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What is the point in "burping" a keg right before force carbonating it? When the co2 hits the keg during carbonation it is going to provide a protective blanket to any air above it.

I am confused about this practice and I believe it is redundant. I can't offer evidence because I always burp the keg myself. :-)

If you want to serve cask ale at home with a beer engine and not have oxygen spoil your beer there is a device which can be connected to your co2 tank which will shoot a burst of co2 into your keg each time the beer engine is pumped and oxygen is shot into your keg. This co2 will protect your beer from the oxygen and this way you can have cask ale at home without having to finish it all in one go. The fact that this works should be reason enough that burping is unnecessary.

Here are a couple of videos showing this device:

Beer Engine at home - Jump to 4:04

Northern Brewer Cask Ale video - Jump to 5:40

And here is the device on Micro Matic: Cask Breather

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    I think you've misunderstood the mechanism of the cask breather a bit. CO2 isn't added along with O2, it's added instead of it. Using one should entirely exclude O2 from the keg/cask. Micro Matic's website is a tad misleading in referring to said blanketing effect. @jsled is absolutely right, the whole idea of the "CO2 blanket" is largely false. – Franklin P Combs Feb 25 '15 at 15:36
  • Traditionally casks allowed O2 to enter as beer was drawn out, but these were served in pubs where the cask would empty within a day or so. Unless you are going through your homebrew that quickly, you will want CO2 replacing the beer. – jalynn2 Feb 26 '15 at 18:21
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Because that's not how gasses work. :)

Gasses in a space, especially under pressure, are going to come into an equilibrium, a homogeneous combination. While during active fermentation, or for a brief period of time after off-gassing CO₂ the (heavier) CO₂ will form some sort of layer/"barrier" on top of the beer, after time or once pressurized, that will not be the case. CO₂, O₂ and other gasses will become uniform, and the beer will be exposed to O₂. Purging/burping the keg vents this combination of gasses, reducing the fraction of O₂ remaining. Purging a few times will further reduce that fraction of O₂ remaining.

I can't speak to cask/beer engines, but I don't have any reason to believe this is not true there, either. I suppose the idea is to have little/no carbonation. Even still, I would opt to replace the keg head space with very low PSI CO₂, rather than O₂.

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    Nice. This article does a good job of explaining why the blanket theory is sort of right for a short period of time, if anyone's interested. – Franklin P Combs Feb 25 '15 at 15:46
  • Thanks your answer makes a lot of sense. This raises another question though. Without the blanket why can we leave a beer in a plastic bucket primary without an air tight lid (i.e we have an air lock but no rubber gasket on the lid) for 3 to 4 weeks? I've done this with low gravity beers where terminal gravity should have been achieved in a week. co2 production would have halted and air should still be seeping into the bucket, albeit slowly. Without the blanket the beer would be slowly oxidizing. My guess is the oxidation is minimal as the air intake is very small. – fthinker Feb 25 '15 at 18:28
  • It's true, you will get O₂ introduction, though the lid and through the permeability of the plastic itself, but that's going to add up to relatively small amounts of dissolved oxygen. Plus, beer in primary still has some active yeast, which will take up some of the dissolve oxygen. – jsled Feb 25 '15 at 19:45

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