Yeast is one of the four main ingredients that go into making beer, beautiful beer. The others are hops, malt, and water (this post is next in my Beer 101 series). There are certainly other ingredients that can be added to the beer process, but these four are the core pillars that hold up the whole thing.
These itty bitty single-celled microrganisms are technically classified as a fungi. They reproduce by an asymmetric division process called budding. Their job is to convert fermentable sugars from the malt into alcohol and other byproducts. There are hundreds of varieties and strains of yeast out there, some of which are commonly used to brew beer.
Yeasts are generally put into one of two categories: ale yeast (top fermenting) or lager yeast (bottom fermenting), depending on how they behave during the fermentation process. There’s also a nebulous third category, known as spontaneously fermenting yeasts, which result when beer is left exposed to the air and is literally infected with wild yeast strains as they wander by – this is what creates sour beers.
Ale yeasts generally sit on top of the beer-to-be, fermenting away between temperatures of 10° to 25°C (though some yeasts won’t activate below 12°C). These guys rise up to the surface, forming a thick raft of a head as they bubble away. These yeasts tend to yield beers higher in esters, which are the chemicals that give fruits their characteristic flavors. In the case of Hefe Weizen beers, the yeast produces the ester iso-amyl acetate, the same one that is found in bananas. Other esters include ethyl acetate, which can be flowery, and ethyl caproate, which is kind of wine-like and fruity. Top-fermenting yeasts are used for brewing ales, porters, stouts, Kölsch, Altbier, and wheat beers.
Lager yeasts create much less of a head and tend to settle at the bottom of the tank as fermentation nears completion. They grow less rapidly than the ale yeasts and don’t create that layer of thick foam on top of the beer. These yeasts work at lower temperatures, around 7° to 15°C.
In addition to making beer the alcoholic beverage that we so enjoy, it also has a large impact on the flavor of the final beer. The flavor and aroma of beer is complex and is influenced by many factors, including malt, hops, and the yeast strain. The synthesis of yeast creates many byproducts, including ethanol (alcohol), CO2 (carbon dioxide), and also some flavor compounds like clove, butterscotch, and green apple.
Yeast may be tiny and invisible to the naked eye, but it plays a huge role in making beer what it is.