Fermentation science
Every sourdough starter contains a small ecosystem — wild yeast and lactic acid bacteria living in close competition. Temperature is the single most powerful variable controlling which of them dominates, and by how much.
There are four key organisms worth understanding:
Wild yeast — peaks around 26–28°C. Active but tolerant of a wide range. Below 10°C it goes dormant; above 35°C activity collapses; above 40°C it starts dying.
Baker’s yeast — the commercial strain (S. cerevisiae), peaks at 32–35°C. Narrower cold tolerance than wild strains. Faster at warm temperatures, which is why it’s used for speed in commercial baking.
Lactic acid bacteria (lactic profile) — homofermentative strains, peak 30–37°C. Produce lactic acid: the mild, creamy sourness. Running a warm prove pushes the starter toward this flavour.
Lactic acid bacteria (acetic profile) — heterofermentative strains, peak 16–20°C. Produce acetic acid: the sharper, more vinegary sourness. Stiff doughs at cool temperatures strongly favour these. The San Francisco style is built on this effect.
The practical upshot: a 6°C shift in ambient temperature changes which organism leads the fermentation. That’s the difference between a mild, open crumb and a sour, tighter one — not technique, just temperature.
The interactive chart is on the Fermentation Charts page — all four curves plotted across the full range, with hover values at each degree.