Edward Buchner discovered, and won a nobel prize for it, that yeast was getting into the juice from the air and it was converting the juice to alcohol. he called these particle enzymes.
Yeast was understood as a substance but then it became known that it was actually a living thing.
Buchner discovered that it was something inside the yeast, not the the yeast itself. he was able to extract a chemical from the yeast and reproduce fermentation with it.