Yeasts are living
organisms and, as a result, not always easy to work with. They are a uni-cellular fungus, and have a rare ability: to live with
or without oxygen. When the presence of air, they multiply. However, in the
absence of oxygen, they ferment sugars into alcohol. That's what makes
brewing beer possible at a brewery or at home.
Beer styles are distinguished by the two main types of yeast used to ferment the wort
(the liquid made from water and malt, flavored with hops), to make ales or lagers.
Ale
yeast is said to be 'top-fermenting' since the yeast cells tend to accumulate
('flocculate') at the top. Lager yeast migrates to the bottom of the tank during fermentation and so is called 'bottom-fermenting'.
In the case of ale yeast,
some interaction with oxygen takes place during fermentation. Ale yeast
ferments quicker - a few days to two weeks - and at higher temperatures
(around 21ºC/70ºF), though this can vary from as low
as 10ºC/50ºF to as high as 25ºC/77ºF. Brews made from it also tend to store
longer and have a higher alcohol content.
Lagers ferment more
slowly (up to a month) and at lower temperatures, sometimes as low as
near freezing. As a result, historically, lagers were often brewed in
the winter and consumed later. Temperature ranges vary, though, and can
easily be in the higher
45ºF-59ºF/7ºC-15ºC range. With
modern refrigeration technology came the option of having lager
year-round.
There's
also a third type of yeast which was used originally almost exclusively in Belgium: Lambic yeast. The
name derives from the West Flanders area in Belgium where the yeasts
grow wild. Today, as a result of importing, it's used in many parts of
the world.
These helpful little creatures
transform malt sugar (maltose) into alcohol (ethanol) and carbon
dioxide - the basic fermentation process. But, as its bread-making
cousins do, yeast also adds distinctive flavors to the brew.
Many
ale yeasts have a
full-bodied, fruity aroma and taste. Others are more nutty or mineral
tasting, suitable for stouts or Belgian ales and other strong brews.
One
variety is used in
Weizenbier, or wheat beer. The primary ingredient in beer is malt, from
the cereal grain, barley. The name 'wheat beer' comes not from the
grain used, but from the yeast used to ferment it. Wheat beer yeast
goes into this ale-style brew, where it helps produce a fruity, intense
character.
Lager
yeasts are often
smoother and dryer, with the taste of cloves, vanilla or a wide variety
of other hints. Pilsner, for example, is a type originating in the
Czech Republic in the town from which the brew gets its name.
Once,
it formed the basis
of 90% of the lagers consumed around the world. But with the growth of
micro-breweries and the expansion of variety in the U.S. and elsewhere,
lagers now come as Dortmunders, Märzens, Bocks and other
styles as well.
Though
carbon dioxide and
ethanol are the two primary products of fermentation, yeast produces
secondary products as well. The different types can add tastes or
aromas as varied as sweet corn to green apple to butterscotch.
Unfortunately, they can also produce sulfur, or phenolics, which have a
medicinal taste.
Control of the added flavor is as much an art as the control of fermentation is a science.
Home Brewing
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