YEAST
Beer is not beer without yeast. Yeasts are
actually microscopic fungi that transform the fermentable sugars
created during the malting and brewing process into alcohol,
carbon dioxide and other by-products. Brewing yeasts can be
generally classified as either ale yeasts (Saccharomyces
cerevisiae) or lager yeasts (Saccharomyces uvarum).
Although both yeasts ferment throughout the entire beer, ale
yeasts are considered "top-fermenting yeasts" as they prefer to
settle at the top of the beer during fermentation. Ale yeasts
perform best when fermenting at 60-75º F and aging at 40-55º F.
Lager yeasts, on the other hand, are considered
"bottom-fermenting yeasts" as they tend to settle on the bottom
of the fermentation vessel during fermentation. Lager yeasts
prefer fermenting at 45-55º F and aging at 32-45º F.
Different yeast strains impart different flavors
on beer. Therefore, the yeast strain a brewer would use to brew
a pale ale is not the same strain a brewer would use to brew a
Belgian dubbel. Likewise, a pilsner yeast strain would not be
used for brewing an imperial stout. Although lambic yeast
strains are available to the homebrewer, true lambics are
spontaneously fermented by the wild airborne yeasts peculiar to
a small area outside Brussels, Belgium.
Yeasts come in several different forms. Dry
yeasts must be rehydrated with warm water prior to pitching.
Dry yeasts are cheaper and tend to be longer lived than liquid
yeasts. Some liquid yeasts may need to be prepared a day or two
in advance of brewing, while others are pitchable directly out
of the tube. As they are living organisms, yeasts, like all
brewing ingredients, should be as fresh as possible when used.
