A Close Look at the Beasts
Yeast is a member of the fungus kingdom. Like mushrooms, they turn out to be very useful to human cooks. Most of the critters that you control are varieties of Saccharomyces cerevasiae, Greek and Latin for “beer sugar mold.” The little (mostly) asexual budding microbes conjure the real magic in the craft.
Look at a yeast cell under a microscope and you'll see a simple, single-celled creature that shares so many cell structures with animals that scientists extensively study them to gain a better understanding of more complex cellular systems.
In the presence of oxygen, yeasts get their groove on, making building blocks to prepare for fermentation. Abundant sugar and depleted oxygen triggers fermentation. Cells reproduce rapidly. When yeast saturates the wort, they consume available sugar. As it is metabolized, yeast creates ethyl alcohol (ethanol) and carbon dioxide. They secrete a host of flavorful compounds.
Every yeast culture produces unique concentrations of flavors based on conditions, including fermentation temperature. Brewers exploit these fermentation characteristics to transform barley malt and hops into a universe of beer.
Reflecting the long association between bread and beer, bakers use S. cerevasiae to give bread its lift. Their breeds are slow fermenters and give very distinctive yeast characters to the dough. In the past, homebrew was fermented with cakes of fresh bread yeast.
Ale Yeast Versus Lager Yeast
S. cerevasiae isn't alone in the pool. As brewers in cold climes discovered, its close cousin S. pastornius (nee S. carlsbergensis) ferments at colder temperatures. They can consume raffinose, a large sugar, making drier beer.
Traditionally brewers refer to ale yeasts as “top fermenting” when they clump together (flocculate) and rise to the top. Skimming efficiently recycles the yeast for another batch. Bottom-fermenting lagers throw less krausen and the yeast settles directly.