The omission wasn't purposeful. It wasn't until the 1800s that Louis Pasteur (yes, the father of pasteurization) discovered the role yeast performed in fermentation.This is an inaccurate, but almost ubiquitous statement. I googled "beer yeast discovery" and they blossomed like mushrooms after a spring rain:
"Without knowing its role or its existence, men have always used yeast." (Lesaffre yeast)
"The German Beer Purity Law of 1516 - The Reinheitsgebot, listed the only allowable materials for brewing as malt, hops, and water. With the discovery of yeast and its function in the late 1860's by Louis Pasteur, the law had to be amended." (John Palmer, How to Brew)
"Until the mid-19th century brewers knew very little about yeast. To make good beer they had to rely on ancient practice. With the aid of a microscope, Louis Pasteur discovered that yeast was responsible for beer fermentation in 1866." (Christopher White, Brew Your Own)And so on. If you think about this for more than five minutes, you realize it can't be true. When you brew, you get gobs of stuff boiling off the top of your fermenting wort and lots of "stuff" pooled in the bottom afterward. The single-celled organism is invisible, but in the billions they are quite a sight. We also know brewers regularly re-pitched that stuff centuries ago. Matthias Trum and I got off onto a long, interesting digression about brewing history in which he illuminated much about the German guild system for me. (He focused on brewing history when he was a student at Weihenstephan.) He pointed out the obvious and then added:
“About the yeast and the purity law. The yeast is in fact not mentioned, that is correct. You have, again, to put yourself in the mind of a medieval brewer. In the middle ages, they had a profession called the 'hefner.’ So they knew exactly. The purity law lists ingredients, right? Yeast I put in there and I get more out of it. I harvest the yeast at the end and I put it into the next batch. And that was actually the job of the hefner.”
I made my “stuff” point and he continued: “Zeug. Zeug was the German word, which is stuff. The hefner’s job was to harvest the yeast from the batches, to press out as much remaining beer as possible, which was sold at a low price to the poor, and then the yeast was added to the next batch. You started with a smaller amount of yeast and then you ended with a bigger amount of yeast.”Indeed, we have records of brewers using yeast from long ago. Richard Unger documents this in his indispensable Beer in the Middle Ages and the Renaissance:
As early as the mid-14th century, a Flemish recipe book mentions adding yeast to beer, and it seems likely that already by 1300 brewers were using some of the foam skimmed off the top of the fermenting beer from the last brew to start fermentation with the next one. By the 16th century, brewers commonly added yeast to wort from cultures which they kept separate and which they controlled and maintained.And indeed, it had to be that early, because that's roughly the time when lagering began--a process not possible without knowledge of the function of yeast. He continues, making the point:
In 1420 a brewer in Munich got permission to use yeast that fell to the bottom and regulations from Nuremberg suggest that bottom yeasts which had been identified and to some degree isolated were already in use in the fourteenth century.Pasteur's role was observing the organism and explaining why it produced sour beer. (He was a big fan of bottom-fermentation and lagering.) It was a watershed. But humans, which even a thousand years ago were pretty clever beings, had a pretty good idea of what was causing the beer to ferment. My guess is that monks were the first to really develop a theory of yeast--they were the first great brewing innovators--but whoever it was came many hundreds of years before Pasteur.