While there are a myriad of faults that brewers discuss, debate, and diagnose, one of the most common is diacetyl, which is a fermentation by-product found in basically all fermenting (and some fermented) beers. Under normal circumstances, yeast cells are happy to go around and clean up the diacetyl produced while creating ethanol, but sometimes they don’t get to it all (usually through our own mistakes).
Today isn’t about how to avoid diacetyl, but here’s a quick and dirty primer:
1. Select a yeast strain that is less likely to produce diacetyl.
2. Hold the initial fermentation temperature in the lower range for the yeast.
3. Bring the fermentation temperature up a few degrees in the last few days of active fermentation and hold it there for 3–7 days.
4. There. Now you know how to avoid diacetyl (in theory).
Today is about how to test whether or not you have diacetyl in your beer, and I’m going to proceed under the assumption that you don’t have ready access to a gas chromatograph.
Passive Organoleptic Testing
Passive organoleptic testing is also known as “smelling and drinking your beer.”
While each palate has a different level of sensitivity to diacetyl, this should (eventually) be your most-common method of detecting it. Diacetyl can be perceived reliably in aroma, taste, and mouthfeel if you know what you’re looking for (and if you throw in some actual palate training!).
The most common descriptor for diacetyl is “buttery” or “butterscotch” (but don’t fall in love with those—more on that in a bit), and the majority associate it with popcorn butter in particular. Give your glass a swirl and see if you detect the aroma of a Friday night at the cinema in your youth, minus the bad cologne and plastic nacho cheese.
Sniff a few times, then reset by sniffing yourself (your sleeve is fine), or if you’re slightly more fancy, a jar of coffee beans. Next get back in there—if your nose isn’t wet, you’re not trying. Swirl the glass to get air into the beer and release more aromas. Cap the glass with a coaster, an identical inverted glass, or your hand (as a last resort, since it may have aromas of its own) to trap/concentrate more of the aroma. You might find that this method alone is sufficient to detect diacetyl (or just “D” if you’re hip, or “VDK” for vicinal diketone if you’re really, tragically hip).
Diacetyl can also be tasted, and tasting will reveal basically the same flavor profile, though in both cases, you’ll want to be aware of how other flavors in your beer might be mimicking or masking diacetyl. Caramel (especially the sweeter variety created by low-Lovibond crystal malts) or toffee can often be mistaken for diacetyl, as can the richness of melanoidins from some character malts (melanoidin malt, Victory, Biscuit, Aromatic, etc.). Diacetyl can also be masked by many roasted malts and highly active yeast strains, which generate lots of esters and phenols.
Luckily, we can do more than just taste for diacetyl: we can feel it. Mouthfeel gives us our last organoleptic tool in detecting diacetyl. Beers with high levels of diacetyl will often have a slick, coating mouthfeel. While this isn’t always an indicator, in and of itself, it can be a good secondary indicator if you’re on the fence in your aroma/flavor perceptions.
But how do you know whether what you’re tasting is really diacetyl? Practice, practice, practice—but also training.
Training Your Palate
I told you we’d get back to training your palate. Butter is only one way we might perceive diacetyl, and every palate is unique in its sensitivity and in the way the brain interprets those perceptions. What might taste like popcorn butter to you might taste like something else to me, depending on the sensory hook or impression that I get from the same compound.
For the record, to my palate, diacetyl tastes like artificial vanilla with a touch of candied sugar—not at all like butter. How do I know? Because I spiked a sample of clean light lager with straight diacetyl, and tasted it against a control of the same beer. Then I repeated the test using lots of other styles—porters, IPAs, Belgians, stouts, and more—to see how my impression changed. The only time I got close to “buttery” was in the roasted beers—they tasted like burnt buttered toast to me.
You can find imitation butter in the baking aisle at most grocery stores, but I recommend that you get yourself an off-flavor kit from a reliable source such as Siebel (usually as part of a group buy with a homebrew club or a group of craft beer people!), since the baking stuff could be one of any number of formulations. Then start spiking. You’ll soon learn how you perceive diacetyl.
The Force Test
One final way to find diacetyl is with a force test. What this does is force diacetyl’s precursor to oxidize rapidly through heating, helping you confirm its presence. You can do this one of two ways.
The simple test is this: pull a sample from your fermenting beer, put it in a microwave-safe vessel, and nuke that thing for about 15 seconds. Pull it out. Test as above. A hot sample is going to be giving up a lot more flavor overall, and as a result, if you have D in there, it will be amplified. The trouble is, so will everything else. This is a good test for when you’re more familiar with picking up diacetyl. Until then, consider a more rigorous method.
From your primary fermentor, draw two samples into two jars and seal the jars. Heat one of those jars (a hot-water bath works well) to about 140°F (60°C). Hold it there for about fifteen minutes, then cool to the same temperature as your unheated sample. Now, smell/taste both. If they taste the same, then you’re in the clear! There’s likely very little chance of a meaningful diacetyl flavor. However, if the heated beer tastes strongly of diacetyl, you should let the yeast continue to work on it (and maybe increase the temperature a bit to goose the yeast cells into activity).
Try, Then Trust
As the poet said, first we try, then we trust. I usually recommend that brewers start by training up their palate before conducting force tests on their beers, until they’re sure they can detect diacetyl reliably. And from then on, they can trust their ordinary sensory faculties. That pattern should serve you well in every off-flavor area: perfect it, repeat for other flavors, and before you know it, you’ll have a properly trained-up palate!
From steeping specialty grains to extract and hops additions to pitching yeast and racking to secondary fermentation, as well as bottling your beer, CB&B’s DVD, Brewing Great Beer Start to Finish, will get you started down the road to making beer that rivals what you get at the local pub.