Sourdough Bulk Fermentation Timer

Common uses for a sourdough bulk fermentation timer

Bulk fermenting a sourdough loaf
Adjusting a recipe to a cold kitchen
Planning an overnight bulk
Timing bulk around stretch and folds

Bulk fermentation is the stretch between mixing and shaping, where the dough does most of its rising and develops most of its flavour. It is the step people most often get wrong, and almost always because they follow a recipe's stated time without noticing that the recipe assumed a warmer or cooler kitchen than theirs.

Fermentation rate is governed by dough temperature far more than by the clock. Yeast and bacterial activity roughly doubles for every increase of around 8–10 °C, so dough at 26 °C ferments in about half the time of the same dough at 18 °C^[1]. This is why the same recipe produces an under-proofed loaf in a cold winter kitchen and an over-proofed one in August. Measure the dough, not the room — they are frequently several degrees apart after mixing.

Watch the dough, not the timer

The reliable signal is rise, not elapsed time. Most sourdough recipes look for something in the region of a 25–50% increase in volume during bulk, along with a domed surface, a few bubbles, and a jiggle when you shake the container.

A straight-sided container makes this readable, and a mark or elastic band at the starting height makes it obvious. Judging rise in a mixing bowl that curves inward is guesswork, which is part of why bulk goes wrong so often.

Under-proofed and over-proofed look different

An under-proofed loaf is dense at the base with large irregular holes toward the top, and it usually bursts unpredictably in the oven. An over-proofed one spreads flat, has a fragile slack feel when handled, and produces a pale, gummy crumb.

If you are getting the second, shorten bulk or cool the dough rather than changing your flour or your starter. Temperature is nearly always the variable actually responsible.

Guide times for a typical white sourdough at around 20% starter. Fermentation roughly doubles in rate for every 8–10 °C increase^[1], so treat these as a starting point and judge by rise.

Approximate bulk fermentation time by dough temperature
Dough temperature	Approximate bulk time	Practical note
18 °C (64 °F)	8–10 h	Cold kitchen; consider a warmer spot
20 °C (68 °F)	6–8 h	Slow, good flavour development
22 °C (72 °F)	5–6 h	Common room temperature
24 °C (75 °F)	4–5 h	Comfortable working pace
26 °C (79 °F)	3–4 h	Fast; check from 3 h
28 °C (82 °F)	2–3 h	Very fast; easy to over-proof

Measure the dough temperature after mixing, not the room — they are often several degrees apart.
A higher proportion of starter shortens all of these; a stiff or wholegrain dough behaves differently again.

Source: On Food and Cooking: The Science and Lore of the Kitchen — fermentation and temperature (ISBN 978-0-684-80001-1)

Sources

On Food and Cooking: The Science and Lore of the Kitchen — fermentation and temperature (ISBN 978-0-684-80001-1) , Harold McGee, Scribner (Revised edition, 2004) — accessed 2026-07-28

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❓ Frequently Asked Questions

How long should bulk fermentation take?

It depends almost entirely on dough temperature — roughly 3–4 hours at 26 °C, 5–6 at 22 °C, and 8–10 at 18 °C. Fermentation rate roughly doubles for every 8–10 °C increase, which is why the same recipe behaves completely differently in summer and winter.

Should I measure the room temperature or the dough?

The dough. Mixing adds friction heat and the ingredients start at their own temperatures, so dough is often several degrees away from the room after mixing. A cheap probe thermometer removes most of the guesswork from sourdough.

How do I know when bulk is finished?

By rise rather than by the clock: typically a 25–50% increase in volume, a domed surface, some bubbles and a jiggle when you shake the container. Use a straight-sided container with the starting height marked — judging rise in a curved bowl is guesswork.

What does an over-proofed loaf look like?

It spreads flat rather than holding its shape, feels slack and fragile when handled, and bakes to a pale gummy crumb. If that is what you are getting, shorten bulk or keep the dough cooler before you start changing flour or starter.

What about an under-proofed loaf?

Dense at the base with large irregular holes toward the top, and it tends to burst unpredictably in the oven rather than opening at the score. It usually means bulk was cut short — often because the recipe's stated time assumed a warmer kitchen.

Can I bulk ferment in the fridge?

You can, and it slows everything dramatically — many bakers do the final proof cold rather than bulk. A cold bulk takes many hours and develops a more acidic flavour, since the bacteria tolerate cold better than the yeast does.