Imagine. You’ve got your hands on a beer at last. It’s been a long day. You’re tired, and you’re thirsty. There’s nothing you want more than to enjoy those first satisfying swallows of beer. But as you lift the glass to your mouth, what’s that you smell? Is it… is it butter!?
This is a sure-fire sign your beer is faulty. The problem is that it contains too much diacetyl. This is a malt-derived chemical that gives beer an aroma like butter. Some people also report aromas of butterscotch or movie-theatre popcorn. And it’s not just the smell. Diacetyl can also cause the beer to feel flat and slick on your palate. All in all it’s not the pleasant experience we expect our beer to deliver.
So how did this happen?
Diacetyl is present in all beer. It’s a natural side-effect of the fermentation process. Usually there’s so little you can’t even taste it. No harm, no foul. The problems I’ve described above only happen when there’s too much of it in your beer.
The problem is incomplete fermentation
There’s a saying in brewing circles that brewer’s don’t make beer. They make wort (unfermented beer) that is food for yeast. It’s the yeast that makes the beer. Brewers are in the yeast-husbandry business.
Well, buttery beer is a sign of poor yeast management. It could be that the brewer has taken the beer off the yeast too soon. Or the brewer allowed fermentation to happen at the wrong temperature — in this case, one that was too cool.
In the normal process of fermentation diacetyl builds up over time. It’s a by-product of the yeast’s activity. But when the yeast reaches a certain point in the fermentation…