Why cask ale is so easy to fall out with

Cask ale is beer that has never quite stopped working. It arrives at the pub live, unfiltered and unpasteurised, gets vented and tapped, and for the next few days it is at the mercy of the cellar, the lines and the pace of trade. A busy Friday night in a Lancashire town can empty three casks of bitter; a quiet Tuesday can leave one sitting half-full and breathing for a week. That gap between fresh and finished is where the faults live.

The good news is that most faults announce themselves before the beer touches your tongue. A quick sniff and a glance at the head tell you whether you are about to drink something lovely or something that should have been pulled through to the drain days ago.

Vinegar and the sour edge of infection

Vinegar is the loudest fault and the easiest to spot. It comes from acetic acid bacteria or wild yeasts getting into a cask that wasn't clean, or into beer that has sat too long with too much air above it. Smell before you sip: a prickle at the back of the nose, a sharp tang like a chip shop, a hint of cider vinegar or pickled onions.

On the tongue it shows at the sides — a sourness that lingers, rather than the clean, purposeful bitterness of hops. A trace of acidity suits an old-fashioned mild, but in a classic English bitter it doesn't. If it makes you wince, that's your answer, and a pint that tastes of vinegar will only give you a headache and a grudge.

Cardboard, paper and the slow creep of oxidation

Oxidation is the quieter, sadder fault. It happens whenever beer meets air: a cask on too long, a part-filled cask with too much headspace, beer racked or served badly. The classic note is wet cardboard — a papery, dusty smell like an old newspaper left out in the rain. Older drinkers call it stale: sherry-like, slightly sweet, with the hop aroma gone flat.

The tell is that the beer smells older than it looks. The colour may still be bright, the head may still be decent, but the aroma has collapsed into something dull and brown. You notice it most in heavily hopped beers, where a fresh grapefruit or grassy note has faded to nothing.

Oxidised beer won't hurt you, but it is joyless. Compare a freshly tapped cask with one that has been on for a week and the difference is obvious.

Sulphur, eggs and the whiff of a brewery floor

Sulphur is the one that catches people out, because it can be honest and it can be a fault. Young beer sometimes throws a matchbox or boiled-egg note as fermentation finishes, and given time it blows off. In a properly vented cask it should be gone by the time the beer reaches the bar.

What you don't want is a pint that smells of struck matches, drains or a boiled egg with the lid left on. That points to a cask tapped too early, vented too briefly, or a struggling fermentation. Sometimes it comes with a rubbery, vegetal edge — cooked cabbage, tinned sweetcorn — which rarely improves.

Give it a minute in the glass. If the sulphur blows off, drink up. If it stays, it won't get better.

Butterscotch, must and other cellar ghosts

Diacetyl is the buttery one: a slick, toffee-popcorn note that coats the tongue and sits on top of a pint that should be crisp. A touch can be part of the style in some dark beers and milds, but a full-on butterscotch bomb usually means fermentation stopped early or the beer was racked off the yeast too soon.

Then there are the taints that have nothing to do with the beer. Musty, damp-cellar smells — wet wool, old sacks, soil — usually mean dirty lines, a mouldy cellar or a cask stored somewhere it shouldn't be. A tinny, metallic tang at the back of the throat often points at old pipework or a poorly cleaned firkin.

Smell the empty glass after a good pint, too. It teaches your nose what clean beer smells like, which makes the dirty version much easier to catch.

Condition, temperature and what to do at the bar

Faults aren't only about flavour. A cask ale should have a tight, lacy head and a soft, natural condition — lightly sparkling, never flat and never fizzing like lager. No head and a thin, lifeless body means the cask has been on too long or the venting has gone wrong. Too cold numbs the tongue; cask ale is best at around 11–13°C. Too warm, above 14°C, and every fault is amplified.

If you get a bad pint, speak up. Ask for a taster first if you're unsure — a third of a pint costs the pub almost nothing and saves everyone a scene. If the beer is clearly off, say so calmly: "This tastes a bit vinegary — could I try something else?" Any decent landlord will swap it, and a good one will want to know, because a cask going over is money leaking out of the cellar.

The same nose serves you beyond cask ale. Cider can turn to vinegar or pick up a mousey, cereal-like taint; home-brewed beer goes cardboardy if it's bottled with too much air in the neck; a badly stored soft drink tastes dull and flat. Learn the vocabulary in the pub and you'll brew and drink better everywhere.