The scientific definition of diacetyl is a vicinal diketone with the molecular formula C4H6O2. In simpleton terms, it is a movie theater butter or butterscotch flavor. Yes, I have heard that movie theaters actually are using diacetyl when they ask you if you want your popcorn buttered. Is this fact? I’m not saying no, but I wouldn’t doubt it. But diacetyl is a common problem in homebrew. I have even had a lot of commercial examples that have loads diacetyl. But this is is not a flaw in some English style beers.

So how does diacetyl get in our beer? Diacetyl is produced during fermentation.  It starts to show up in the low krauesen phase. Huh? Low krauesen? This is the phase of fermentation when the yeast has finished growing and you’ll start seeing a foam wreath develop in the middle of the surface.  The yeast has not completely adapted to the environment and ready to start metabolizing those sugars you worked so hard to create.  Ok, maybe I need to go in depth on the yeast development cycle one day.  It is quite interesting what all those yeasts do during fermentation.  Back to diacetyl.  It can start showing up in the low krauesen phase and the yeast will start cleaning up by products that were developed during late krausen phase.

Ok, so now we know when diacetyl can show up in our beer. Now why am I tasting it in my beer Well, it can be a lot of things.

• If you have a long lag phase (from when you pitch to when you get to the growth phase of fermentation) which can be caused from poor yeast health or insufficient aeration.
• Some bacteria strains can cause diacetyl production. Here is where I use that sanitation word. Any homebrewer has heard sanitation probably 3.7 million times. Seriously. Sanitation, so that’s 3,700,001 times.
• Premature racking out of primary. See earlier? You might not be able to see when your beer in late krauesen. So make sure your beer is done fermenting by checking your final gravity for a couple days straight to make sure your gravity is not dropping.
• Under pitching. Huh? So you’ve never made a yeast starter? You’ll improve your beer ten fold by making yeast starters. Take the plunge, get a stir plate and you can make sure you’ll have enough yeast to pitch.
• Too much oxygen. Wait one minute. I bet you remember me mentioning earlier about insufficient aeration? Well of course I did, there can also be too much. Yeast absorb all the oxygen it can during the growth phase. Well if there is too much oxygen there will still be oxygen lingering when the fermentation is over. Most homebrewers don’t filter their beer. If there is still oxygen left over, the yeast will still be feeding off of it and still trying to go through fermentation phases. Also minimize oxygen exposure after fermentation started, e.g. while racking to the keg or bottling bucket.
• Increase your fermentation temperature. That is use a diacetyl rest. A diacetyl rest is a common practice for lager beers. When you warm up the yeast it becomes a bit more active and it will help clean up the yeast. You can also use a diacetyl rest for ales, but most of the time you are fermenting at the correct temperature for the diacetyl to be cleaned up.
• Use a less flocculant yeast strain. Flocculation is the state of yeast of being clumped together and falling out of solution. If the yeast is still in suspension it will be a little more efficient when cleaning up diacetyl.

Diacetyl will most of the time be a flaw in lagers, but not in every ale style. It is acceptable in a Czech Pale Lager, English and Scottish style beers and also a dry stout.

I’m sure we have all had this problem show up in our beers. This is one of the major issues that I fight with, especially since I started doing lagers.

Relax, don’t worry, have a homebrew.

References:
How to Brew
Beer Judge Education Course