One of the most important skills for any beer nerd — and absolutely essential for exams like Cicerone or BJCP judging — is learning to detect off-flavours. Let’s dive into one of the most common: diacetyl.

Diacetyl is a compound produced naturally during fermentation. In small amounts, it can smell and taste like butter, butterscotch, or even buttery popcorn. Sounds tasty, right? But in beer, it’s usually considered a fault. That slick, buttery character can overwhelm the clean profile brewers are aiming for, especially in lagers and hop-forward styles.

So how does diacetyl get there? Normally, healthy yeast will reabsorb and break it down at the end of fermentation. But if fermentation is cut short, if the yeast is stressed, or if certain bacteria like pediococcus or lactobacillus sneak in, diacetyl can hang around. That’s why many brewers use a “diacetyl rest” — warming the beer slightly at the end of fermentation to help the yeast clean it up.

Now, here’s the catch: not all diacetyl is bad. Some traditional styles — like English bitters, some Scottish ales, and a few Czech lagers — may have a low level that adds complexity. But in most lagers, IPAs, or American ales, it sticks out like a sore thumb.

Training your palate to recognise diacetyl is a game-changer. The best way is through spiked tasting kits used in sensory training, where a neutral beer is dosed with diacetyl so you can lock in the flavour memory. And yes, it’s not the most pleasant sip — but once you’ve identified it, you’ll never miss it again.
So next time you get that unexpected buttered popcorn note in a beer, you’ll know exactly what’s going on.

That’s The Certified Beer Nerd — levelling up your tasting skills, one sip at a time.