Why Hammer a Peg Into the Barrel? The Beer Needs to Breathe a Little

Beer racked into a cask still contains live yeast and some fermentable sugar, so fermentation continues slowly in the cellar and produces carbon dioxide. Some of that dissolves in the beer and gives it the gentle liveliness that cask beer is meant to have, and the rest accumulates as pressure in the space above. A sealed cask would build pressure until it leaked or burst, and an open one would lose all its gas and go flat. The peg controls the balance, and managing it is the central skill of cellar work.

Two different pegs are used at different stages:

• •A porous soft one is driven in first and lets gas escape freely
• •That vents the initial vigorous pressure over a day or so
• •Foam rising through it indicates the beer is still working hard
• •Once it settles, a hard non-porous one replaces it and seals the cask
• •That holds the remaining gas in and keeps the beer lively
• •The hard one is loosened before serving and reseated afterwards

The judgement involved is why cellar work is a skill rather than a procedure, and the failures are recognisable in the glass. Venting too little leaves the beer over-gassy, which makes it pour as foam and taste sharp. Venting too much leaves it flat and lifeless. Leaving the hard peg out between servings loses the condition overnight and lets air in, which oxidises the beer within a day or two and gives it a stale cardboard character. Disturbing the cask after it has settled stirs the sediment back into suspension and makes the beer cloudy, which is why casks are left to stand for a day or more before they are broached.

Preparing a cask involves a sequence in which each step has a reason, and knowing it explains why cellar practice looks fussy. The cask is stilled on its side on a stand with a slight tilt, so sediment settles at the low end away from the tap. It is left undisturbed for at least a day, and longer for a strong or a freshly moved beer. Venting follows, then the tap is driven in with a single firm blow, since a hesitant one lets beer spray and lets air in. Finings may be added to drop yeast out of suspension. Only then is it sampled, and a cask that is not right is left rather than served.

The whole arrangement follows from a decision about how the beer is finished and separates cask beer from everything else on a bar. Beer in a keg is filtered or pasteurised to stop the yeast working, then sealed and served under added gas, which makes it stable for months, consistent and easy to handle. Cask beer is unfiltered, unpasteurised and still alive, which gives it a character that many drinkers prefer and makes it a perishable product with a life of a few days once broached. That difference is why cask beer requires a trained cellar and why it varies between pubs serving the same brew.

Beer racked into a cask keeps fermenting, so gas must be allowed to escape without losing all of it, which a porous peg does at first and a sealed one manages afterwards. Too little venting gives foam and sharpness, too much gives flatness, and leaving the cask open overnight admits air and stales the beer within days. The whole arrangement follows from the beer being unfiltered and still alive.