Beer history · French
Louis Pasteur
Established that fermentation is biological rather than chemical — which is not the same as discovering yeast, though that is what he is usually credited with.
Credited with
Discovering yeast, and discovering that yeast makes beer ferment.
What the record supports
Established that fermentation is carried out by living organisms, against a strong contemporary consensus that it was a chemical decomposition which yeast merely accompanied. That is a much harder claim than the one he is credited with, and a much more consequential one: it makes contamination a preventable cause rather than bad luck, makes cleaning a technique rather than a courtesy, and makes it rational to select and propagate yeast at all. He also patented heat treatment for beer, before the more famous application to milk. He did not discover yeast — Leeuwenhoek described it under a microscope in the 1680s and brewers had used it deliberately for millennia — and he was substantially wrong about the cause of lager spoilage, which Hansen later showed was frequently other yeasts rather than bacteria.
Why it matters
The point at which brewing stopped being a craft with unexplained failures and became a process with diagnosable ones. His brewing work was commissioned and commercially framed — the Études sur la bière is a practical book about beer that does not go off — which is a useful corrective to the image of disinterested laboratory science.
Pasteur is the clearest case in beer history of a real achievement being displaced by a simpler one. The sentence people repeat is smaller than what he did, and repeating it obscures how contested the actual question was: serious chemists held that fermentation was a decomposition, and yeast was thought to be a by-product of the process rather than its cause.
The technology
- Fermentation understood as biology
The shift from knowing that yeast works to knowing what yeast is — which made spoilage a problem with causes rather than a run of bad luck.
- Pasteurisation of beer
Heating packaged beer enough to kill what would spoil it — the technology that let beer be sold a long way from where it was made, at a cost to how it tastes.
The claims attached to this name
Questions
Did Pasteur discover yeast?
No. Yeast was described under a microscope by Leeuwenhoek in the 1680s, and brewers had been deliberately saving and re-pitching it for centuries before that. What Pasteur established is that fermentation is performed by living organisms rather than being a chemical decomposition — which was genuinely disputed at the time and is the foundation of brewing microbiology.
Connected
Dated events: Pasteur establishes that fermentation is microbial
Last reviewed 2026-08-29.