The most important ingredient in brewing was the last one discovered, because yeast is a single-celled organism that is invisible to the naked eye. Still, brewers have long known that some unseen agent turned a sweet liquid into beer. Long ago, the action of yeast was such a blessing, yet so mysterious, that English brewers called it “Godisgood.”

Modern brewers usually brew with purified strains of yeast that give exactly the result they want. Some yeast strains are fairly neutral, creating alcohol and little more. Others add a whole range of complex side flavors that make beer more interesting.

How does yeast work? When it is added to a sugar-rich solution, it immediately begins to consume the sugars and create more yeast. But from the brewer’s point of view, the important thing is not the growth of more yeast, but the waste products of yeast metabolism: alcohol and carbon dioxide, that gives beer its fizz.

As the food supply runs down and the alcohol levels rise, the environment becomes literally toxic to the yeast, which becomes dormant. The brewer may draw off some of the yeast for the next cycle of brewing.

Ale or Lager?
Different strains of yeast behave differently, so that it’s possible to divide the world of beer according to the yeast. The sixty or more defined beer styles in the world can all be sorted by their yeast into two broad families: the ale family and the lager family.

Beers in the ale family are produced by yeast strains that operate better at warmer temperatures. Ales are ready to drink in days rather than weeks, and the yeasts produce extra flavors in addition to creating alcohol: fruity, spicy, or earthy flavors are not unusual. Ales are the traditional beers of England and of Belgium.

Beers in the lager family are fermented by yeast strains that operate better at cooler temperatures. These beers need to be conditioned or cellared (“lager” is German for a storage place) for several weeks or more to reach peak drinkability. The lager beers are the traditional beers of Germany, the Czech Republic and central Europe.

The action of yeast can generate a range of interesting beer flavors and aromas as varied as apple, pepper or apricot. Some, such as banana or clove, are the typical flavors of particular beer styles; others, such as butterscotch, may be considered defects.

Yeast may be invisible, but without it, there would be no beer.

Learn about the role of water in beer.