Why Fermentation Temperature Is the Real Variable You Should Be Controlling

⏱ 7 min read

Spring is a funny time for sourdough bakers. The kitchen is no longer the cold, sluggish environment of January, but it is not the reliably warm room of summer either. One day your bulk ferment is done in five hours, the next it is dragging past seven. Your starter seems erratic. Your results feel inconsistent. And it is genuinely difficult to know what changed.

The answer, almost every time, is temperature. Not your technique, not your flour, not your starter being moody. Temperature is quietly running the show in the background, and once you understand how it works, your baking gets dramatically more predictable.

Fermentation is just biology, and biology cares a lot about warmth

Your sourdough starter is a living culture of wild yeast and lactic acid bacteria (LAB). Both of these organisms consume the sugars in your flour and produce things you want: carbon dioxide for lift, and organic acids for flavour. The rate at which they do this is directly tied to temperature.

As a rough working principle, warmer temperatures speed fermentation up and cooler temperatures slow it down. But it is not a simple linear relationship, and the two organisms do not respond to temperature in exactly the same way. That distinction is where things get genuinely interesting.

Yeast and bacteria respond to heat differently

Wild yeast is most active somewhere in the range of 25 to 30 degrees Celsius. Below that, activity slows progressively. Above around 35 degrees, it starts to struggle. Lactic acid bacteria are a broader family, but the two main types in sourdough behave quite differently at different temperatures.

Homofermentative LAB, which primarily produce lactic acid, tend to thrive at warmer temperatures, somewhere around 30 to 37 degrees. Heterofermentative LAB, which produce both lactic and acetic acid, prefer cooler conditions. Acetic acid is the sharper, more vinegary component of sourdough flavour. Lactic acid is the milder, more yoghurty note.

So when you retard your dough overnight in the fridge, you are not just slowing things down. You are selectively favouring the organisms that produce more acetic acid. That is why a cold-proofed loaf often has a more complex, pronounced sourness than one baked on the same day.

The concept of dough temperature, and why it matters more than room temperature

Most home bakers track room temperature, and that is a reasonable starting point. But the actual variable that matters is dough temperature, specifically what bakers call the final dough temperature (FDT). This is the temperature of your dough right after mixing, which determines how fermentation begins.

Dough temperature is affected by three things: the ambient temperature of your kitchen, the temperature of your flour (which tends to match the room), and the temperature of your water. Water is the one you can actually control easily.

A simple formula that professional bakers use is this: multiply your target dough temperature by 3, then subtract the temperature of the room and the flour. The result is approximately the water temperature you should be using.

So if you want a final dough temperature of 24°C, and your kitchen and flour are both at 20°C, you would aim for water at around 32°C. In summer, that might mean using room temperature water. In a cold January kitchen, you would be reaching for something noticeably warm.

How to spot under and over-fermentation (and why temperature is usually the cause)

Over-fermented dough tends to be slack, sticky, and difficult to shape. It may have a harsh or unpleasantly sour smell, and the gluten structure will feel weak, like it has given up on you. In the oven, it tends to spread rather than spring and often produces a dense, gummy crumb.

Under-fermented dough is tighter, holds its shape almost too well, and has a pale, bland interior when you cut into it. The crumb is often tight and uniform. It may have an uneven rise in the oven, with the score blowing open aggressively because the gas is all trying to escape at once.

Both of these problems, nine times out of ten, come down to misjudging fermentation progress because the temperature was different from what you expected. This is especially common in spring, when kitchens start warming up and bakers are still working from winter habits.

The most reliable way to judge fermentation is not the clock. It is the dough itself. You are looking for a 50 to 75 percent volume increase, visible bubbles on the surface and sides, a slightly domed top, and a dough that feels airy and jiggly when you gently shake the container.

Using temperature deliberately as a tool

Once you understand that temperature drives fermentation speed, you can start using it intentionally rather than just working around it.

A warmer bulk ferment (26 to 28°C, perhaps near a radiator or inside a switched-off oven with just the light on) means faster activity, slightly milder flavour, and a loaf that is ready to bake sooner. Useful when you are working on a time constraint or want a more approachable, everyday loaf.

A cooler bulk ferment (18 to 20°C, on a cooler worktop or in a slightly ventilated room) slows things down, extends the window for fermentation, and tends to develop more complex flavour. This is the kind of approach that suits a relaxed weekend bake, where you are not watching the clock.

The cold retard in the fridge is the third lever. Shaping your dough and then proofing it overnight at around 4°C gives you enormous flexibility: a longer window before baking, better crust development, and that distinctive tang from acetic acid production.

I have been using this approach a lot lately for pizza dough as well. The DoughRise Pizza Making Kit comes with everything you need to make the most of a cold-fermented pizza base, and understanding the temperature science behind it genuinely transforms the result. A dough that has had a slow, cool ferment behaves completely differently at high heat: better browning, more flavour, a base that is crisp underneath and properly airy through the middle.

Spring specifically: what to watch for

March and April bring genuinely unpredictable kitchen temperatures in the UK. A cold week followed by a mild weekend can mean your starter behaves completely differently from one Friday to the next. This is the time of year to be especially observant.

Feed your starter and watch how long it takes to peak. If it used to take eight hours and it is now peaking in five, your kitchen has warmed up and your dough will ferment faster too. Adjust accordingly. Slightly cooler water, a cooler proofing spot, or just a shorter bulk ferment will bring things back into balance.

It sounds like a lot to track, but honestly, once you have a feel for it, it becomes second nature. Like most things in baking, the learning curve is steepest at the beginning.

Happy baking! Find everything you need at doughrise.store

Photo by Balint Mendlik on Unsplash