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.

4 Answers 4


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.


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.


Suggestion: Instead of repriming with sugar or carbonation drops, try using corn syrup instead. (Make your own, don't buy commercial corn syrup.) Adding the syrup eliminates much of the nucleation and loss of carbonation. Its even better if you chill the syrup before adding to your under carbonated brew.

I have found that when you dissolve 4 oz corn sugar in 1/2 cup of water and boil for one minute it yields almost exactly 200 milliliters (ml) of syrup.

For initial priming a new 5 gallon batch at 65 °F. you would use the full amount (200 ml) to get 2.4 vol CO2. This equates to about 4 ml per 12 oz bottle. (This gives you an idea of the upper limit.)

Since you have already primed your beer, but it's under carbonated, you don't know how much carbonation is in it or how much you need to add, so you will need to experiment with the syrup just as you did with the carbonation drops. I suggest you start with 1 milliliter in one bottle and see what happens in about a week. Whatever you do, don't put 4 or more milliliters of the syrup in a bottle unless you want bottle bombs.

PS If you don't have any small metric spoons or cups, you can safely experiment with 1/4 teaspoon syrup which is about 1.25 ml.


I needed to sweeten and carbonate my flat cider. Done like this:

  • carbonated 1 liter with the chinese carbonator
  • dissolved a few teaspoons of sugar inside 150 ml of still cider
  • mixed the two above, no more foamy mess HTH
  • I agree with your comment and your process. This is an effective way to prevent gushing. However from the standpoint of StackExchange, this is not a direct answer to the Question above, but rather a comment. There's a link for commenting immediately beneath the original Question. That's how we do things on StackExchange. Cheers.
    – dmtaylor
    Commented Feb 26, 2021 at 11:48

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