machine) | Finer, more porous granules | Mix straight into the flour |
| Fresh (cake, compressed) | Moist block, about 70% water | Crumble into liquid; keeps 1–2 weeks refrigerated |
| Osmotolerant instant | Instant yeast bred for high-sugar dough | Sweet, enriched breads like brioche |
Rough equivalents by weight: 1 g instant = about 1.25 g active dry = about 3 g fresh. More in the yeast conversion chart.
Temperature and yeast activity
| Temperature | What yeast does |
|---|---|
| Below 4 °C (39 °F) | Nearly dormant (fridge retarding) |
| 24–27 °C (75–80 °F) | Steady rise, balanced flavour |
| 32–35 °C (90–95 °F) | Fast rise, less flavour |
| 40–46 °C (105–115 °F) | Ideal water for blooming active dry |
| 55–60 °C (130–140 °F)+ | Yeast dies |
Slower, cooler fermentation produces more flavourful bread. Fast, warm rises taste flatter.
What slows yeast down
- Salt moderates yeast in normal amounts. Avoid dumping undissolved salt directly onto yeast in a small bowl.
- Sugar above roughly 10% of flour weight pulls water out of yeast cells by osmosis. Sweet doughs rise slowly unless you use osmotolerant yeast.
- Cinnamon, garlic, and high fat inhibit yeast slightly, which is why cinnamon swirls are added after the first rise.
Chef Fizz says: Store opened dry yeast airtight in the freezer. It stays lively for months, and you can use it straight from frozen.
Testing if yeast is alive
Stir active dry or instant yeast into 60 ml (¼ cup) water at 40–46 °C (105–115 °F) with a pinch of sugar. If it isn't foamy in 10 minutes, it's dead, and no amount of waiting will make your dough rise. See blooming.
Questions
What is the difference between active dry and instant yeast?
Active dry yeast has larger granules and is traditionally dissolved in warm water first. Instant yeast is finer, can be mixed straight into flour, and is a bit more potent, so you need about 25% less by weight.
What temperature kills yeast?
Yeast starts to die around 55–60 °C (130–140 °F). For blooming, keep water at 40–46 °C (105–115 °F), which feels warm but not hot on your wrist.
How much yeast is in a packet?
A standard packet or envelope holds 7 g (¼ oz), about 2¼ teaspoons, of active dry or instant yeast. That's enough for roughly 500 g (4 cups) of flour in a typical bread recipe.
Does salt kill yeast?
Salt in direct, concentrated contact can slow or damage yeast, but in normal amounts, about 2% of the flour weight, it simply moderates fermentation. Mixing them into the flour separately is plenty.