Continuing our discussion of off-flavors, we'll take look at diacetyl it's great on microwave popcorn, but as all brewers know, this "buttery" off-flavor has no place in beer. 

Diacetyl (2,3-butanedione) is well known as the “butter” compound in microwave popcorn.  It presents itself as a buttery or butterscotch flavor in beer.  It’s in the vicinal diketone (VDK) category of organic compounds.

How does it form in beer?

Diacetyl is a compound that naturally occurs during fermentation as alpha-acetolactate is oxidized and turns into 2,3 butanedione (diacetyl).  As yeast complete the fermentation process they begin to take diacetyl back up into their cells and reduce it (chemically and quantitatively) through enzymatic action into acetoin and 2,3-butanediol – both of which do not impact flavor.  The other way that diacetyl can form is via bacterial infection.  Lactobacillus and Pediococcus are well known beer spoiling bacteria.  In addition to souring the beer with lactic acid, they can also produce diacetyl in large amounts.    

How do you solve the problem?

With ales, diacetyl usually presents itself if the beer is taken off of the yeast cake too early in fermentation.  Yeast needs to be present in order to re-uptake the diacetyl in solution and metabolize it into flavorless compounds (namely, 2,3 butanediol).  To help the yeast along, most breweries institute a “diacetyl rest” in which they raise the temperature of the fermentor a few degrees to stimulate the yeast into degrading the diacetyl.  This diacetyl rest process is especially common with lagers since fermentation occurs at lower temperatures and their metabolism is sluggish as a result.  Kraeusening with actively fermenting wort also will assist in the uptake of diacetyl by yeast.