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I've just been playing with some treatments for an undercarbonated batch in bottles. The beer is slightly carbonated, it bubbles up a little when poured, just builds no head & dissipates instantly.

I thought I'd try opening a couple of bottles & adding some carbonation drops, re-seal & see what happens after a week or two. I put 1 drop each in 2 bottles and 2 drops each in another 2 bottles.

When the sugar was added to the beer, it fizzed up. The 1-drop bottles bubbled a little but I was able to get the cap on quickly enough, however the 2-drop bottles bubbled quite viciously, much like with the Diet Coke & Mentos trick.

What could possibly have caused this effect? Obviously it only occurs after conditioning has started, otherwise bottling day would be a huge mess.

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up vote 2 down vote accepted

The bubbles in beer (and other fizzy drinks) only form when the CO2 has particles to attach to. (Related phenomenon occur in cloud formation and boiling water, among other things.) This causes a chain reaction where more CO2 is attracted, and bubbles are formed. When you add sugar to the beer, suddenly there's a lot more for the CO2 to grab on to. The reason this happens so violently with mentos is that they have very fine microstructures on the surface that encourages more bubbles to form, maybe the same phenomenon (but to a lesser degree) applies to the carbonation drops.

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The process is called nucleation. At the microscopic level the surface of the sugar is very rough. This roughness creates a place for the CO2 dissolved in the liquid to force itself out of solution and appear as bubbles.

Its the same principle that applies to etching in glassware to help promote the appearance of bubbles in the beer. It can also be debated that the constant stream of bubbles helps get the aromatics out of the beer for enjoyment.

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