Did Pasteur discover yeast?

Yeast was seen under a microscope two centuries before Pasteur, and brewers had used it deliberately for millennia. What Pasteur established — against a strong scientific consensus — is that fermentation is performed by living organisms, which is a harder and more consequential claim.

Yeast was known

Antonie van Leeuwenhoek described yeast cells under a microscope in the 1680s. Brewers had been saving and re-pitching yeast from batch to batch for centuries before that without needing a name for it, and the practice is old enough that its origins are not recoverable.

So the discovery Pasteur is credited with had already happened twice over: the thing had been seen, and it had been used.

What was actually in dispute

The scientific question of the mid-nineteenth century was not whether yeast existed but what it was doing. The dominant chemical view, held by serious chemists, was that fermentation was a chemical decomposition and that yeast was a by-product of it — present at the scene rather than responsible for it.

Pasteur’s contribution was to establish that fermentation is carried out by living cells. That reframing is what makes contamination a preventable cause rather than bad luck, makes cleaning a technique, and makes it sensible to select and propagate yeast at all. Everything in modern brewing microbiology sits on it.

And he got a significant thing wrong

Pasteur held that spoiled beer was principally the work of bacteria. Emil Christian Hansen at Carlsberg showed in the 1880s that a major cause of the lager "beer sickness" was contamination by *other yeasts* — which is why the pure-culture method Hansen developed was necessary and why it worked when better cleaning alone had not.

That correction is worth keeping in view. The point is not to diminish Pasteur but to show what the period actually looked like: a rapidly moving field in which the most famous figure was substantially right about the framework and wrong about an important detail.