Welcome to The Certified Beer Nerd with Nigel Ayling — your guide to the geeky side of beer.

Today’s off-flavour is acetaldehyde. If you’ve ever picked up a green apple, raw pumpkin, or freshly cut grass aroma in your beer, chances are you’ve come across it.

Acetaldehyde is a natural by-product of fermentation. Yeast produces it on the way to making alcohol. Normally, the yeast reabsorbs it and converts it into ethanol. But if fermentation is rushed, the yeast is stressed, or the beer is packaged too early, acetaldehyde can stick around. It can also reappear later in a beer’s life due to oxidation.

In most beer styles, acetaldehyde is considered a flaw, giving the impression of beer that’s “unfinished.” But there are exceptions: in American light lager, a low apple-like note is sometimes acceptable.

To train yourself, start with spiked samples or simply compare to a fresh Granny Smith apple. Once you’ve got that green-apple character in your sensory memory, you’ll always be able to identify it.

So next time your beer tastes more like cider than ale, it’s probably acetaldehyde at work.

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