Neutralizing citric acid from barrel holding solution

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?

You can neutralize the hydrogen ions that come with the citric acid but you can't 'neutralize' the citrate ions which have, as you have discovered, a distinct flavor. They have apparently worked their way into the pores of the wood and you will have to try to draw them out. All I can think of is to fill the barrel with water and add lye to get the pH up to the level where all the ions are deprotonated - the ions ought to be more soluble that the acid - and let it soak for days or weeks or however long it takes to get the flavor out. Then rinse and rinse with water to get the lye out.

Thanks. I'm more concerned with salvaging the beer than the barrel, as I believe the last aging in the barrel plus a few overnight soakings should have drawn out the remaining citric acid.

Researching a little more, I believe I may be able to form calcium citrate (which is virtually insoluble) through the addition of a calcium salt -- either calcium hydroxide (slaked lime) or calcium chloride. Any opinions on those? I'm leaning Ca(OH)2 since I won't be left with chloride ions.

Via calcium hydroxide mentioned here:
http://en.wikipedia.org/wiki/Calcium_citrate

Via calcium chloride:
http://www.jbc.org/content/123/2/405.full.pdf

I don't know how soluble calcium citrate is except that it is described, as you have done, as 'virtually insoluble' so I suppose you have a chance. Especially if you use an excess of calcium as that will 'push' the reaction from the left (LeChatelier's principle):

3Ca++ 2Cit.--- ----> 2Ca3Cit2

You'll want to have enough of the hydroxide to raise the pH to about 8.5 to 9 to make sure all the citrate is deprotonated and available to form the salt but I would also think you would want enough calcium chloride to push from the left. Assuming this works you will precipitate calcium citrate but now must wait for that to settle out or filter it out which may take some time. Now you have beer at pH 9 which can't taste too good so you will need to add acid (not citric!) to get the pH back to 4.5 or so. It will contain more calcium and more chloride that it did to start but chloride usually improves things up to a point.

I think your hypothesis is solid enough to warrant some testing with small quantities of the beer. Start by adding the lime, waiting and then bringing pH back with acid. You may not need the extra chloride.

No promises in any of this!

Thanks, I have the chemicals on order and I'll let you know how it turns out. I wish I had actual lab equipment but I don't so I'll probably just ballpark some numbers and go by trial and error.