The chances are pretty good that you’ve ingested quite a bit of diacetyl over your life. Diacetyl is a very common food additive not only due to it’s butter/butterscotch-like taste, but also because of the “slickness” of feel to your mouth it provides. That flavored oil that soaks your movie popcorn is absolutely full of it. It’s very commonly added to candies and pastries. That buttery Chardonnay you tried last week had some in it. As you might guess, it’s often in present in beer.

Diacetyl is one of the by-products of yeast respiration. As yeast gobble away at the sugars in a fermenting beer, they are constantly releasing diacetyl into the surrounding liquid. Thankfully, most of this diacetyl is reabsorbed by the yeast for use as a secondary energy supply once the sugars they have been feeding on run out. Brewers are very familiar with the concept of a “diacetyl rest”, or resting your newly fermented beer a short period of time to allow the yeast break down all of that nasty butter taste.

There are a few beers out there that actually take advantage of diacetyl in order to produce a unique taste, most notably some Belgian lambics and a well-known brew by Samuel Smith’s. But for the most part, diacetyl is not a wanted flavor agent in beer.

So why am I talking about this on a site about cellaring beer? Well unfortunately there is another way to get diacetyl in your beer. A nasty little bacteria named Pediococcus Damnosuscan also produce diacetyl, and it’s a sure sign of a compromised beverage.

Say you just cracked open that stout you’ve been saving for a couple of years and you get a whiff of butter that wasn’t there before. And damn it tastes like ass! Chances are that your beer’s defences have been breached, and there’s a full-on Pediococcus infection in your glass. There’s not much you can do, just put the beer down and grab another.

I just want you to be aware that your other beers will most likely be just fine, the pronounced taste is not a sign that the beer is not aging well, you just got a dud. Such is life. Shit happens.