Cask or Keg: What Actually Changes in the Glass

The difference between a cask beer and a keg beer is not the recipe. It is what happens to the beer after it leaves the brewery, and how it gets from the container to the glass. A cask beer keeps fermenting gently inside its own container and reaches the glass by gravity or a hand pump, with no added gas. A keg beer has already finished fermenting, is filtered and usually pasteurised, and is pushed to the tap by extraneous carbon dioxide or nitrogen. Everything a festival pourer or a drinker notices, from the texture to how fast a line can serve a crowd, follows from that one split.

What does cask-conditioned actually mean?

A cask-conditioned beer, often called real ale, undergoes a secondary fermentation inside the very container it is served from. The beer is racked into the cask while still cloudy with live yeast, and that yeast keeps working at cellar temperature, slowly producing the natural carbonation the beer will have when it is poured. The beer in the glass carries the gas it made itself, not gas added afterward.

Because that fermentation is still live, a cask beer also needs time to settle. A cask that has just been moved or tapped is cloudy; staff let it rest so the yeast and any finings drop to the bottom, and a vented spile lets the excess pressure escape instead of building up. Skip that step and the pour looks hazy and tastes rougher than the brewer intended.

How is a cask beer actually dispensed?

Cask beer reaches the glass two ways, and both avoid pushing the beer with outside gas. The traditional method is gravity: the cask sits on a stillage and a tap at its base lets the beer run straight into the glass. The more familiar method on a festival bar is the hand pump, a lever-operated pump that draws the beer up from the cask through a line, using suction rather than pressure from a cylinder. Either way, nothing but the beer's own gentle carbonation is in the glass.

This is also why a cask line runs slower than a keg line. A hand pump needs a few strokes per pint, and a gravity tap only flows as fast as the beer wants to fall. A long festival queue in front of a cask stall is partly a sign of popularity, and partly just the physics of pouring without pressure.

How is that different from a keg beer?

A keg beer has already stopped fermenting by the time it reaches the festival. It is filtered to remove the yeast, and usually pasteurised, which means the beer that leaves the brewery is a stable liquid with no living culture left inside it. Because the natural carbonation was filtered out along with the yeast, the brewery adds carbon dioxide, or a mix of carbon dioxide and nitrogen for smoother styles such as stout, before it leaves.

At the festival, that same gas is used again to push the beer out. A cylinder connected to the keg keeps the beer under pressure, forcing it along the line to the tap whenever a valve opens. A keg line can therefore pour continuously and quickly, because the gas does the work that a hand pump otherwise has to do stroke by stroke. That speed is one reason a keg bar can serve a long queue without the wait a cask bar needs.

Why does a festival pour both differently?

A festival pour of cask beer is usually served a little below room temperature, close to a traditional cellar temperature, with a loose, naturally soft carbonation that gives a rounder mouthfeel and little or no head. A keg pour arrives colder and with the tighter, more persistent fizz that comes from forced carbonation, closer to what most drinkers expect from a bottled or canned beer.

Neither method makes a better beer on its own. A hoppy pale ale built to show its aroma often favours the gentler cask carbonation, which does not strip scent the way a hard fizz can. A style built around crispness, or one travelling a long way to a festival in warm weather, often keeps better and pours more consistently from a keg. A programme that lists both, side by side, is telling the drinker something true about the beer before it is even poured: how it was finished, and how it is about to behave in the glass.

What to look for at the bar

Three cues tell a drinker which they are being handed before the first sip. A visible hand pump or a cask on a stillage behind the counter means cask; a line running from a cylinder to a tap font means keg. A slower queue with a rounder, softer head usually means cask; a quicker queue with a tight, lasting head usually means keg. And a beer served noticeably warmer than a lager, carrying a light natural sparkle rather than a hard fizz, is almost always the cask pour doing exactly what its second fermentation was supposed to do.