Dear Mr. Wizard,
Last week I brewed a German Alt using Wyeast 1007. It’s been fermenting at 58° F for 9 days now and it’s nearing the end of the primary. I will then transfer it into a keg and let it condition for about 8 weeks. My question deals with diacetyl.
Normally this is associated with lager strains, but I’m wondering if I have to worry about it with these special low temperature ale yeasts? Is diacetyl only associated with lagers or is the temperature more of a key factor?
Mr. Wizard replies:
The real question at issue is not whether diacetyl is only associated with lagers, but rather if it is only unacceptable in lagers. Diacetyl is naturally found in all beers during fermentation and some beers can contain perceptible levels of diacetyl after aging.
Diacetyl, also known as 2,3 butanedione, is produced outside of the yeast cell when the compound alpha-acetolactate is oxidized by metal ions or dissolved oxygen. During aging, yeast cells absorb diacetyl, use it as a hydrogen donor in biochemical reactions and in the process convert into a flavorless alcohol called 2,3 butanediol.
Style gurus all agree that diacetyl should never be found in lagers.Its presence is a sign of insufficient aging and is simply not acceptable in a type of beer named after its long, cold storage!
Diacetyl can also originate from beer spoilage bacteria, especially Pediococcus, and any off-flavor associated with bacterial contamination is bad, regardless of the fermenting strain. ("Wild beers", such as lambics, are of course held to a different set of rules.)
Oddly enough, if you asked the question what is the easiest type of beer to brew without having to worry much about diacetyl, I would answer lager. In a classic lager fermentation there is plenty of time for diacetyl to be produced from alpha acetolactate and for yeast to absorb diacetyl and reduce it to 2,3 butanediol. The fact that lager yeast strains are typically less flocculent than ale strains also helps since contact between the yeast cell and beer is essential for beer-yeast interactions to occur. Although Alt yeast is an ale strain, it is very non-flocculent and the cool, protracted method used to produce Alt beers makes Alts more similar to lagers with respect to yeast behavior and fermentation technique than ales. In other words, I would not worry much about diacetyl in your Alt.
Thanks to advances in brewing science over the past few decades, diacetyl is very well understood. The underlying biochemistry has allowed brewers to accelerate the aging process by manipulating beer temperature after fermentation. This has become known as the "diacetyl rest". The diacetyl rest is a step in the fermentation process designed to reduce diacetyl levels. Some breweries have gone as far as to use immobilized yeast to speed up aging by increasing the contact between yeast and beer. Globally, the result of this understanding has been a dramatic reduction in diacetyl levels in all sorts of beers.
Diacetyl is one of those compounds some people really like in small doses because it adds to the complexity of some beer (and wine) styles. Most brewers see it as a defect because of its association with beer spoilage organisms as well as its satiating effect on the consumer. For those brewers who wish to get more diacetyl in your beers, take the opposite route of alt brewing. Use a flocculent yeast, room temperature fermentation, short aging and good racking techniques to minimize the amount of yeast carried into the bottle.