CAMRA's raison d'etre, a quintessentially British method, but why is cask conditioning any different to other methods of packaging beer?
Prior to forced carbonation by CO2 from a bottle, the production of CO2 by yeast would've been the only way to get CO2 into solution in beer. In some places this process hung around for longer than others; a lot of Belgian brewers still use yeast to bottle condition their beers, and in the UK cask conditioning survived the technological revolution that brought us force carbonated kegged beer. CAMRA can largely claim credit for this, though at the time it was also a push against filtered, pasteurised insipid product and cask conditioning was a good place to draw a line.
Cask beer can't be sterile filtered or pasteurised as it needs to be packaged with live yeast to allow the production of CO2. This has benefits and drawbacks. On one hand, packaging cask beer becomes fairly cheap as there is no need for filtration, pasteurisation (which is expensive) nor does it require as much protection against oxygen ingress as the yeast will metabolise a fair amount of oxygen pickup. On the other hand there are pitfalls which you don't encounter with filtration and pasteurisation. Too much yeast will lead to over-carbonation, difficulties with clarity and potential flavour issues, too little yeast can cause problems with under carbonation and oxygen related spoilage and off flavours. Without pasteurisation in the container, the beer needs to be free of contaminants from the packaging process, otherwise it can pick up off flavours, acidity or over-carbonate. The beer also needs to have the right amount of residual sugar levels for the yeast to metabolise, too much and the beer overcarbonates, too little and beers will not carbonate at all. Because of the required levels of yeast, the beer is also not clear in the package, and thus we get to the cellar care required, and the need for settling the cask for a period to drop the yeast out of suspension.
Historically care in the cellar of a pub would have extended right out to controlling the amount of carbonation in the beer, by venting the cask and releasing CO2, however in the modern era this is generally not required with many beers. Indeed the requirement from pubs in the modern era is generally to reduce the need for cellar conditioning, and the cellar care tends to largely be reduced to settling beer and venting is rarely required to the levels it would have been in the past. Stuart Howe, originally head brewer at Sharps actually attributed much of the success of Doom Bar to it being conditioned in the brewery, and packaged with minimal yeast, allowing pubs to settle, tap and serve in a matter of hours
"Doom was popular with landlords because it was racked with a lower yeast count and the correct CO2 level for dispensing. This meant you could put the beer on about 4 hours after it was delivered and it would be crystal clear and full of condition. To achieve this the beer was conditioned in the brewery before racking and of course the elements which affect how it would perform in trade were measured and controlled. There weren’t many breweries in the UK doing this at the time." Boak and Bailey
Doom Bar became the hugely successful beer it is, and also helped change peoples expectations of what a cask should act like in the cellar. When I first started tapping casks, people like Brodies would have you running for cover as the foam poured out of the cask. Even something as ubiquitous as Timothy Taylors Landlord needed this traditional venting procedure to get the beer into its best condition for serving.
At Iron Pier we try to balance the two. We know that cellar space, particularly for our micropub customers, can be at a premium so venting times are preferably minimal. We also know that whilst cellar staff can vent excess CO2 off from a lively beer, you can't put CO2 in if the beer is flat.
To ensure we have the right levels of residual sugar in the beer we run forced ferment tests to find the expected final gravity of the finished product. We count yeast in suspension to ensure adequate yeast cells are present, but low enough for the beer to clear, at this stage we also look for signs of microbial contamination. We run finings trials on every batch at packaging to ensure we have the right ratio of finings to aid in the settling of the yeast as quickly as possible without over-fining.