I know Reinheitsgebot permits only water, hops, and malt as ingredients in beer but if you want to make beer, mead, cider or any other alcoholic brew, you need to add yeast. In the 21st Century, many homebrewers just rip open an envelope of dried yeast from the local homebrew shop and add it to the wort. This is certainly not how it was done in the 18th Century. Lets look at some of the common sources.
Strube, Lees, Dregs, or Sediment: Many brewers would save the sediment, or lees, from a previous batch of beer and add that to a new batch. An advantage of adding lees is that it contains minerals and nutrients that nourish the yeast. Lees can be added to the fermenter as is, or the living yeast can be washed from the dregs. To do this pour the lees into a wide jar or basin with a lid, add water, let the dead yeast cells sink to the bottom and then carefully pour off the top layer. The living yeast float while the dead yeast and waste matter sinks. Removing the dead yeast prevents adding unpleasant flavors to the new fermentation.
Barm: The frothy scum of top fermenting ale, can be scooped off and added to another batch. This is a very popular technique for brewers who use open fermenters.
Saved Yeast: Period, bakers typically saved yeast is made of a mother culture of bread yeast, which would be mixed with flour dried and shaped into little cakes, to be reconstituted as needed. Bread yeast is usable for brewing but often not as efficient as brewer’s yeast. Most brewers, however, got their “mother culture” from other brewers, just like bakers did.
Stirring Stick and Wood Barrels: Throughout history, the most common means of adding yeast to beer was to simply use the same wood stirring stick and the same wood fermenting barrels which would have the yeasts hiding away in the grains of the wood. European Abby ales and sour beers commonly use this method today.
Wild Fermentation with Ambient Yeast: Yeast is everywhere in the environment and a very interesting and exciting way to inoculate your beers is to capture naturally occurring yeast from the air. Many brewers in the 18th Century and a few modern Belgian brewers use Kühlschips for cooling their beer. These large open trays not only allow the beer to convectively cool, they make great tools for capturing yeast that settle from the air. Some fruits have yeast growing on the skin (especially grapes, currants, raspberries) and adding these to your cooled wort effectively inoculates your beer. Most places have great yeast strains naturally occurring in the air so if you use ambient yeast, your beer will be unique for every location at which you brew. Who knows, perhaps you will find the ideal location, like the Belgium caves and basements which create wild fermented brews that are now widely recognized some of the world’s highest quality beers and your yeast strains can become a trade secret.
Brewers and bakers know that yeast is the critical ingredient. Cultivate your yeast with care, don’t just rip open an envelope.