Yeast and fermentation basics

Short answer

Yeast feeds on sugars in the flour and produces carbon dioxide, which inflates the dough, along with alcohol and other compounds that contribute flavour. Less yeast with more time — often in the fridge — and more yeast with less time can both work; the trade-off is mainly flavour development and convenience.

What yeast does

Baker's yeast is a living fungus. In dough it breaks down sugars, many of them released from starch by the flour's own enzymes, and produces carbon dioxide and ethanol. The gas is trapped by the gluten network, which is what makes the dough rise and what later expands in the oven to give the rim its lift.

Fermentation also changes flavour and texture. Over longer periods, by-products such as organic acids and aromatic compounds build up and the gluten relaxes, which is why a slowly fermented dough usually tastes more complex and stretches more easily than a same-day one. Fermentation is not only about rise.

The main types of yeast

Instant yeast is a dried yeast that can be mixed straight into the flour. Active dry yeast is also dried; it was traditionally dissolved in warm water first, and King Arthur notes that modern active dry can often be used the same way as instant. Fresh (compressed) yeast is moist, perishable and keeps for a short time in the fridge. Sourdough, or natural leaven, is a culture of wild yeasts and bacteria maintained by the baker; it ferments more slowly and less predictably and adds noticeable acidity.

As a rule of thumb, fresh yeast is used at roughly three times the weight of instant yeast; King Arthur's conversion is fresh × 0.33 to get instant or active dry. Between instant and active dry, sources disagree. King Arthur and the manufacturer Red Star treat them as interchangeable 1:1 by weight, while many recipes still call for roughly 1.25–1.5 times as much active dry as instant. King Arthur attributes the old gap to earlier manufacturing that left more dead cells in active dry yeast. Treat any conversion as a starting point, and judge the dough rather than the clock. Sourdough cannot be converted by a fixed factor because starters vary so much in strength.

Time, temperature and yeast quantity

Yeast activity rises with temperature over the range a kitchen is likely to see, so warmer dough ferments faster and colder dough slower. That gives three levers that trade against each other: yeast quantity, time and temperature. A same-day dough uses more yeast at room temperature; a dough meant to ferment for one to three days uses much less yeast and spends most of that time in the fridge.

Cold fermentation is popular because it slows gas production enough that the dough can ferment for a long time without over-proofing, while flavour development carries on. It also makes timing flexible. The cost is planning: cold dough usually needs time back at room temperature before it stretches well — often an hour or more, depending on ball size and the kitchen. Room-temperature fermentation is faster and needs no fridge space, but there is less margin for error on a warm day.

How to tell when dough is ready

Recipe times are only guides, because flour, water temperature, kitchen temperature and yeast all vary. Ready dough balls have usually grown noticeably, look smooth and slightly domed, and show some bubbles at the sides or base of a clear container.

The poke test is a common check: press a floured finger about a centimetre or two into the dough. If the dent springs back immediately the dough probably needs more time; if it springs back slowly and partly, it is likely ready; if it does not spring back at all or the dough sags, it may be over-proofed. The test is a rough guide. It is less reliable on cold dough, which feels firm and springs back even when well fermented, and on very wet doughs, which barely spring back at all.

Common misconceptions

“More yeast makes better pizza dough.”

More yeast mainly makes dough rise faster. Beyond what the schedule needs, it shortens the window before over-proofing and can give a more pronounced yeasty or alcoholic smell.

“Yeast types convert by exact, agreed factors.”

Fresh at about three times instant is widely used, but the instant-to-active-dry ratio is disputed between 1:1 and roughly 1.25–1.5:1. Any conversion is an approximation.

“Cold fermentation stops fermentation.”

It slows it. A dough left in the fridge too long, or made with too much yeast, can still over-proof.

What this page cannot tell you

This page does not give yeast quantities for specific schedules, because the right amount depends on flour, temperatures and the yeast itself; use the fermentation schedule tool and adjust from experience. Sourdough is covered only in outline. Yeast conversions are approximations and the sources quoted disagree.

Related

Evidence

• How do I bake with fresh yeast? — King Arthur Baking Company
• Active dry vs. instant yeast: Why we prefer instant — King Arthur Baking Company
• Saf-instant Red Yeast — Red Star Yeast (Lesaffre)
• The Science of Fermentation: The Secret to Good Pizza — Baking Steel
• How to tell if bread dough has risen enough — King Arthur Baking Company
• How to Use the Dough Poke Test — The Perfect Loaf