Every bread book has a temperature section. Warmer for a faster rise and more lactic character, cooler for a longer one and more acetic sharpness. It is plausible, widely repeated, and much less well documented than its confidence suggests.
The one quantified comparison
A 2024 review reports the time needed to reach a target pH at two temperatures: 11 hours at 25 degrees Celsius, 9 hours at 35.
Ten degrees warmer, eighteen per cent faster. That is the whole of the quantitative temperature-timing evidence I was able to locate in accessible literature.
It is worth registering how modest that acceleration is. Biological rates often roughly double for a ten degree rise, and much fermentation advice implicitly assumes something in that range. A reduction from eleven hours to nine is not a doubling of rate; it is a shortening of about a fifth.
Two caveats. This is a single reported pair, not a temperature series, so it cannot be extrapolated to the 4 to 20 degree range that cold retarding occupies or to the low 30s where some bakers work. And a target pH is a specific endpoint; the time to a different endpoint could scale differently.
Why the fermentation quotient is the more interesting number
The same review gives an ideal fermentation quotient of 2.0 to 2.7. The fermentation quotient is the molar ratio of lactic to acetic acid.
That ratio, rather than total acidity, is what most people are actually reaching for when they talk about sourdough character. Lactic acid is comparatively mild and rounded; acetic acid is sharp and volatile. A dough at the same pH can taste noticeably different depending on which acid dominates.
So a target range of 2.0 to 2.7 means roughly two to nearly three times as much lactic as acetic acid for a well-balanced result. Below that range, the loaf reads as sharp. Above it, as flat or merely tangy.
And why the temperature-to-quotient relationship is missing
Here is the gap. Having established that the quotient is the meaningful variable, I could not find published data giving the quotient as a function of fermentation temperature.
A 2021 review states that temperatures above 30 degrees tend to favour bacterial activity and organic acid production, and is candid that much remains to be studied. That is a direction, not a curve. It does not tell you whether 22 degrees gives a quotient of 1.5 or 3.
Searching for temperature-series studies measuring lactic and acetic acid separately returned work on specific strains and specific raw materials rather than temperature gradients in ordinary dough. If such a dataset exists in accessible form, I did not find it.
The consequence is direct: any chart telling you that 24 degrees produces lactic-dominant fermentation and 18 degrees acetic-dominant is presenting craft consensus in the visual language of measurement.
What to control instead of time
If the timing relationship is weak and the quotient relationship is undocumented, what does a baker use?
The most defensible answer is an endpoint rather than a duration, and the endpoint should be measured. That means either pH or titratable acidity, and the choice between them matters.
A survey of traditional starters found pH varying by only 3.9 per cent across samples while titratable acidity varied by 22.6 per cent. pH sits in a narrow band because buffering by flour components resists change; total acid content moves freely. Titratable acidity is measured by suspending a weighed sample in water and titrating to pH 8.3 with dilute alkali, which is more work than dipping a meter but reports the quantity that actually changes.
For following your own dough day to day, either works as a relative indicator. For deciding how much acid a dough carries, only titratable acidity answers the question.
What temperature does that is not in dispute
Two effects are solid enough to act on.
Temperature changes timing, in the direction everyone expects and by a smaller factor than most assume. If a schedule needs to shift by an hour or two, temperature is an appropriate lever. If it needs to halve, temperature alone will not do it.
Temperature also changes which organisms are favoured, and the 2021 review's observation that above 30 degrees bacterial activity is favoured is consistent with what one would expect from the differing optima of yeasts and lactic acid bacteria. Traditional starters, the same review notes, are kept at ambient temperatures of 20 to 30 degrees, which brackets the range in which both populations remain active.
There is also a downstream benefit of acidification worth knowing: the 2024 review notes that the lactic and acetic acid produced lower dough pH, and that this acidic environment inhibits the reassociation of starch molecules and so delays retrogradation. Fermentation therefore buys keeping quality, not only flavour, and the acid content rather than the pH is what does it.
What is worth measuring in your own kitchen
Given how little of this is settled in the literature, the useful move is to generate your own data rather than to import someone else's chart. Two readings taken consistently are enough to make a schedule reproducible.
Record dough temperature at the end of mixing, not room temperature. Mixing adds heat, flour and water arrive at different temperatures, and the dough is what ferments. A dough that finishes mixing at 26 degrees in a 21 degree room is on a different clock from one that finishes at 22.
Then record one endpoint indicator at the point you decide to divide. Titratable acidity is the more informative measurement, and if the titration is impractical, a pH reading taken the same way each time still tracks your own dough usefully even though it compresses the variation.
The practical position
Pick a temperature you can hold, since consistency matters more than the specific value when the underlying relationships are this poorly quantified.
Judge the dough by a measured endpoint rather than by the clock, because the clock relationship you would need is a single data pair.
Expect a ten degree change to move your schedule by a fifth or so, not by half.
And treat flavour-versus-temperature guidance as inherited practice. It may well be right. It is not, on the accessible evidence, measured.
