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Why Does My Beer Smell Like Butter?

Jeremy Short · March 26


If you're here, you may have just cracked up in a bottle of homebrew and took a big whiff of butter from your pint glass. Yum, butter? Wait, this isn't supposed to smell like butter. What happened to my beer? And just maybe you took a swig of that beer and there is is again: butter. Not cool. You, my friend, have diacetyl. Adding a butter aroma and flavor, Diacetyl is one of the more common off-flavors found in beer. So what causes this thing? Can you fix it? How do you avoid? But, hey, I like butter isn't it cool if it's in my beer? (the answer to the last question is no). The good news is that diacetyl is a pretty easy fault to avoid.

Where does diacetyl come from?

Yeast. See, easy. Wait, do I have to say more? I guess just saying “yeast” isn’t super useful. Diacetyl (CH3CO)2  and also known by its sciency name butanedione is produced by yeast during fermentation. As the yeast chill out for a while the cells then absorb the diacetyl back up leaving you with no more butter.  

If it’s super butter or even butterscotch like that suggests there could have been yeast health issues and/or a bacteria jumped into the action and spoiled all the fun. A bacteria infection that causes this flavor can also be caused in dirty tap line.   

How do I prevent diacetyl?

The key to preventing diacetyl is time. While it may seem like your yeast have finished their duties there’s still some activity going on in that carboy. Relax, let your yeast kick back and chill for a bit. It’s like a delicious Thanksgiving meal they just enjoyed and they need time to absorb all that diacetyl back up. To see where your beer stands you can always use a thief to get a little taste. If it smells like butter let it sit longer.

Yeast tends to work better at absorbing diacetyl at warmer temperatures. If you are fermenting at low ale temps (low 60s) or at lager temps it’s a good idea to use what’s called a diacetyl rest. When the yeast are done converting sugars to booze you will then want to raise the temp 10 degrees to encourage the yeast to clean up any diacetyl. It’s important to wait to do this at the end of fermentation or you may get more than you asked for and the yeast will produce additional esters or phenols.  

Hey, I like butter!

Oh, wait, you say. You want some butter flavor in your beer? Fine. There are actually a few styles out there that are cool with butter, too. English Pale Ale, Oatmeal Stout and even Czech Pilsners are fairly chill with a bit of diacetyl. But all of them are also cool with no diacetyl and in general I would avoid it all together if you’re entering your homebrew into a competition.  

Diacetyl in wine

One thing that’s helped me increase my perception of diacetyl is drinking Chardonnay especially from Burgundy. These wines, sometime called butter bombs by the cool kids, can be a showcase of the flavor produced by diacetyl. But just like with beer, it’s generally considered a flaw in wine.

How to increase your perception of diacetyl

If you want to get better at picking up this flavor you can swing by the grocery store and pick up some butter extract (yum?). Then use a few drop in a 12oz beer and boom, butter baby! Make sure to save one beer without the butter so you can compare the two.  

Don’t go sniffing bags of this stuff

There once was a time when diacetyl was the main flavoring in microwave popcorn. Well, no more. Turns out the stuff is not good to breath in and is known to cause bronchiolitis obliterans or "popcorn worker's lung.” As a result many popcorn manufacturers have stopped using diacetyl altogether.

Everyone is Different

There’s a chance you might not ever perceive this flavor. It’s interesting to be at a table of judges tasting beer and one will taste diacetyl and another judge won’t pick up a hint. It can be frustrating to get that sort of mixed feedback. Even if you can’t pick up the aroma or flavor of diacetyl using the tips above will make all the difference when your beer ends up in the hands of someone who is very sensitive to diacetyl.  In the end, all that really matters is that you want to drink your own beer.



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