When beer goes bad: Diacetyl

If you’ve ever experienced a butter or butterscotch flavour in your beer than you’re tasting diacetyl. The interesting thing about diacetyl, I think, is that not everyone can detect it as a flavour; of those who can detect it, some like it or don’t mind it, while others can’t stand it.

Diacetyl is a natural product of fermentation and is created and given off by the yeast cells early in fermentation. The yeast cells then take it back within themselves towards the end of fermentation and convert it into something almost tasteless – if a brewer stops the fermentation process before the yeast has finished its conversion, or if the fermentation stalls, then diacetyl will still be there (stopping it early could be the result of a need for brewing speed or just a slack move by the brewer; if it stalls early then the yeast is probably not healthy enough).

Lower temperatures towards the end of fermentation could see lazy yeast leaving behind diacetyl. One way to combat any leftover diacetyl is to include a diacetyl rest in the fermentation process, which involves slightly raising the temperature to poke the yeast into a final burst of action and leaving it for a further day. Diacetyl is an easy thing to prevent or control unless you’ve got a yeast infection.

An excess of diacetyl can be the result of a contaminated yeast which isn’t up to the job of converting the buttery flavour into a flavourless one, so the yeast needs to be fit and healthy. It could also be the result of an infection or bacteria in the yeast or the presence of wild yeast – some of these produce diacetyl. However, some strains of yeast just throw off more diacetyl that others when they ferment (pale ale and lager yeasts often do).

Beer shouldn’t taste like butter but small amounts are appropriate to some beer styles – pale lagers and some pale ales. In larger volumes diacetyl can give a fatty, slick mouthfeel (as if there is some melted butter in your pint...) which is very unpleasant. If your pint tastes like butter then you might not want to finish it, but that’s down to individual tastes and it’s not a fault that will do you any harm.

Diacetyl is one of the off-flavours which many find easy to recognise and can sniff it out an arm’s length away. I used to enjoy the flavour in beer (mmm, this has a delicious buttery taste!) until I realised that it shouldn’t be there and now my inner beer geek just screams out DIACETYL! whenever I taste it. I now only get on with it if it’s in small volumes. Are you sensitive to diacetyl? Do you like it, tolerate it or hate it?

This is the first in a series of posts looking at when beer goes bad. Being neither a brewer nor a scientist I’ve tried to keep it simple and from a drinker’s point of view. If I’ve missed anything interesting or important out then please add it below.

I used Beer Sensory Science (a great blog!) to help with this post.