yeast. Top-fermenting yeast, or ale yeast, works in warmer temperatures and settles on the top of the beer after fermentation. Bottom-fermenting, or lager yeast, is a newer discovery, not having been scientifically isolated until the 1800s. Lager yeast prefers colder temperatures, is slower to ferment, and settles at the bottom of the fermenter. Both types of yeast have hundreds of strains that give beer slightly different characteristics. They can be used in dried or liquid form.
