Beer Lecture! Diacetyl

Continuing with the trend of last week's post about beer glassware, I am going to talk again about some beer nerd stuff that you may not have known about. This week I’m going to teach you about everyone’s favorite off flavor, diacetyl. You have probably already experienced it and never knew what to call it. If you can’t tell, I’m brushing up on some Cicerone knowledge right now. I've got to study in preparation for the Bacon and Beer Classic, a beer festival in Seattle where I will be helping host the “Cicerone’s Corner.” A fun byproduct of that is going to be beer fun fact posts where I get to dump lots of oddly specific beer knowledge on you! So go ahead and buy your tickets to the Bacon and Beer Classic and let’s get this road on the show.

Diacetyl is the “movie theater” fake butter flavor in some beer. If you have had local Seattle favorite Mac and Jacks African Amber then you have tasted lots of it. Due to a *ahem* negative experience I once had with that beer (let’s leave it at "there was a keg and a challenge was issued") I can’t stand the signature buttery flavor of that beer so I am always hyper-aware of it when it appears elsewhere. In that beer and in many others it is a welcome flavor, deliberately cultivated out of yeast to give it a pleasant mouth-coating slickness and candy-like butterscotch taste. However, in many other styles, it is an unwelcome byproduct of fermentation and is sometimes imparted by dirty tap lines. It is usually caused by wimpy fermentation at too low of temperatures. Yeast produce diacetyl as a natural part of fermenting and once they run out of sugar to eat, they tend to go about cleaning up after themselves which usually includes munching down on all the buttery diacetyl they accidentally pooped out (yeast are a fungus, so they don’t technically munch or poop anything, so forgive my simplification) but if it gets too cold, they go to bed before cleaning up. Basically, the same thing you would do if you lived in alcohol. This is especially common in beers that are fermented cold, like lagers, so more steps need to be taken by the brewer to make sure the beer comes out right. 

Yeast going to town in a fermenting beer. Done too cold or ended too soon, this might result in the buttery off flavor of diacetyl.  I pilfered this photo straight from wikipedia: By Ildar Sagdejev (Specious) - Own work, GFDL,

Yeast going to town in a fermenting beer. Done too cold or ended too soon, this might result in the buttery off flavor of diacetyl.

I pilfered this photo straight from wikipedia: By Ildar Sagdejev (Specious) - Own work, GFDL,


It is often produced by British beer strains that like to go to bed early in the fermentation process, so this flavor is especially common in British styles. I perused the Beer Judge style guidelines to see where it is an acceptable flavor - turns out it’s not super common in most styles. In Czech lager styles, it is usually allowed in very low amounts. These lighter beers would showcase even a very small amount of diacetyl. Many British styles can also tolerate a small amount including British porters, milds, bitters, and stouts. Also, some Scottish and Irish beers tend to sneak some in. Looks like they have been using the same family of yeast over there. There’s no mention of it being common in some of the beers I have found it in here like some Ambers, so that is just a testament to how you can drink whichever beer you like and who cares if it is an off flavor or not! If you like Mac and Jack, Shiva, then go ahead and drink it.  

Sometimes the flavor is not produced by yeast but instead by bacteria that have infected dirty tap lines. Nothing is worse than getting a beer that you know shouldn’t have a particular off flavor and knowing that it’s bad because it literally went swam through filth to get to you. Breweries know this is a problem and clean their tap lines frequently, however, your favorite dive bar with a middling number of craft beers on tap might not. Plenty of the beer books I have read suggest that if you suspect the beer you are drinking is coming through a tainted tap line, then you have every right to send the beer back and mention it to the bartender. That sounds like a thoroughly awkward situation to me. God, imagine being on a date with someone and sending a beer back because it was bad. No one would believe you. The bartender would scoff at the blatant insult and your date would hang their head in despair and embarrassment. Better to just finish the beer, try not to reveal to your date that you are such a massive beer nerd that you can’t go out in public, and then leave a shitty Yelp review. Just kidding, don’t do that. In the name of beer, it’s better to risk the embarrassment and point out the spinach in the bar’s tapline-y teeth. 

This week you learned about diacetyl, an off flavor and sometimes on flavor in beers. It tastes like butterscotch and is usually a yeast byproduct often created by cold fermentation. Sometimes it is produced by the bacteria in dirty tap lines and if you are comfortable with it, you should point out said tap lines to your bartenders when you encounter them. If you want to know more about diacetyl (and let’s face it, who wouldn't?!) the information in this post came from Evaluating Beer, a multi-authored book of magic featuring famous homebrewer Charlie Papazian, and from Yeast by Chris White and Jamil Zainasheff. Some info also came from the BJCP 2015 Beer Style Guidelines.  Thank you for reading! Share, like, and check out my new brewer interview podcast in yesterdays post