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

Today’s fault is infection — a catch-all term for unwanted microbial activity in beer.

When wild yeast or bacteria take hold, they can create a range of off-flavours. Think sour, lactic, or vinegary notes, unexpected funk, or even harsh plastic-like compounds. Lactobacillus and pediococcus can sour a beer unintentionally, while wild Brettanomyces might leave earthy, barnyard flavours. Acetic acid bacteria can turn your beer into vinegar if oxygen is present.

Now, here’s where it gets interesting: what’s a flaw in one context is a feature in another. The tartness from lactobacillus in a Berliner Weisse or the funk from Brett in a lambic? Absolutely intentional. The same flavours in a pale ale or lager? That’s an infection.

Infections often happen when sanitation slips, when equipment isn’t cleaned properly, or when beer is exposed to oxygen during packaging.

To train yourself, compare intentionally sour or Brett-fermented beers with clean examples of the same base style. Once you can separate “intended” sourness from “oops, something went wrong,” you’ll have the skill nailed.

So if your clean lager suddenly tastes like vinegar or farmyard, infection has crashed the party.

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