Bulk Fermentation — What It Is and How to Get It Right

Bulk fermentation is the most important phase of the sourdough bread-making process. It's the long rest period between mixing the dough and shaping it — when the levain ferments the dough, gluten develops, gases accumulate, and flavour builds. Getting bulk fermentation right is the single biggest factor in whether your bread turns out well.

What Is Bulk Fermentation?

After you mix your dough with the levain (and any salt), you leave the combined dough to ferment as one large mass — hence "bulk." During this time:

• Wild yeast consumes sugars and produces CO2, making the dough rise and become airy
• Lactic acid bacteria produce organic acids that give sourdough its flavour
• Gluten continues to develop and strengthen through the stretch and fold sets

This phase typically lasts 4–8 hours at room temperature, though it can be significantly shorter or longer depending on temperature, levain percentage, and flour type.

How Temperature Controls Bulk Fermentation

Temperature is the dominant variable. The same dough can bulk ferment in 4 hours at 26°C or 8 hours at 20°C:

• 18°C: 8–12 hours (common in Australian winters)
• 20°C: 6–8 hours
• 22°C: 5–6 hours
• 24°C: 4–5 hours
• 26°C: 3–4 hours (common in Australian summers — watch carefully)

This is why recipes that say "bulk ferment for 5 hours" are only accurate for a specific temperature. Learn to read the dough, not the clock.

What Does Properly Bulk-Fermented Dough Look Like?

At the end of bulk fermentation, the dough should:

• Have increased in volume by 50–75% (not doubled — that's usually over-fermentation for sourdough)
• Feel noticeably lighter and more airy than at the start
• Show bubbles on the surface and at the sides of the container
• Jiggle slightly when you shake the container — a sign of gas development
• Have a smooth, slightly domed top
• Pass the poke test: poke with a wet finger, the indent should spring back slowly but not completely

Under-Fermented Dough — Signs and Consequences

Under-fermented dough hasn't had enough time to develop:

• Dense, tight crumb with no irregular holes
• Little oven spring — the loaf doesn't rise much in the oven
• Bland, underdeveloped flavour
• The dough may feel heavy and dense when you try to shape it

Fix: Extend bulk fermentation or warm up the environment. If you suspect under-fermentation, give the dough another hour at a warm temperature before shaping.

Over-Fermented Dough — Signs and Consequences

Over-fermented dough has gone too far:

• The dough feels very slack, sticky, and hard to shape
• It may smell very sour or alcoholic
• When you score the baked loaf, it spreads sideways rather than rising
• The crumb can be gummy or dense despite having large holes

Fix: Shape immediately and keep the proof short. Over-fermented dough can still produce acceptable bread if baked quickly — it won't be your best loaf, but it's not a total loss.

How Much the Dough Should Rise

The "double in size" rule often cited for bread-making doesn't apply as neatly to sourdough. Most recipes target 50–75% volume increase during bulk fermentation. A full doubling usually means the dough has over-fermented, though this varies with levain percentage and flour.

Mark the starting level with a rubber band on a clear container. This makes it easy to track the rise without guessing.

Bulk Fermentation in the Fridge

You can bulk ferment in the fridge (called a cold bulk ferment) for a slower, more controlled fermentation. This takes 8–16 hours at 4°C and is useful if you want to start the dough in the evening and shape in the morning. The dough should still increase by around 30–50% during cold bulk fermentation.

For the full process from levain through to baking, see our Classic Sourdough Bread recipe. And for a starter to bake with, browse our sourdough starter kits.