As off-flavors go, diacetyl is an odd duck in that it's fairly benign but also infuriating. A little diacetyl (which comes across as butter, butterscotch, or maybe vanilla in flavor) is acceptable in some styles, and not all that off-putting in others, but many brewers struggle to identify it (or pretend they can!).
This week at Beer Simple, we're going to talk through a fool-proof method for testing for diacetyl that will allow even the most butter-insecure person to confidently state whether or not they're out of the diacetyl woods, and what to do if you're not.
A Quick and Dirty Overview of "D"
Diacetyl is a fermentation byproduct (specifically, a vicinal diketone, or VDK) that is present in almost every beer. The trick is preventing it from ending up in your finished beer, above detection levels. A thousand other articles have discussed how to limit it, so feel free to read those, but here's the short version:
- Pitch enough healthy yeast
- Choose a yeast strain that produces less diacetyl (unless you want it)
- Control initial fermentation temps
- Increase temperature in the latter stage of fermentation to encourage the yeast to clean up after themselves
- Avoid contamination
Simple enough. The real question is whether you've met that goal, and what to do if you haven't.
The Force Test
Once you build up some confidence in identifying diacetyl you'll be able to just take a sample, give it a swirl, and sniff/taste/feel your way to a conclusion. To help you build up that confidence and train up your palate, you might want to consider the Diacetyl Force Test.
Ferment your beer, and pull a four-to-five ounce sample. Divide the sample between two microwave-safe vessels (most coffee mugs work well), and cover them both. Put one sample in the microwave for about 20 seconds, and pull it.
The heated sample is going to be jamming with aroma at this point. Take a sniff and compare it to the unheated sample. If they smell identical (just at a different magnitude), then you almost certainly don't have diacetyl remaining! Heating oxidizes and drives out AAL (the precursor to diacetyl) and will create a rich, buttery aroma that will be both distinct from traditional "beer-y" flavors like caramel or melanoidins (which can be mistaken for D) and also turn up the volume on it.
This is a terrific way to train your palate because it will demonstrate the difference between what are and are not "diacetyl" flavors, and teach yourself how you perceive them.
We often discuss beer flavors as though they're monolithic, but they're not: I perceive diacetyl, oxidation, and acetaldehyde differently than you (artificial vanilla, old books, and wild grasses, respectively). Hell, to me Isovaleric Acid (gym socks) tastes like raspberry. The terminology we use describes common perceptions and creates a functional language for identifying beer faults, but that doesn't mean that your brain will process these flavors/aromas in exactly that way.
So, yes, to many, diacetyl is patently identical to popcorn butter - but it's better to know for sure.
Knowing is Half the Battle
Now that you know whether or not you still have a diacetyl issue, what can you do about it?
There are two simple solutions. OK, I take that back, one simple solution, and one kind-of-harder solution.
The first is to increase the temperature of your fermenting beer slightly, and wait. This is the famous "diacetyl rest," and it will encourage your yeast to go back and clean up/break down any AAL or VDK remaining in your beer. Time alone might do the trick, but time plus a little warmth will encourage your yeast to stay active and clean up after themselves.
The other is to add actively-fermenting wort to your finished beer, a process known as "krausening." The highly-active yeast will be hunting for anything they can get their hands on to process, and as a result will drive you to a fuller attenuation, jump-start the carbonation process, and clean up diacetyl. It's not a bad practice to get into in general!
Diacetyl is nothing to fear, and as I said above, there are certainly worse brewing faults to contend with. As you get a better sense of how you perceive it you can abandon the force test altogether and trust your senses, and between that and some minor process tweaks you'll soon have your VDK problems in the rearview mirror.
Keep it simple.