Yeast is a single celled fungus that
takes sugars extracted from grain and transforms them into alcohol and carbon
dioxide in a process called fermentation.
That's a pretty general overview of
what yeast does, and to become a Certified Cicerone, you'll need to understand
fermentation in slightly, but only slightly, more detail.
There are two main types of yeast: ale yeast and lager yeast.What is the difference between the two types?
Ale yeast
Ale yeast have the Latin name Saccharomyces cerevisiae (Sack-er-oh-my-uh-ses Ser-vee-see-yay). Ale yeast prefer warm temperatures and float to the top of the fermentation vessel.
The higher fermentation temperature allows ale yeast to process sugars faster, and in doing so, create byproducts that add flavor and aroma to the beer.
Lager yeast
Lager yeast have the Latin name Saccharomyces pastorianus (Sack-er-oh-my-uh-ses Pas-tor-ee-an-us), but are also known as Saccharomyces carlsbergensis.
Lager yeast prefer cool temperatures and sink to the bottom of the fermentation vessel.
Cool temperature fermentation allows lager yeast to fully process the available sugar without adding significant byproducts of their own.
Lager is German for storeroom or cellar, and refers to the practice of storing, or lagering beer at near-freezing (35-45°F, 2-7°C) temperatures for up to a month after fermentation has completed.
Lager yeast have the Latin name Saccharomyces pastorianus (Sack-er-oh-my-uh-ses Pas-tor-ee-an-us), but are also known as Saccharomyces carlsbergensis.
Lager yeast prefer cool temperatures and sink to the bottom of the fermentation vessel.
Cool temperature fermentation allows lager yeast to fully process the available sugar without adding significant byproducts of their own.
Lager is German for storeroom or cellar, and refers to the practice of storing, or lagering beer at near-freezing (35-45°F, 2-7°C) temperatures for up to a month after fermentation has completed.
To summarize:
| Ale yeast | Lager yeast |
|---|---|
| warmer temperatures ≈(65-70°F) ≈(18-21°C) | cooler temperatures ≈(45-55°F) ≈(7-13°C) |
| quick fermentation | slow fermentation |
| significant yeast derived aromas and flavors | little yeast derived aroma and flavor |
| quick to make | requires additional storage at cold (35-45°F, 2-7°C) temperatures |
| tastes great! | less filling! |
Yeast can contribute many byproducts to beer. Some yeast byproducts taste good, some do not.
Here are some of the by products that yeast may contribute to beer:
Esters are aroma compounds that are fruity at low concentrations, but in higher amounts come across as nail polish remover.
Diacetyl and 2,3-Pentanedione are flavor and aroma compounds. Diacetyl comes across as movie popcorn butter or butterscotch candy, while 2,3-Pentanedione tastes more like honey.
Acetaldehyde tastes like green apples.
Sulfur compounds are acceptable in some beer styles at low levels, but in higher levels give a rotten egg flavor.
Dimethyl Sulfide (DMS) also acceptable in some beer styles at low levels, but in higher levels adds the taste of cooked corn.
Fusel Alcohols come from higher temperature fermentations.
Organic Acids give a sour or salty taste to beer. Some organic acids are: Acetic, Citric, Lactic, Pyruvic, and Succinic acid.
Fatty Acids often give a goaty or soapy taste to beer and decrease the foam stability.
Nitrogen compounds can give beer a fuller body.
It is the brewer's job to manage the action of the yeast, typically by controlling the temperature of the fermentation vessel.
Esters Haiku
by Seth Wright
Yeast stress shall dictate
Levels of isoamyl acetate
Brewers salivate
An Ode to Esters
by Jason OliverEsters, esters, are good for the beer.
How you ferment is how they’ll appear.
Much less in a lager than in an ale,
Swirl your glass around and pleasantly inhale.
Some beer styles rely on specialized yeast strains to create their unique flavors.
Hefeweizen uses a yeast that produces 4-vinyl guaiacol and isoamyl acetate, which produce clove and banana aromas, respectively.
Saison uses a strain of ale yeast that can survive fermentation temperatures of up to 90 deg F. This yeast provides phenols that give saison an earthy, black pepper spiciness.
Other types of microorganisms can contribute to beer flavor.
Sour beer relies on flavors that are derived from sources other than yeast. Sometimes these microorganisms are unintentionally introduced to other style beer where their flavor contributions are considered a fault.
Brettanomyces (Brett-an-no-my-uh-sees) is a strain of yeast that produces 4-ethyl phenol, which contributes a barnyard, or horse blanket character.
Lactobacillus (Lack-toe-bah-sill-us) and
Pediococcus (Pee-dee-oh-cock-us) are bacteria that produce lactic acid, which gives a sour, acidic flavor to beers like Berliner Weiss.
Acetobacter (Ah-see-toe-back-ter) is a bacteria that converts alcohol (ethanol) into vinegar (acetic acid). This is an important component of some sour beer styles.
For further research:
Read it online.
by Chris White with Jamil Zainasheff
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