When we drink beer, there’s a fungus among us, a one-celled beast: yeast. Yeast is a live organism with a unique trait. It can thrive in the presence of oxygen, or without it. Given air—specifically, oxygen—yeast multiply. The condition makes dough rise. Shut off from oxygen, yeast can ferment sugar into alcohol. Thank yeast for your next beer.
Brewers primarily rely on three types of yeast. Although each of the three main types catalyzes the fermentation of the wort, the liquid made from water and malt and flavored with hops, different yeast types will heavily influence the style of the product, such as whether it becomes ale or lager.
Types of Yeast Used in Beer
Ale yeast is “top-fermenting.” Top-fermenting yeast cells tend to accumulate or, in brewer’s terminology, flocculate, at the top of a tank of liquid. Lager yeast does the opposite: it migrates to the bottom of the tank during fermentation. Lager yeast is bottom-fermenting.
During fermentation, ale yeast is permitted some access to oxygen. This limited access to oxygen encourages a quicker fermentation than would occur without oxygen. The process consumes a few days to two weeks, and is carried out at higher temperatures than the same brewer would ferment with lager yeast. Brews made with ale yeast generally have longer shelf life and higher alcohol content.
Lagers ferment slowly, a couple of weeks to a month, and temperatures that dip to near freezing, which explains why, historically, lagers were frequently brewed during winter months. However, lager yeast can become active at higher temperatures and still produce quality beer. Modern refrigeration, of course, eliminates the need to confine lager brewing to winter.
The third main type of yeast for beer making was, for a time, virtually unique to Belgium: Lambic yeast. In Belgium’s West Flanders vicinity, Lambic yeasts grow wild. Today, Lambic yeast is exported to brewers around the world.
Yeast Can Do More Than Ferment Beer
Yeast catalyzes the basic fermentation process. It transforms malt sugar (maltose) into alcohol (ethanol) and carbon dioxide. Beyond that, however, yeasts have distinctive flavor characteristics to contribute.
Many ale yeasts have a full-bodied, fruity aroma and taste. Others, however, are nutty or make one think of minerals; makers of stouts or Belgian ales and similarly strong brews rely upon these types.
Lager yeasts tend to be smoother and dryer. They typically taste of cloves and vanilla. With the proliferation of micro-breweries in the U.S. and elsewhere, lagers have become more varied. We now talk of, for example, Dortmunders, Mrzens, or Bocks, rather than simply lager. Yeast contributes to the distinctive flavors of each.
Use Yeast to Give Beer Added Flavor and Aroma
Beer makers can also get creative, such as when they concoct Weizenbier, or wheat beer. Beer’s primary ingredient is malt, made from barley. Malt for wheat beer is not made from wheat; it is named for the yeast used to ferment it. Wheat beer yeast goes into this ale-style brew, revered for its intense, fruity character.
Although yeast stimulates fermentation, it has a highly important secondary function. It influences flavor and aroma, adding tastes that vary from sweet corn to green apple to butterscotch. Controlling yeast’s flavor contributions is as much an art as the control of fermentation is an engineering process.