Bulk Fermentation in Sourdough: A Beginner's Guide (UK)

Bulk fermentation is the most important step in sourdough baking, and the one most home bakers get wrong. It's the period between mixing your dough and shaping it, when the wild yeast and bacteria do their work — building flavour, structure, and gas. Stop bulk too early and you get dense bread. Stop too late and the dough collapses. Get it right and the rest of the bake almost takes care of itself. This guide covers everything.

What bulk fermentation actually is

Bulk fermentation is the period — typically 4–8 hours at room temperature — between mixing the dough and dividing/shaping it. During this time, three things happen simultaneously:

1. Fermentation: Wild yeast eats the sugars in the flour, producing CO₂ (which inflates the dough) and ethanol (which contributes to flavour and is mostly cooked off in the oven).
2. Gluten development: The gluten network strengthens through the stretch-and-folds you perform, holding gas more efficiently as the dough develops.
3. Acid production: Lactic acid bacteria produce the tang that defines sourdough, while also conditioning the gluten and contributing to crumb softness.

If you stop bulk before all three have progressed enough, your bread will be dense and bland. If you push past the optimal point, the gluten breaks down and the dough collapses.

The five signs your bulk is ready

Recipe times are guidelines. The dough is the truth. Five signs to look for:

1. Volume increase of 50–80%

The dough should grow noticeably in the bulk container. Use a clear container (Tupperware works well) so you can see the rise. Mark the starting level with tape or a rubber band; bulk is done when the dough has grown by 50–80%. Less than 50% means under-bulked; more than 100% means over-bulked.

2. The jiggle

Tip the bowl. The dough should wobble like soft jelly — bouncy, slightly slack, full of air. A tight, smooth dough that doesn't wobble is under-bulked.

3. Visible bubbles

Look at the side of the bowl through the dough. There should be visible bubbles distributed through the dough, with some larger bubbles starting to form near the top.

4. The poke test

Wet your finger and poke the dough about 1cm deep. The dimple should:

• Spring back fully and quickly: under-bulked. Wait longer.
• Spring back slowly, leaving a slight indent: ready. Shape now.
• Stay where it is and not spring back at all: over-bulked. Shape immediately and skip the cold retard.

5. The smell

The dough should smell mildly fermented — yeasty, slightly tangy, sweet. A strong alcoholic or vinegary smell means over-bulked.

The temperature factor

Bulk time is dramatically affected by temperature. The same dough behaves very differently at different temperatures:

Dough temperature	Bulk time (70% hydration, 20% starter)
18°C	8–10 hours
20°C	6.5–8 hours
22°C	5.5–6.5 hours
24°C	4.5–5.5 hours
26°C	3.5–4.5 hours
28°C+	2.5–3.5 hours

Most UK kitchens drift between 18–22°C across the year. A bulk schedule built for 24°C summer kitchens will be 2 hours short in February. Trust the dough, not the clock.

The aliquot jar trick

This is the professional baker's trick that's only recently caught on with home bakers. After mixing, take 50g of dough and put it in a tall narrow jar (a small olive jar works perfectly). Mark the level. The jar sits next to your bulk bowl in the same spot.

When the dough in the jar has grown by 70–80%, your main dough has too — and the small portion is much easier to read against the jar's straight sides than the irregular surface of a bowl.

This works because the jar gives you a clear, calibrated way to measure rise. You're not relying on visual estimation of an irregular shape; you're reading a percentage off a glass tube. Once you've used an aliquot jar a few times, you'll find your judgment about the main bulk improves too.

Stretch-and-folds during bulk

Stretch-and-folds are gentle gluten-development sessions performed during bulk. They build strength without aggressive kneading. Standard schedule:

• Set 1: 30 minutes after mixing.
• Set 2: 30 minutes after Set 1.
• Set 3: 45 minutes after Set 2.
• Set 4: 45 minutes after Set 3.
• Then leave the dough alone for the remaining 1.5–2.5 hours.

How to perform a stretch-and-fold: wet your hand. Reach under one side of the dough, lift it up high until it stretches, then fold it over the top. Rotate the bowl 90° and repeat three more times for the four sides.

The dough should feel rough at Set 1 and silky-smooth by Set 4. If it still feels rough at Set 4, add another set.

How hydration changes bulk

Higher hydration ferments faster. The yeast has more access to water and nutrients. Standard guidelines:

• 65% hydration: longer bulk, slower rise. Easy to under-bulk.
• 70% hydration: standard pace. Most beginner recipes here.
• 75% hydration: 30 minutes faster than 70%. Watch closer.
• 80% hydration: 1 hour faster than 70%. Easy to over-bulk if you're not paying attention.
• 85%+ hydration: very fast bulk. Watch every 30 minutes.

Common bulk-fermentation mistakes

1. Going by the clock instead of the dough

The single biggest cause of failed sourdough. Recipe says "6 hours bulk" — your kitchen is at 19°C, the dough barely moves, but you shape on schedule because the recipe said so. Result: brick.

2. Cold dough temperature

Most recipes assume 24°C dough. UK kitchens at 18–20°C produce dough at 20–22°C. The bulk takes 2+ hours longer than the recipe says. Probe-thermometer your dough; adjust accordingly.

3. Weak starter

If your starter is sluggish, the dough won't bulk no matter how long you wait. Confirm your starter is doubling reliably in 4–8 hours before mixing dough.

4. Insufficient stretch-and-folds

The gluten won't develop without folds. A bulk dough that's never been folded is a slack mess. Three sets minimum, four for higher hydrations.

5. Over-bulking

Less common than under-bulking but happens, especially in summer. The dough grows past 100%, the gluten weakens, and shaping produces a slack mess that never recovers.

Reading the dough at peak

The peak of bulk is a 30–60 minute window. The dough is at maximum gas retention without yet starting to deflate. Visually, it's domed, jiggly, and just starting to show signs of softening at the surface.

Once you've baked twenty loaves, you'll know peak by feel. Until then, trust the poke test: it should spring back slowly and incompletely. That moment is your shaping cue.

What if you have to leave?

Life happens. The dough is in bulk and you have to go out for 4 hours. Two options:

• Refrigerate the bulk container. Slows fermentation by 75%. A dough that would over-bulk in 2 hours can sit in the fridge for 8.
• Push to shape early and cold-retard immediately. Less ideal but works in a pinch.

The fridge is your friend. It pauses fermentation rather than stopping it, so the dough continues developing flavour without losing structure.

The final temperature target

Aim for a final dough temperature (immediately after mixing) of 24–26°C. This is the sweet spot for most recipes — not so warm that it ferments too fast, not so cool that it drags on for hours.

Adjust water temperature to hit it:

• Cool kitchen (under 22°C): use water at 28–32°C.
• Warm kitchen (over 24°C): use water at 16–18°C.
• Standard kitchen (22–24°C): use water at 22°C (room temperature).

FAQ

How do I know if I've under-bulked or over-bulked?

Under-bulked: dough is tight, smooth, slightly grown, low volume; the loaf is dense and tight-crumbed. Over-bulked: dough is slack, very bubbly, high volume; the loaf is flat and spreads wide.

Can I rescue an under-bulked dough?

Yes — give it more time. If you've already started shaping when you realise, leave the shaped dough at room temperature for an extra 30–60 minutes before fridging.

Can I rescue an over-bulked dough?

Sometimes. Skip the cold retard, shape immediately, give it a short final proof (30 minutes), and bake. The result will be flatter than ideal but better than nothing.

How long should bulk fermentation be?

4–8 hours at 22–24°C with 20% starter. Longer with less starter or cooler temperatures; shorter with more starter or warmer temperatures.

Should I bulk in the fridge?

Some bakers do, but it makes timing harder. Most home bakers bulk at room temperature and fridge after shaping (the cold retard).

Can I shape mid-bulk to break it up?

You'd disrupt the gluten development too much. Stretch-and-folds during bulk, but don't pre-shape until bulk is done.

What's the most reliable bulk indicator?

The poke test combined with a visible volume increase. Together, they're more reliable than any single sign.

The bulk fermentation routine

Once you've nailed bulk fermentation, your sourdough becomes consistent. Most home bakers we know who graduate from "sometimes good" to "reliably good" do it through better bulk reading. The same recipe, the same starter, the same flour — but the loaves are now consistently great because bulk is being managed properly.

Spend the time on bulk. It's the most important 5 hours of your bake. Everything else flows from getting this right.

The skill ladder for bulk

If you're new to sourdough:

1. First 5 loaves: follow the clock; bulk for the time the recipe says.
2. Loaves 6–10: introduce the poke test alongside the clock.
3. Loaves 11–20: use the poke test as the primary indicator; clock as backup.
4. Loaves 21+: trust your judgment entirely. The clock is just a rough guide.

By the time you've baked 20 loaves with attention to bulk, you'll know peak by feel. Until then, use the tools and trust the dough over the timer.

What happens biochemically during bulk

For the technically curious, here's what's actually happening in your bowl. The wild yeast in your starter consumes simple sugars (glucose, maltose) released by enzymes naturally present in flour. As they consume sugars, they produce CO₂ and ethanol — the CO₂ inflates your dough; the ethanol contributes to flavour and is mostly cooked off in the oven. The lactic acid bacteria simultaneously consume the same sugars and produce lactic acid (mild yoghurty flavour) and acetic acid (sharper vinegary flavour). Different conditions favour different organisms — warm bulk favours yeast; cool retard favours bacteria.

Meanwhile, proteolytic enzymes (proteases) slowly weaken the gluten network, while the gluten itself is being rebuilt by your stretch-and-folds. The balance between weakening and strengthening is what gives a properly bulked dough its perfect texture — soft enough to be airy, strong enough to hold shape.

Bulk fermentation in different recipes

Not every recipe wants the same bulk profile. Different breads thrive on different fermentation regimes:

• Country loaves (this guide's main focus): 5–6 hours bulk, 70–80% growth.
• Open-crumb sourdough: 4–5 hours bulk, 70–80% growth, then long cold retard for flavour.
• Sandwich bread: 4 hours bulk, 50–60% growth (less open crumb is the goal).
• Pizza dough: 3 hours bulk at room temperature, then 24+ hours cold ferment.
• Wholemeal: 4–4.5 hours bulk (faster than white).
• Rye: 2–3 hours bulk (rye ferments very fast).

Match your bulk to your recipe. Don't apply country-loaf bulk timing to a pizza dough — they want different things.