It is said that brewers make the wort, but the yeast make the beer. Without yeast, there cannot be beer.
Yeast is a single celled micro-organism and is a form of fungus. There are at least 1,500 different strains of yeast (about 1% of the fungus world). But most of the time when people refer to yeast, they are talking about theSaccharomyces cerevisiae family, which is what bakers use to make bread and brewers use (most of the time) to make top-fermented ales, or the Saccharomyces carlsbergensis family that is used to make bottom-fermented lagers. Brettanomyces is another member of the same family of yeast and was traditionally in beer, but was seen as a “spoiler” since its discovery (it causes sour or off flavours). In recent days, the use of Brett, as it’s known, in beer has seen a serge in popularity in the craft beer movement.
Yeast take up oxygen for reproduction (self replicating through a process known as budding), but when the yeast have finished reproducing, they start to ferment the sugars. Yeast consume the sugars in the wort and produce carbon dioxide and alcohol. This is the magic. They also produce many different compounds that give flavours to the beer and vary with the different offshoots of yeast within their family. Some yeasts give very “estery” (fruity notes in English ales), some are “phenolic” (banana and clove in hefeweizen), and others are “clean”, producing very little esters or phenolic compounds (lagers and many American ales).
When we say that yeast “make the beer” its understandable as they produce the fizz, the alcohol kick, and many of the flavour compounds that give the beer its taste. We just could not have beer without yeast.