The Science of Fermentation Temperature: Why a Few Degrees Changes Everything

⏱ 7 min read

Spring is a funny time for sourdough bakers. The kitchen is warming up, the starter is suddenly bouncing back with more energy than it had all winter, and then you go and mix a dough that ferments twice as fast as you expected and ends up over-proofed before you have even had lunch. Sound familiar?

The culprit, almost every time, is temperature. Not technique, not your flour, not some mysterious flaw in your process. Just a few degrees Celsius that completely shifted the timeline. Understanding what is actually happening inside your dough when the temperature changes is one of those things that takes your baking from guesswork to something you can genuinely control. So let us get into it.

What fermentation actually is (the quick version)

When you mix flour, water, and a sourdough starter together, you are setting off a biological process driven by wild yeast and lactic acid bacteria. The yeast eats sugars in the flour and produces carbon dioxide, which inflates the gluten network and makes your dough rise. The bacteria produce acids, mainly lactic and acetic, which give sourdough its characteristic flavour and help preserve the loaf.

Both of these processes are enzymatic. And enzymes, like pretty much everything in biology, are extremely sensitive to temperature. Too cold and they slow to a crawl. Too warm and they accelerate, or eventually denature and stop working entirely. That sensitivity is exactly why your dough behaves so differently in January versus April.

The sweet spot: what happens around 24 to 26 degrees

Most sourdough recipes are written with a dough temperature somewhere between 24°C and 26°C in mind. At this range, wild yeast activity is steady and reasonably predictable, and the bacteria are producing a balanced mix of lactic and acetic acids. You get good rise, decent structure, and a flavour profile that is tangy without being sharp.

This is why experienced bakers obsess over their dough temperature at the point of mixing. It is not pedantry. If your flour is sitting on a cold worktop and your water comes straight from the tap in March, your dough could easily start at 19°C or 20°C, and everything in your recipe timings will be off.

What heat does to your dough

As temperature rises above around 27°C or 28°C, fermentation accelerates noticeably. The yeast becomes more active, CO2 production speeds up, and your dough can appear to be doing brilliantly. The trap here is that fast fermentation does not always mean good fermentation.

When things move quickly, the bacteria do not have as much time to do their work before the gluten network is already under strain. You can end up with a dough that has risen but lacks the acid development and flavour complexity you were hoping for. Worse, if the temperature climbs much above 30°C, the gluten starts to weaken faster than the structure can keep up, and you risk a sticky, soupy mess that will not hold its shape during shaping.

There is also the matter of acetic versus lactic acid balance. Cooler, slower fermentations tend to favour acetic acid production, which gives that sharper, more pronounced tang. Warmer fermentations lean towards lactic acid, which is softer and more yoghurt-like. Neither is wrong, they are just different, and understanding this gives you a real lever to pull when you want to dial in flavour.

What cold does (and why it is actually useful)

Drop the temperature down and everything slows. Below about 10°C, yeast activity becomes very sluggish, though it does not stop entirely. The bacteria, particularly the ones producing acetic acid, continue working at cooler temperatures longer than the yeast does. This is the science behind cold proofing overnight in the fridge: you are essentially pausing the rise while letting flavour development continue quietly in the background.

This is also why spring baking can feel a bit unpredictable. Your kitchen might be 18°C in the morning and 23°C by mid-afternoon if you have got the windows open and the sun comes out. That six-degree swing over a few hours can meaningfully alter your bulk ferment timeline, especially if you mixed your dough before heading out and came back expecting it to be where you left it.

A useful rule of thumb: for every 5°C rise in dough temperature, fermentation roughly doubles in speed. It is not perfectly linear, but it is a decent mental model. Going from 20°C to 25°C? Your bulk ferment could take half the time.

Enzyme activity: the other temperature story

Yeast and bacteria are not the only things temperature affects in your dough. Enzymes called amylases are breaking down starches in your flour into simpler sugars from the moment water hits the flour. Proteases are gently weakening gluten bonds. Both of these processes are temperature-dependent too.

Protease activity, in particular, is worth knowing about. At warmer temperatures, proteases work faster, which can soften gluten and make dough feel silkier but also less strong over time. This is part of why a long, warm bulk ferment can leave you with dough that is difficult to shape: the gluten has been gradually broken down, not just stretched and inflated.

Whole grain flours are especially prone to this, because bran particles contain more enzymatic activity. If you are baking with a high percentage of wholemeal or rye this spring, keep that in mind and err on the cooler side for your bulk.

Tracking it all without losing your mind

Once you start paying attention to temperature, you quickly realise how much you want to be able to log it. If you baked a great loaf last week and want to recreate it, knowing that your dough temperature was 24°C, room temp was 21°C, and bulk took four and a half hours is genuinely useful information. Without that record, you are starting from scratch every time.

This is exactly where I find Doughrise Pro useful. Being able to save unlimited formulas alongside full bake history means you can actually build up a picture of how your dough behaves across different conditions, rather than relying on memory. When spring temperatures finally stabilise, you will have a reference point to work from.

The practical upshot

Temperature is not something to be stressed about. It is just a variable, like hydration or flour type, and once you understand what it is doing to your dough, you can work with it rather than be caught out by it. Get a probe thermometer if you do not already have one. Take your dough temperature after mixing. Note it down. Pay attention to how your dough feels and looks rather than just watching the clock.

A few degrees does not ruin a bake. It just moves things around. And the more you bake, especially through a season change like this one, the better your instincts get.

Happy baking! Find everything you need at doughrise.store

Photo by Nik on Unsplash