Off Flavor Focus - Diacetyl

This is the presentation Bobby gave at the May 2010 meeting during the Siebel kits tasting. The pdf document attached has better formatting, but the text is pasted here so that the website search can get to it.

Off Flavor Focus: Diacetyl

Presented by Bobby M

Sounds like : Dahy-uh-SEET-l, Dahy-uh-SET-l, or Dahy-AS-etl

Webster: a compound (CH 3 CO) 2 that has an odor like that of quinone, that is chiefly responsible for the odor of butter and contributes to the aroma of coffee and tobacco, and that is used as a flavoring agent in foods (as margarine) called also biacetyl.

Diacytel is a natural byproduct of the fermentation process and under the right conditions is nearly completely consumed by the yeast after it is produced. It smells and tastes like microwave popcorn and creates a slick feeling on the tongue.

BJCP styles? Scotch Ales, Bitters, Dry Stouts, Czech Pils, Oktoberfest allow for minimal residual Diacytel while considered a flaw for all others and especially in moderate to high concentrations.

Typical Homebrewing Cause

Brewer pitches yeast warmer than recommended for that strain.  Exothermic fermentation causes temp to rise and remain high which maximizes Diacytel production. As active fermentation slows, beer returns to (lower) ambient temperature and starts knocking the yeast activity down. Brewer racks Diacytel-rich beer out of primary and away from the yeast that would have cleaned up the diacetyl. Diacetyl remains in the beer.

Diacetyl can also be caused by Pediococcus bacterial contamination due to poor sanitation practices.

Some yeast strains are more prone to diacetyl production than others.

Recommendations to reduce diacetyl in your beer:

Chill wort to 2F LOWER than the recommended ferment range prior to pitching. Pitch yeast, then allow to warm to desired ferment temp. As activity dramatically slows, slowly ramp temp up to the upper end of the range and hold it there for at least an extra week and up to 3 weeks for higher gravity beers.

Make sure you have enough healthy yeast to begin with: If you use Whitelabs or Wyeast liquid yeast, you should be making at least a one liter starter for ales between 1.050-1.070 OG. If the OG is higher or the yeast is a lager strain, the required starter size can go up to 2-4 liters!! Dry yeast should be rehydrated in water per Danstar/SAF instructions to retain viability. Be sure you aerate or oxygenate the wort prior to pitching.

Sources and additional info: (John Palmer)