What is Yeast
Yeast is a living single-cell organism that uses sugar as food in its life cycle. During fermentation, brewer's yeast (scientific name: Saccharomyces cerevisiae) converts sugar from the malt into alcohol and carbon dioxide. Yeast is added to begin the fermentation process after the wort has been reached a suitable temperature, depending on the style of beer.
Yeast Formats
Brewer's yeast generally comes in two formats: dry yeast and liquid yeast. Adding either format of yeast to the wort is commonly known as "pitching" the yeast.
Liquid yeast is easy to pitch since it does not require rehydration. However, improper handling and storage of liquid yeast can negatively impact its viability.
Dry yeast is less susceptible to improper storage but requires a rehydration process to ensure a healthy fermentation. To rehydrate dry yeast, simply add the yeast to warm, sterile water and allow time for the yeast cells to become fully saturated by water before adding to the wort.
Yeast Varieties
There are two primary types of brewer's yeast: ale yeast and lager yeast. Both have a multitude of varieties.
Ale yeast is top-fermenting yeast and is fermented at warmer temperatures, typically 20°C - 22°C (68°F - 72°F), though some varieties such as Kveik yeast perform better at temperatures as high as 40°C (104°F). Ale yeast is used to produce ale-style beers such as pale ales, IPAs, stouts, porters, and blonde ales. Ale yeast varieties include:
Lager yeast is a bottom-fermenting yeast and is fermented at colder temperatures, typically 10°C - 13°C (50°F - 55°F). Lager yeast is used to produce lager-style beers such as pilsner, Helles, Vienna lager, and bocks. Lager yeast varieties include:
Wild yeast and bacteria like Brettonamyces, Lactobacillus, and Pediococcus are used in the production of sour beers, like Berliner Weisse, Lambic, Oud Bruin, and Flanders Red. Not a single yeast variety, but rather a mixed culture of microorganisms, these blends are traditionally left to age, often in oak barrels, to develop unique flavours.