Buttered Up: Identifying and Treating Diacetyl In Your Beer

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.


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Quick (Good) Lagers for Lazy Brewers

I wouldn't say I'm obsessed with fast brewing, but there's no doubt that one of the virtues of "simple" brewing is that it tends to be quicker.  That's why it always bugs me when I hear people say that they won't/can't brew lagers because they don't have the time and/or can't temp control for long enough.

Of course you do, and of course you can.  

Most anything we want to do in brewing can be done (and done well) with the right method.  I'm a believer in the idea that the hard part is figuring out what you want - figuring out how to do it can be surprisingly easy once you settle on your desired outcome.

So, this week, let's assume that you want to brew lagers, but that you've convinced yourself (or have been convinced by others) that they take too long or that you can't hold the proper temperature.

Work Backwards

What makes a lager a lager?

Well, if you want to get all historical, lager (from the German lagern) literally just refers to beers that were stored, usually in caves, and therefore at cool-ish temperatures (12C or so - it's German, so we're using metric).  

Moving into modern lager style characteristics, though, we generally think of these as "clean and clear" beers.  Ester and phenol production is low, alcohols are restrained even when the beer has a high ABV, and they're usually brilliantly clear with a nice jewel tone.

Why?  Because that's what you get when you make them the way earlier brewers made them.  They're low in esters and phenols and feature "cool" alcohols because they were fermented cooler than, for example, summer farmhouse ales.  They're brilliantly clear because they've been sitting and precipitating out solids for weeks, or months.  

But that doesn't mean that's the only way to get those characteristics.  Why don't we just work backwards and see if we can get this done while shaving time off, preferably in simple ways?

Set the Board for Success

First things first: let's make sure you're stacking the deck in your favor here.

Choose a yeast with a reputation for "clean" fermentations.  Not all lager yeasts are equally clean (just like not all ale yeasts create a riot of fermentation characters), so read reviews and product descriptions to get something as flavor-neutral as you can.  You might even (cover your ears, orthodoxy-lovers) consider the cleaner ale strains in case your temp control "ceiling" is a little on the high side.

Also, pitch big.  Esters and phenols are the result of work, and the less your yeast have to work and the sooner they finish fermentation, the less they'll put out detectable flavors.  Take the recommended lager yeast pitch rate, and bump it up by about half.

Consider your style, too.  If you're concerned about being able to make a fast lager without creating fermentation flavors, go with something that'll cover them up, at least a bit.  Doppelbock and Baltic Porter hide a lot more than Helles and Pilsner.

Last, give your yeast plenty of air.  This goes to rapidity in moving through the lag/log phases of the yeast life cycle in which the flavors we don't want are produced: the more oxygen there is in your beer at the start, the quicker they'll settle down and stop producing the stuff we're trying to avoid.  Esters and phenols are usually a reaction to heat and/or stress.  Reducing at least one of those is a good way to get clean beer.

Now that you've given yourself some structural advantages, let's talk process.

A Question of Time

Lagers don't need to chew up a lot of time, either in your fermentation fridge or in your finished-beer fridge (we all have one or three of those, right?).  This is probably the second-most-commonly-cited reason I hear for why people don't make lagers: "I don't have the time to brew lagers because I need the space in the fermentation fridge for other beers."  Fine - why are you leaving them in there for so long, then?

The things we want to avoid - principally esters, but other compounds as well - are formed (or their precursors are) pretty early in the fermentation process.  How early?  Well, if it isn't there by the time we finish the lag phase and growth phase, it probably won't ever appear in levels sufficient to be noticeable.  That means that if you start cool and stay cool for about 72 hours, you can pull that beer and leave it at any steady room temperature and still avoid the things that make your lager seem like not-a-lager to your palate.  

Voila - free space in the fermentation fridge.  I mean, don't leave it in a hot garage or anything, but just your normal basement temps (even if they're in the high-sixties Fahrenheit) aren't likely to cause any real trouble for you.  Hell, it might even help you avoid incomplete fermentations and/or increase blowoff of things like sulfur, making your beer even cleaner.

"But what about the extended aging process?  I might brew it faster, but I still need to age it..."

Why?  Get aggressive with the gelatin (or your preferred clarified) and it'll be bright and clear before you know it.  I once turned around a Helles in nine days for a 500-entry competition held three weeks from brew day, and it won a silver medal with a 40+ score.  

This isn't really about time.  Again, if you have it, it helps, but not having it isn't disqualifying.

A Question of Temperature

A much more valid concern is when people tell me they want to brew lagers but don't have any real form of temperature control.  

This one is hairier, because there's no "simple" way to set up an evaporation rig.  It's not assembling an aircraft engine, but there's no doubt that it's a bit of a pain.  

If you can't get your hands on a chest freezer and temp controller (though thanks to the secondary market and falling prices on products like the Inkbird, those are much more affordable now!), and aren't willing to drape t-shirts and towels and set up a fan, I do have at least one solution that takes minimal effort: ice jugs.

Take six one-gallon plastic jugs.  Fill with water.  Freeze.

Chill your beer down as cold as you can, put it in a large vessel (bathtubs work), and fill with cold, groundwater-temperature water, as high on your fermenter as you can get it.  Except for the deepest parts of the deep south, that will give you a starting temperature in the high-50s or lower (even better in winter, but I'm assuming we're thinking "summer" here).  

Immediately add three of your ice jugs.  Thermal mass is your friend here.  You don't want to cool water down - it's far easier to keep water cool.  Do this morning and night, cycling your melted jugs back into the freezer and replacing with the others, for three days.  After that, just let it slowly come up to whatever temperature you can hold it at using nothing but water replacement (drain the tub, refill with cold water) for another day or two.

If the three-jug method doesn't keep you below 60F, increase your total to eight and add four at a time - if it's too cold, dial it back.  But you want to try to maintain a steady temperature for those 72-96 hours.

After that, you're out of that lag/log phase flavor-production window, and just like your temp-controlled colleagues you can pull your beer and hold it at room temperature!

Lager Away

There's nothing magical about brewing lagers.  And, for that matter, then recommendations noted here work just fine for ales, too.  

Don't let time or temperature be your reason to not make lagers, though - you've got this!

Keep it simple.


Please help support BEER SIMPLE by visiting the Support page and saving the links there as your bookmarks, especially this Amazon link!  Every dollar you spend will help keep BS coming your way, and more often (which is at least as much a threat as a promise).

Stuck: Managing a Troubled Fermentation

Brewers make wort - yeast make beer.  In light of that, it's a bad idea to focus too much on recipes and wort production and neglect your fermentation process, which is why so few good brewers do it.  But even the best brewer will occasionally have a problematic fermentation.

Diagnosing and treating a failed, slow, or stuck fermentation takes a bit of guesswork, but in the end you should nearly always be able to get things moving.  This is a robust process, and once you get the yeast rolling they'll usually get back to what they're bred to do.  After all - apparently it's cliche day here at Beer Simple - wort wants to become beer.

The questions are these: do you have a problem, how can you address it, and is it worth it?

Do You Have a Problem?

Step One is figuring out if you actually have a problem.  

Lots of new brewers panic when they don't get quick action in the fermenter.  I like to see some krausen forming within 12-24 hours (though with a good dose of oxygen I've had it in as few as 6), but I don't start sweating until about 72 hours, and maybe not even then, even if I don't see action in the airlock.  Don't panic - just take a gravity reading.  Sometimes your yeast crank through primary fermentation very fast and leave little evidence of it - these "phantom fermentations" are rare, but they happen.  Your gravity reading will tell you.  If it's still at your OG, then you have a failed pitch.

Diagnosing a stuck fermentation follows this same basic method, but asks for some analysis.  It requires taking gravity readings at two times (say, 12-24 hours apart); if both are identical, then you may have a stuck fermentation.  It's not certain, though.  The gravity might be static, and higher than you anticipated, but it's possible you are finished.  If you mashed warm, or had a recipe that included a lot of unfermentables, or over-yielded on your efficiency and started with a higher-than-expected OG then your FG calculations might be off (consider this possibility if you're over 50% attenuated).  Maybe you're done and you don't even know it.  How do you know?  Taste your beer - if it's not particularly sweet, then you could very well be finished.  Those residual sugars often don't taste as sweet as simple sugars, and your palate is very sensitive to sweetness.  Sweet beer is usually still-fermenting (or fermentable) beer.

And then there's diagnosing a slooooooooow fermentation.  Sometimes a beer is still fermenting, just very, very slowly.  Sluggish yeast are a pain.  You take your gravity readings, and it's dropping...but only by a couple of points every couple of days, even with 20-30 points to go.  

If you have any of these, then you might consider taking action.


For a failed fermentation (no activity, and no movement on your OG):

  • Step one is to check your temperatures.  If you're freezing your yeast, they might have simply gone dormant - anything below 50F is a risk for that, though I've fermented at 45F without problems.  If you're flirting with that number, put your fermenter somewhere a little warmer and see what happens..  
  • If nothing happens then, make sure your wort isn't too hot (over 90F) and re-pitch.  You can wait it out, but if you don't see activity within 4-5 days and there's no bubbling in the airlock and your gravity hasn't dropped, then your yeast are likely dead (or so few are alive that they'll struggle like crazy, creating lots of off-flavors), and they need the cavalry to come to the rescue.  If you wait too long, every other thing in your house will try to get in there and establish a foothold.

Stalled fermentations invite all kinds of tricks, but they have some uncertain results.  They can work, and you should try, but don't get your hopes too high (though this isn't the end of the world, as we'll discuss a little later).  Your options include:

  • Just like a failed start to fermentation, check your temps - and increase them.  Warmer yeast are more-active yeast, and if you catch them in time this might get them off the picket line and back to work.  Go all the way to 90F - if you're already through the initial fermentation stages, the hot temps won't be nearly as likely to produce off-flavors.  
  • Shake it up.  You could also try rousing the yeast, either by shooting CO2 into the sediment or old-fashioned swirling or agitation.   
  • Repitch.  This can be with the same yeast strain, a more-aggressive yeast like champagne strains, or even bugs that might keep on chewing (especially if you think the problem was caused by an excess of unfermentables).  
  • Re-feed.  If you think that the problem is a bunch of unfermentables and you're not comfortable introducing Brett or its ilk into your brewery, you can also spike your beer with simple sugars (honey, maple syrup, table sugar, etc.) that the yeast will consume.  You'll add alcohol, thinning out the beer, but at the possible cost of new/off flavors and hotter alcohols.

And for slow fermentations, well...

  • Increase temperature and wait.  It will end someday.  Go on vacation.  May I recommend Campobello Island, New Brunswick, home to an Joint US-Canadian International Park that FDR used as a retreat?

Do you REALLY Have a Problem?

Before you take any of these steps, though, ask yourself if you really have a problem - or, at least, one that's worth fixing.

Taste your maybe-unfinished beer.  If it's soured or funked already because the yeast never took hold, then you might consider dumping it, cooking with it, or making some vinegar depending on the flavor.  

If it's a stuck fermentation but it tastes OK, then consider just carbonating it, claiming victory and departing the field - the odds that someone can taste the difference between a 1.030 and a 1.020 beer are pretty slim (just be sure you're actually stuck - otherwise you could be making bottle bombs). 

I've seen a lot of brewers fight their beer for those last few points.  It's not always worth it, and the cure can be worse than the disease, so ask yourself some tough questions about whether you even want to try.  

But what you definitely shouldn't do is what we tried with an early group-brewed batch before we knew much about brewing: don't dilute your beer with club soda.  I mean, it tasted alright eventually...about eight years later.

Keep it simple.


Please help support BEER SIMPLE by visiting the Support page and saving the links there as your bookmarks, especially this Amazon link!  Every dollar you spend will help keep BS coming your way, and more often (which is at least as much a threat as a promise).