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I've been to several bars recently that feature a "What's on cask?" section of the beer menu. The menu does not, however, suggest why that particular beer is being held in a firkin and served via the hand-pulled tap.

I do not mean this as a repeat of this question. I am interested in the reasons bars, specifically, might wish to have and serve from firkins, and not just the general difference in beer varieties.

I understand there is a historical nature to this, which might be amusing for some purposes. But I want to know if there are specific effects on the beer itself: storage timing, retention of flavor, temperature needs, pouring effect (slower, no CO2, whatever), and so on.

If the bar offered the exact same beer, one in a cask and one on a regular tap, should there be noticeable differences in the resulting served beer?

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Yes the same beer from a cask and a keg will be VERY different.

To your first point, breweries and bars will often have specially treated casks in ways that are impossible to do with kegs. Stuff like dry-hopping the beer in the cask at the bar so it's at its freshest and most intense. With a keg you'd need to put this between the tap and the glass like a Randall but with a cask you can just pop the cap and shove whatever you want in there.

Casked beer also generally still have active yeast in it, at least according to CAMRA's rules, while kegs are filtered and have no yeast in them. Because of this there's a lot of preparation involved in a proper real ale cask.

  1. The cask will come sealed from the brewery. First a soft spile or peg must be driven into the top hole. This is just a porous plug that allows air out. Based on how foamy this gets the bar can see how active the yeast is.

  2. When the yeast dies down the soft spile gets replaced with a hard spile which stops gas exchange. The cask then has to sit for a while so all the yeast can fall to the bottom as well as any other particulates or large proteins in the beer. This is called "Dropping Bright"

  3. Finally, once the beer has dropped bright the cask can be tapped which involves removing the hard spile and also either hooking up the side hole to a beer engine or a simple tap.

Because of all this and traditionally being served from a cellar or bartop at "cellar temperature" vs kegs which are refrigerated the beer that comes out will be slightly warmer (40-50F vs 30-40F), much slower because of the pumping and off-gassing from the pour, and way less carbonated. Some beer engines will also use "sparkler" taps that fizz up the beer even more on the way out to generate more head but also results in a less carbonated beer.

The result is a beer that will generally be smoother and rounder, you may get more flavor and more sweetness since it's warmer. Some flavors will come out more in the warmer and less carbonated and some will be overshadowed. Mouthfeel will be affected a lot by both factors as well. It's a hugely different animal and sometimes people like it.

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So... the short answer is novelty... and possibly, less beer loss per firkin if the CO2 is causing the beer to be head-y and a lot of beer gets wasted when bartenders pour through the foam.

The more complex answer is - as long as a beer is in a metal firkin... there should be no discerning difference in quality/taste/etc. The difference would come in the form of wooden firkins. If, for example, you had a Stout on tap and the same stout served in a wooden firkin, the longer the beer stayed in the wooden firkin, the more the flavors would change. Especially if the wooden firkin was previously used to make a spirit (like bourbon).

If the latter - bourbon barrrel - the longer the beer stays in, the more of the bourbon flavor shines through.

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