A Craft Beer & Brewing Magazine reader recently asked us the following question:
Why do some of my beers have a buttery flavor to them?
That buttery flavor is an off-flavor usually caused by fermentation problems. It is created by a compound called diacetyl, which is a by-product of fermentation. Diacetyl can produce a flavor like buttered popcorn or a slightly butterscotch flavoring. Diacetyl is one of two major vicinal diketones (VDKs) produced during fermentation. The other is pentanedione, which has a honey flavor to it. Both are present in all beers, although usually they are well below the threshold where they can be detected. Lighter ales and lagers are more susceptible to diacetyl problems simply because the compound is easier to detect in a light-flavored beer.
Diacetyl is produced during active fermentation, but yeast can actually mop up diacetyl during the later phases of fermentation. To aid yeast in cleaning up diacetyl, it is important that you do a diacetyl rest, which involves raising the temperature of the finished beer by a few degrees at the end of fermentation. This can be done with both ales and lagers, and a healthy yeast population can clean up diacetyl in as little as a few hours, although usually the diacetyl rest is maintained for a day or two.
A healthy yeast population is critical for managing diacetyl as well as other off-flavors, so you should make sure you pitch enough healthy yeast, aerate your wort before pitching the yeast, and manage your fermentation temperatures. It is also common with some lagers to pitch additional yeast during the diacetyl rest to aid in mopping up any remaining diacetyl.
In addition, both bacterial infection and oxygen can cause diacetyl problems in your beer, so proper sanitation is critical. You also need to minimize oxygen exposure once fermentation has started as oxygen can cause a variety of off-flavor and stability issues. It is also common to have diacetyl form in dirty keg lines and taps where both oxygen and bacteria are present. This is a big problem at many craft breweries and pubs, so it is important that keg lines and taps be cleaned regularly.
If you have a question for the experts or want to share your expertise, email us at [email protected] or visit our website at beerandbrewing.com.