Why Temperature Matters More Than You Think

Sourdough is a chemistry experiment that runs at room temperature, and your kitchen's temperature is the single biggest factor in how that experiment turns out. A 4°C swing — say from 20°C in winter to 24°C in summer — can halve or double your bulk fermentation time. The bakers who get consistent results aren't following the recipe more carefully than you. They're tracking temperature.

Why temperature is the master variable

Yeast and bacteria — the two organisms doing the work in your starter and dough — are temperature-sensitive in a non-linear way. A 10°C jump roughly doubles their metabolic rate. So a dough that takes 6 hours to bulk at 22°C takes about 3 hours at 30°C, and around 12 hours at 12°C.

Three things change when temperature changes:

1. Speed — fermentation accelerates with heat.
2. Flavour balance — warmer ferments favour yeast (mild, sweet, breadier flavour); cooler ferments favour bacteria (sharper, more tangy, more sour).
3. Structure — warmer doughs get gummier; cooler doughs hold structure longer.

The bakers chasing perfect open crumb at 28°C ferment fast and bake quickly. The bakers chasing complex sour flavour ferment cold and slow. Neither is right or wrong — they're working different sides of the same equation.

The four temperatures that matter in your bake

1. Starter temperature

Affects how fast your starter peaks and how strong the rise is. 22–26°C is ideal; below 18°C the yeast dramatically slows.

2. Final dough temperature (FDT)

The temperature of your dough after mixing, measured with a probe thermometer. This sets the pace of bulk. Most recipes assume 24–26°C; below 22°C, bulk takes much longer than the recipe says.

3. Ambient room temperature

Where your dough actually bulks. UK kitchens range from 16°C in winter to 24°C in summer — a huge spread.

4. Fridge retard temperature

Most fridges run 3–6°C. Cold enough to halt yeast; bacteria still slowly work. This is what gives cold-retarded sourdough its complex flavour.

How to measure each one

Buy a £6 probe thermometer. It's the single best investment in your sourdough kit, and it transforms guesswork into routine.

• Starter temperature: stick the probe in. Record at peak.
• Final dough temperature: stick the probe in the dough immediately after mixing.
• Ambient temperature: leave a thermometer near the dough. Smart-home thermostats usually work too.
• Fridge temperature: stick a probe in for 10 minutes. Most home fridges sit at 4–5°C.

Hitting your target final dough temperature

Final dough temperature is the variable you actually control. Adjust the water temperature to compensate for everything else.

Rough formula:

Water temperature ≈ (Target × 4) − Flour temp − Room temp − Friction

For most home mixes, friction is around 2°C. So if you want a 25°C final dough temperature in a 22°C kitchen with 22°C flour:

(25 × 4) − 22 − 22 − 2 = 100 − 46 = 54°C ÷ 2 ≈ 27°C water

That sounds complex. The shortcut: in a cool kitchen (under 22°C), use water at 28–30°C. In a warm kitchen (over 24°C), use cold tap water (15–16°C). The dough will land somewhere near 25°C.

Bulk fermentation time vs temperature

This is the table that matters most. Approximate bulk fermentation times for a standard 70% hydration country loaf with 20% starter:

Dough temperature	Bulk time	Notes
18°C	8–10 hours	Slow ferment, good flavour, easy to mistime
20°C	6.5–8 hours	Cool UK kitchen norm in winter
22°C	5.5–6.5 hours	Comfortable spring/autumn pace
24°C	4.5–5.5 hours	Sweet spot for most home bakers
26°C	3.5–4.5 hours	Fast — watch closely
28°C+	2.5–3.5 hours	Bakery pace; easy to overproof

These are guidelines — individual starter strength, exact hydration, and how strong your gluten development is all shift the times. Use them as a starting point and trust your eyes for the final call.

Where to bulk in a cold UK kitchen

If your kitchen sits below 21°C, here are the warm spots that work:

1. Top of the fridge. The motor heat keeps it 2–4°C warmer than room temperature. Always our first choice in winter.
2. Inside the cold oven with just the light on. Most ovens hit 24–26°C with the light on. Excellent for bulk.
3. Near (not on) a radiator. About 30cm away gives you a few degrees of lift without baking the dough.
4. In a proofing box. Worth buying if you bake every week. £30 for a basic one.
5. Wrapped in a tea towel inside a cool box. The poor-man's proofer; works surprisingly well.

How temperature changes flavour

Warmer ferments favour yeast and produce mild, slightly sweet, breadier flavours. Cooler ferments favour the bacteria and produce more lactic acid (yoghurty) and acetic acid (sharper, vinegary). The cold retard you do overnight in the fridge is mostly a flavour-builder — that's where the deep tang of a 24-hour-retarded country loaf comes from.

If your bread tastes flat or under-developed, lengthen your cold retard. If it tastes too sharp, shorten the retard or push the bulk a bit warmer.

Seasonal adjustments

Summer (kitchen 24–28°C)

• Use cold tap water (15°C).
• Watch bulk closely — it'll be 1.5–2 hours faster than usual.
• Reduce starter to 15% to slow things down.
• Cold retard is essential — without it, your dough tastes flat.

Winter (kitchen 16–20°C)

• Use warm water (28–30°C).
• Bulk in a warm spot (oven with light, top of fridge).
• Increase starter to 25% if you want to keep your usual bake schedule.
• Cold retard can be shorter (8–10 hours) — the dough is already cool.

Spring/autumn (kitchen 20–23°C)

• Tap water as it comes.
• Standard recipe times work.
• The easiest sourdough season — small adjustments only.

Common temperature mistakes

Following timings without checking temperature. A 5-hour bulk in a 22°C kitchen is fine; the same recipe in a 17°C kitchen needs 8+ hours. Going by the clock instead of the dough is the biggest single cause of dense loaves.

Trusting the room thermostat. The thermostat is usually in the hallway. Your kitchen is often 2–3°C cooler. Measure where your dough actually sits.

Putting dough on a marble counter in winter. Marble runs 4–5°C cooler than the room. It silently slows bulk. Use wood or plastic in cold months.

Heating the dough directly on a radiator. Spot-heating creates pockets that ferment too fast and skin that dries out. Use ambient warmth, not direct heat.

The temperature troubleshooter

Use this when something's not working:

• Dough hasn't grown after 6 hours of bulk: probably cold. Probe the dough — if under 22°C, move to a warmer spot.
• Dough proofed too fast and is now slack: probably warm. Reduce starter % next time, or use cooler water.
• Bread tastes flat, no tang: retard longer in the fridge.
• Bread tastes too sour: shorter retard, or bulk warmer for a faster yeast-dominated ferment.
• Open crumb won't develop: may be too cold during bulk; gluten doesn't extend as well below 22°C.

FAQ

What's the ideal temperature for sourdough?

24–26°C for bulk fermentation; 4–6°C for cold retard.

Why is my dough so slow in winter?

Because it's cold. Use warmer water and a warmer spot. Don't add more starter unless you want to fundamentally change the timing.

Can I just leave the dough in the oven with the light on overnight?

For bulk, yes — but check temperature first. Some ovens hit 30°C with the light on, which is too warm. 24–26°C is ideal.

What temperature should the water be?

Most of the year, room temperature is fine. In winter (kitchen under 22°C) use 28–30°C water. In summer (kitchen over 24°C) use cold tap (15°C).

Does the temperature of the flour matter?

Yes, but only a little. Flour stored at room temperature is fine. Flour stored in a cold cupboard or garage cools the dough — let it warm up before mixing in winter.

Scaling up: what professional bakeries do differently

Commercial sourdough bakeries operate at scale and need consistency, so they treat temperature with surgical precision. Their kitchens run at 26–28°C year-round, controlled to within a degree. Their flour, water, and bowls are tracked individually for temperature. Their bulk fermentation happens in retarder-proofers — refrigerated cabinets that hold dough at exactly 4°C, then ramp up to room temperature on a schedule. The recipes don't change between January and August because the environment doesn't change.

You won't replicate that at home, and you don't need to. But it explains why your friend's bakery loaf tastes the same every week and yours might vary — they've removed the temperature variable entirely. The closer you can get to a constant 24°C bulk, the more your home loaves will match each other week to week.

The temperature toolkit

The kit that pays for itself within a month:

• Probe thermometer (£6). Stick it in dough, stick it in starter, stick it in water. Stop guessing.
• Ambient thermometer (£8). Sits beside your dough. Tells you the actual room temperature where the dough is, not the room temperature on your phone.
• Inside-fridge thermometer (£5). Most home fridges run colder than expected. Knowing your fridge actually runs at 3°C versus 6°C changes how long your retard should be.
• (Optional) Proofing box (£25–60). Folds away. Maintains a constant 24–28°C. Worth it if your kitchen is unreliably warm.

Three tricks for cold-kitchen bakers

1. Pre-warm your bowl. Rinse your mixing bowl with hot tap water and dry it before mixing. Adds a degree or two of warmth that helps the first hour of bulk.
2. Wrap the bulk container. A tea towel around the bowl insulates against draughts. Surprisingly effective.
3. Use the dishwasher trick. Some home bakers run a dishwasher cycle, then place the dough on top after the cycle finishes. The residual heat keeps the surface warm for hours.

Why probe thermometers change everything

The single most life-changing purchase for a home baker is a £6 probe thermometer. Before, you're guessing — "the dough feels about right, the kitchen feels warm enough." After, you're measuring — "dough at 23°C, room at 21°C, bulk will take 6 hours." Suddenly every variable becomes legible. Suddenly bulk fermentation isn't a mystery. Suddenly you can predict when your loaf will be ready, instead of hoping. Buy the thermometer. Use it on every bake. The difference is genuinely transformative.

Temperature and the cold retard, in detail

The cold retard is where most flavour development happens, and the temperature of your fridge directly controls how much. Most home fridges run between 3–6°C; that's the standard food-safe range. The flavour outcome differs slightly within that band:

• 3–4°C: very slow fermentation, mostly bacterial. Sharp, lactic, intensely tangy after 24 hours. Best for those who want a really sour loaf.
• 5°C: the standard sweet spot. Balanced flavour development, easy to time, dough is firm enough to score well.
• 6°C+: the dough continues fermenting noticeably. Risk of overproofing if retarded longer than 18 hours.

If you're baking for flavour, push the retard to 24+ hours at 4°C. If you're baking for convenience and just need the dough to wait overnight, 8 hours at 5°C is plenty.

Common temperature questions in a UK home

My house has underfloor heating. Lucky you — extremely consistent ambient temperature. Bulk anywhere on the floor.

My kitchen is at the back of the house, always cooler than the front. Move the dough to a warmer room. Bedrooms are usually 1–2°C warmer than kitchens.

I bake on a holiday cottage break. Pack a probe thermometer and a small thermometer for the room. New environments throw timings off completely.

The house is heated overnight, cool by morning. Bulk before bed; the warm overnight house ferments the dough; shape and fridge in the morning. Many home bakers have rediscovered this is actually the easiest schedule in winter.

The single rule

If you remember nothing else from this guide: bake by feel and temperature, not by clock. The recipe gives you a guideline. The dough tells you the truth. Stick a thermometer in it, look at how it's grown, poke it. Trust the dough over the timer every single time.