Trying to explain yeast and what it does is much more difficult than trying to explain the other components of beer. Because of this, I have broken up my discussion of yeast into several posts. This is the first post in which I describe what yeast is.

Yeast is the unsung hero in beer making. An old brewing saying goes: “Brewers make wort, but yeast makes beer.” This quote stems from the fact that yeast has the most important job and without it, there is no fermentation and no alcohol. Prior to the discovery of what yeast is, ancient civilizations found the fermentation process to be a gift from the gods since the process was seemingly out of their control. You can’t force the yeast to ferment, it is a living organism and can at times be difficult to deal with.

So what is yeast? Yeast is a unicellular microorganism in the fungus family. While there are 1,500 different species of yeast, brewers are concerned only with 2: Saccharomyces cerevisiae and Saccharomyces pastorianus. They are also known by their easier to pronounce nicknames: ale yeast and lager yeast.

All beers are either an ale or a lager. The differentiating factor between the two is determined by what species of yeast is used, saccharomyces cerevisiae being the ale yeast and saccharomyces pastorianus being the lager yeast. Within these two species are many strains of yeast.

To understand what the differences are between these two species of yeast are, it is best to understand what it is that yeast does to wort and what affects its ability to do so. In the next article, I’ll detail the purpose of yeast.