The reaction
Yeast prefers to breathe. Given plenty of oxygen it burns sugar completely to carbon dioxide and water, and it grows fast doing it.
Deprived of oxygen — inside a dough, under a liquid, in a sealed vessel — it switches to fermentation instead: it breaks sugar down only partway, into ethanol and carbon dioxide, and gets far less energy out of it. That partial breakdown is the useful one, and every yeasted food and drink depends on it.
In bread, the carbon dioxide is the product and the alcohol boils off in the oven. In beer and wine, the alcohol is the product and the carbon dioxide is either vented or kept for fizz. Same organism, same reaction, opposite priorities.
What controls the rate
Four levers, and every dough problem is one of them.
- Temperature. The dominant control. Yeast works faster as it warms, roughly doubling in pace for a modest rise in temperature across the normal kitchen range, until it gets hot enough to start killing the cells. Cold does not kill it; it slows it down, which is what a fridge prove is for.
- Sugar availability. Yeast cannot eat starch. In a lean bread dough it depends on the flour's own enzymes cutting starch into sugars, which is one reason a slow ferment develops flavour a fast one does not.
- Salt. Slows yeast down by drawing water away from it. This is a feature: salt in bread dough is partly there to keep the rise controlled and even. Dough made without it rises fast and collapses.
- Sugar in quantity. Counter-intuitively, a very sweet dough ferments slowly for the same osmotic reason, which is why enriched doughs take longer and often use more yeast.
- Alcohol. Yeast eventually poisons itself with its own ethanol. This is what sets the natural ceiling on the strength of a fermented drink.
Where the flavour comes from
Carbon dioxide has no taste and ethanol mostly leaves the bread. The reason a long-fermented loaf tastes better than a fast one is everything else the yeast makes.
Alongside the main reaction, yeast produces esters (fruity), higher alcohols, aldehydes and organic acids, in quantities that depend on temperature, time and how stressed the culture is. It also breaks down and reassembles compounds in the flour. A cool, slow ferment accumulates far more of this than a warm, fast one, which is the entire argument for an overnight prove.
In sourdough there is a second population — lactic acid bacteria — working alongside the yeast and producing acid and its own aromatics, which is why sourdough tastes of considerably more than yeasted bread. See lactic fermentation.
The yeasts a cook meets
- Fresh yeast
- Compressed cake, perishable, crumbled straight into dough. Traditional, and still preferred by many bakers. Keeps days, not months.
- Active dried yeast
- Granules that need rehydrating in warm liquid before use. Skipping that step is a common reason a dough fails to rise.
- Instant or fast-action yeast
- Finer granules that go straight into the flour. The most forgiving and the most common domestically.
- Sourdough starter
- A live mixed culture of wild yeasts and lactic acid bacteria, maintained by feeding. Slower, more variable, and responsible for a flavour no packet yeast produces.
- Brewer's and wine yeasts
- Strains selected for alcohol tolerance and flavour rather than for gas production. Related organisms, different jobs.
Common questions
Does bread contain alcohol?
A trace. Most of the ethanol evaporates during baking, and the crumb temperature exceeds ethanol's boiling point well before the loaf is done.
Why does my dough rise faster in summer?
Temperature. Yeast speeds up substantially as the kitchen warms, which is why the same recipe behaves differently across the year and why bakers time by dough rather than by clock.
Does salt kill yeast?
No, it slows it by osmosis. Direct contact between undissolved salt and fresh yeast is worth avoiding, but salt in a dough is doing a useful job.
Why prove dough in the fridge?
Because cold slows the gas production much more than it slows flavour development, so you get a better-tasting dough for the same amount of rise.
Can I use less yeast and more time?
Yes, and it is usually an improvement. Less yeast and a longer ferment gives more flavour and a more even crumb.
Try this next
- Yeast the organism itself
- Lactic fermentation what works alongside it in sourdough
- Bread the thing it exists to make
- Bread not rising when it fails to happen
- Gas expansion what the bubbles do in the oven