DIACETYL

di·a·ce·tyl – /dī-ə-ˈsēt-əl/ OR /dī-ˈas-ət-əl/ – noun

Now that we’ve covered nearly everything you’ll find on a beer’s label, it’s time to get into some stuff that goes a little deeper. In that spirit, I’ve decided we should talk about a flavor in beer that’s caused by either the brewing process OR the serving process and is seldom (if ever) wanted in a beer: Diacetyl.

Simply put, diacetyl tastes like butter. Like straight up movie theater popcorn butter. It can also be described as butterscotch but c’mon, does that sound much better? Regardless of if it’s like butter, butterscotch or Butterbean, you can see how it may not be the best thing to smell/taste in a beer.

As we typically do, we’ll keep this fairly simple so if you want to know more than what I say below, there are plenty of resources that will be happy to get technical with you. For those of you ready for the short and sweet, keep reading.

Diacetyl from Brewing

It’s no mystery how this gets into your beer at the brewery because it naturally occurs through the process of fermentation. What happens is yeast produce diacetyl early on in their fermentation lifecycle and then before they’re done completely fermenting the wort, they clean that chemical up on their own and die in peace. That’s why if you place beer into the secondary fermenter while the yeast is still slightly active, it’ll do its job to make that butter go bye-bye.

While there are plenty of yeast related things that cause diacetyl in beer, stressed yeast is the most common culprit behind the production of this chemical. The yeast strain you use and the temperature its comfortable at are very, very important factors to keep in mind when fermenting your beer. There are actually some ales, English Bitters and Scottish Ales for example, that are actually allowed to have very, very subtle diacetyl as part of their style. That being said, it should not be off-putting or truly even noticeable because that’s a LOT more than should be in there.

Lagers on the other hand LOVE to be fermented colder than ales. If you get lager yeast up to a higher temperature than it wants to be at, you can be nearly guaranteed there’s going to be diacetyl as well as a host of other flavors formed that the yeast simply doesn’t have the capacity to clean up.

Since lagers need more time to ferment due to their cold temperature, brewers actually do something called a “diacetyl” rest at the end of the fermentation cycle (typically when the beer has just a couple days of fermentation left). Because there’s a lot of diacetyl in the beer that the yeast is moving too slow to clean up, they raise the temperature of the beer so it’s closer to an ale’s fermentation range. This “wakes the yeast up” and gets them more active and ready to clean up all the stuff you don’t want in your beer.

Diacetyl from Serving

The other thing that causes this taste/aroma to get into a beer is a dirty draft line. Again, for your science-free lesson we’ll avoid the “how” but just know that improper or infrequent cleaning of a draft line creates a world of trouble for drinkers and bar owners alike. I was at a wedding recently and one of the draft beers was like a butter coated butterscotch candy. The lesson learned that day was that telling a bartender at a “regular” bar as opposed to a craft beer bar that your beer tastes like butter makes you a crazy person. If you run across this in the wild, I would just tell them the beer “tastes funny” and ask them to try it for themselves.

The last thing that I wanted to point out is that this flavor (like many others) is something that some folks are more sensitive to than others. I for one can taste subtle diacetyl when present whereas my brewing partner, Jake, is much less sensitive. I notice this not because I’m better at tasting things than Jake is, it’s just because I’m more sensitive to that particular flavor. There are master beer judges that admittedly don’t have the ability to taste certain flavors as easily as others so don’t feel bad if you can’t pick it out. Actually, you should feel good because diacetyl sucks.

Do you have a beer term that you’d like to know more about? Email me at justin@saintbrewis.com or comment below with something you’d like to see explained in a future Vocabrewlary Lesson.