Tuesday Beer Trivia: Fermentation Flaws

The following explanations were taken from “Foiling Fermentation Flaws” by Steve Holle, originally featured in the March/April 2006 issue of Zymurgy.

Question 1: Yeast must metabolize other wort constituents besides sugar to support cellular activities that ensure yeast growth and health, among which are fusel alcohols and amino acids. In general, conditions that promote rapid yeast growth foster fusel alcohol production.

Question 2: Esters are responsible for fruity aromas in beer and are important flavor contributors, particularly in top-fermenting ales. However, high levels of fruity esters are usually unwanted when producing lager beers. At excessive levels, esters become unpleasant and can even take on a solvent-like character.

Question 3: Because acetaldehyde production is an intermediary reaction, the concentration tends to increase and then decline. This makes high levels of acetaldehyde a common green beer flavor and a good indicator of beer maturity.

Question 4: Diacetyl is an important flavor compound produced by yeast during fermentation which has a butterscotch aroma and a slick mouthfeel.

Question 5: Yeast requires sulfur for the synthesis of cell protein. Wort contains sources of sulfur including inorganic sulfate in water and sulfur-containing amino acids, peptides, and proteins from malt. Concentrations of these yeast-produced compounds increase with as fermentation conditions deteriorate. Among these compounds is sulfur dioxide, which can resemble the smell of a struck match.