I recently put some unsweetened cocoa powder into my secondary and got an erruption ......... bubbled up and came out the carboy like a volcano going off. Don't understand what went on.
I used 8 ozs cocoa powder in 7 gallons. Because the carboy was pretty much full, instead of using water to make the cocoa into a paste/liquid, I took beer out of the carboy (a couple of ounces at a time), and mixed it in a bowl with the cocoa. It took 20 ozs to get the powder into a liquid. I used 24 ozs total to make it flow better when pouring back in. I mixed it before adding, because just dumping in the powder, wouldn't have dissolved it.
I started to pour it through a funnel, and when I had about half in, I noticed bubbling in the fermenter. I stopped pouring, removed the funnel, and reached for a handy towel to wrap around the carboy, just in time to prevent a major mess. It came up like lava, and when it stopped it left a 'mountain' of bubbles about 4 inches high above the carboy.
I cleaned it up and added the rest slowly (about an ounce at a time), and got it in in the end. So I don't actually know how much cocoa I got in the fermenter.
Not sure what happened. The cocoa mix was alkali, while the beer is acidic. It was a Brett beer so may have been more acidic than normal. I don't know if that had anything to do with it.