Packaging
Cask
A vented container in which beer finishes fermenting and from which it is served — a live product with a working life of days.
Also known as firkin.
Cask beer is unfiltered and unpasteurised, and it finishes its fermentation in the container it is served from. A small amount of live yeast remains, producing gentle natural carbonation, and the cask is vented to the air so the beer is served at atmospheric pressure by hand pump or gravity.
That venting is what makes cask both distinctive and demanding. A firkin is nine imperial gallons, about 40.9 litres, and once it is tapped and vented, air replaces the beer drawn off. The working life is measured in days rather than months — typically three at best. Cask is a live product handled by the pub as much as by the brewery, which is why the same beer can be excellent in one establishment and tired in another.
What replaces the beer as it leaves
- The cask is vented. A peg is driven into the top and the beer inside sits at atmospheric pressure. As each pint is drawn, air enters through that vent to take its place — and because oxygen is what stales beer, the working life of a tapped cask is measured in days, typically three at best.
- The keg is sealed. Nothing enters but carbon dioxide from a cylinder, which pushes the beer out. Air never touches it at any point between the brewery and the glass, and the beer stays stable for months. That is a consequence of the seal rather than a judgement about which beer is better.
How well it protects the beer
| Light | complete. Opaque, so no light reaches the beer. Lightstrike is not a risk in this format at any point. |
|---|---|
| Oxygen | high. Substantial exposure to air. The format assumes the beer will be drunk quickly, and it should be. |
| Conditioning | Beer can referment in this container, which builds carbonation naturally and leaves a yeast sediment. |
| Material | steel |
| Typical volume | 40910 ml |
Other formats
- Aluminium can — complete light protection, very low oxygen ingress.
- Brown glass bottle — good light protection, low oxygen ingress.
- Clear glass bottle — none light protection, low oxygen ingress.
- Green glass bottle — poor light protection, low oxygen ingress.
- Keg — complete light protection, very low oxygen ingress.
- Mini keg — complete light protection, low oxygen ingress.
See the side-by-side table on the packaging index, or how this plays out over time in storage and freshness.
Last reviewed 2026-08-15.
Evidence
What BeerHQ knows about this, and how. Every statement carries the state of the evidence behind it rather than being presented flatly as fact.
A cask admits air as beer is drawn from it, which is a design consequence of dispensing without applied gas rather than a defect — and is why cask beer is intended to be sold within days of broaching.
Backed by a source
Sources: Campaign for Real Ale; Institute of Brewing & Distilling technical publications