Process: Yeast & Fermentation

After the wort is boiled, cooled, and aerated, yeast is added, or pitched, into the sugary solution to begin the wort's transformation into beer.

While fermentation can take place in any type of vessel, many modern fermentors have a conical bottom. The conical bottom provides several advantages - one advantage is giving sediment a place to collect.

Generally speaking, fermentation is the process where yeast consume sugars in the wort and excrete alcohol and carbon dioxide. Yeast also produce a variety of other chemical compounds, and we'll get to those soon.

You may recall that there are two main types of yeast, and each type prefers to ferment at a different temperature, and each type effects different qualities in the finished beer.

Lager yeast prefer to ferment at cooler temperatures (46-55° F, 7–13°C) and settles to the bottom of the fermentation vessel.

Ale yeast prefer to ferment at warmer temperatures (65-70° F, 18-22° C) and floats to the top of the fermentation vessel.

Both types of yeast consume the sugars in the wort and produce ethanol and carbon dioxide in addition to a variety of other flavor compounds that influence the final flavor of the beer. 

In small quantities, these compounds add complexity to beer flavor, but in large quantities they can taste unpleasant. Think about salt in your food: unsalted food can taste bland, a little salt improves the flavor, but too much salt can ruin a dish. 

Yeast create these compounds as a reaction to temperature, and they don't care what the flavor compounds taste like. The brewer guides the yeast to create the desired flavors primarily by regulating the temperature of the fermentation.


Fermentation temperature is the main influence on flavor compound production.  
 
Yeast produced flavor compounds can be classified in several groups:

• Esters
• Alcohols
• Phenols
• Sulfur Compounds
• Other (Aldehydes & Ketones) 

Esters

Esters give fruity flavors to beer. Different beer styles require different levels of esters, and different yeast strains produce different types and amounts. Higher fermentation temperatures generally cause the production of more esters.

Alcohol

One of yeast's main byproducts of fermentation is ethyl alcohol (which is also called ethanol), but several other types of alcohol are also produced when ethanol combines with esters. These larger molecule alcohols are called fusel alcohols, and in high concentration they can remind you of nail polish remover or paint thinner.

Higher fermentation temperatures can increase the production of fusel alcohols.

Phenols

Phenol is a broad range of chemical compounds. Most are undesireable, but some define certain beer styles. Phenols differ from esters in that phenols tend to be perceived as medicinal or smoky, where esters tend towards fruity.

Phenol production increases at higher temperatures, but so does ester production, so the brewer must create a balance.

Two noteworthy phenols contribute to Bavarian wheat beer's unique flavor profile: 

1. 4-Vinyl Guaiacol smells like cloves  
2. Isoamyl Acetate smells like bananas (yes, bananas are a fruit - don't dwell on it)  

Sulfur Compounds

Yeast naturally create sulfur compounds that smell like rotten eggs (hydrogen sulfide) or burnt matches (sulfur dioxide). Sulfur compounds are volatile and will eventually evaporate, and this happens faster at warmer temperatures.

Other

I've grouped aldehydes and ketones together because:

• They are chemically sorta similar. 
• The Cicerone Certification exam is not a chemistry quiz.    

Some of the compounds created by yeast during fermentation can be reabsorbed by yeast after fermentation has completed.

The most important "other" compounds to know about are acetaldehyde [as-i-TAL-duh-hide] and diacetyl [die-uh-SEE-tul or die-AS-it-tul].

Acetaldehyde is an intermediate step in the formation of ethanol. In high concentrations it can taste like green apples or fresh cut pumpkin. Given enough time, yeast will convert acetaldehyde to ethanol.

Diacetyl tastes like movie popcorn butter or butterscotch candy. It is produced as a normal byproduct of fermentation, but it is later reabsorbed by yeast.

Diacetyl can also end up in beer as a result of bacterial infection in draft (draught) systems, but we'll cover that later. First we have to finish brewing the beer.
  
When the yeast have consumed all the available sugar in the wort, fermentation is complete.

But be patient, the beer is not ready yet...



For further research:

Identifying yeast flavor in beer

Yeast – The Practical Guide to Beer Fermentation
by Chris White with Jamil Zainasheff
Section 4

Mastering Homebrew
by Randy Mosher
p 200-203