In between uses, I attempted to keep my barrel sterile by filling it with "holding solution" of citric acid and metabisulfate that I read about on this More Winemaking article and cross-checked with a couple other sources (I've since switched to burning sulfur discs).
Before aging my next batch, a Scotch ale / wee heavy, I rinsed the barrel several times and let it soak overnight, but apparently this wasn't enough -- the beer has a citric acid tartness (definitely not acetic).
I've conducted a couple experiments neutralizing the effects with calcium carbonate (chalk) and sodium bicarbonate (baking soda), but found that chalk left a slight astringency and baking soda raised the pH significantly (don't have my notes in front of me but from around 4.2 to 5.4 maybe). I might try a dilute sodium hydroxide (lye) solution but at this point I'm basically just throwing things at the wall to see what sticks.
Does anyone have other suggestions about what could I use to neutralize citric acid that would result in the least noticeable byproducts? Maybe something that would form a precipitate?